diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 23d3e71424b2b..2770bded30aec 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -71,6 +71,11 @@ jobs:
         uses: actions/checkout@v3
         with:
           fetch-depth: 2
+      - name: "checkout the `master` branch for tidy"
+        uses: actions/checkout@v3
+        with:
+          ref: master
+          fetch-depth: 1
       - name: configure the PR in which the error message will be posted
         run: "echo \"[CI_PR_NUMBER=$num]\""
         env:
@@ -485,6 +490,11 @@ jobs:
         uses: actions/checkout@v3
         with:
           fetch-depth: 2
+      - name: "checkout the `master` branch for tidy"
+        uses: actions/checkout@v3
+        with:
+          ref: master
+          fetch-depth: 1
       - name: configure the PR in which the error message will be posted
         run: "echo \"[CI_PR_NUMBER=$num]\""
         env:
@@ -600,6 +610,11 @@ jobs:
         uses: actions/checkout@v3
         with:
           fetch-depth: 2
+      - name: "checkout the `master` branch for tidy"
+        uses: actions/checkout@v3
+        with:
+          ref: master
+          fetch-depth: 1
       - name: configure the PR in which the error message will be posted
         run: "echo \"[CI_PR_NUMBER=$num]\""
         env:
diff --git a/Cargo.lock b/Cargo.lock
index 9581099c210ea..7a86a1012a761 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -259,6 +259,10 @@ dependencies = [
  "toml",
 ]
 
+[[package]]
+name = "build_helper"
+version = "0.1.0"
+
 [[package]]
 name = "bump-stage0"
 version = "0.1.0"
@@ -2675,9 +2679,9 @@ dependencies = [
 
 [[package]]
 name = "owo-colors"
-version = "3.4.0"
+version = "3.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "decf7381921fea4dcb2549c5667eda59b3ec297ab7e2b5fc33eac69d2e7da87b"
+checksum = "c1b04fb49957986fdce4d6ee7a65027d55d4b6d2265e5848bbb507b58ccfdb6f"
 
 [[package]]
 name = "packed_simd_2"
@@ -5203,9 +5207,9 @@ dependencies = [
 
 [[package]]
 name = "termcolor"
-version = "1.1.2"
+version = "1.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2dfed899f0eb03f32ee8c6a0aabdb8a7949659e3466561fc0adf54e26d88c5f4"
+checksum = "bab24d30b911b2376f3a13cc2cd443142f0c81dda04c118693e35b3835757755"
 dependencies = [
  "winapi-util",
 ]
@@ -5304,11 +5308,13 @@ dependencies = [
 name = "tidy"
 version = "0.1.0"
 dependencies = [
+ "build_helper",
  "cargo_metadata 0.14.0",
  "ignore",
  "lazy_static",
  "miropt-test-tools",
  "regex",
+ "termcolor",
  "walkdir",
 ]
 
diff --git a/Cargo.toml b/Cargo.toml
index 1cec4a84480c4..ce08d4edb567d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,6 +4,7 @@ members = [
   "library/std",
   "library/test",
   "src/rustdoc-json-types",
+  "src/tools/build_helper",
   "src/tools/cargotest",
   "src/tools/clippy",
   "src/tools/clippy/clippy_dev",
diff --git a/compiler/rustc_target/src/abi/call/m68k.rs b/compiler/rustc_target/src/abi/call/m68k.rs
index c1e0f54af5f1e..1d4649ed8678e 100644
--- a/compiler/rustc_target/src/abi/call/m68k.rs
+++ b/compiler/rustc_target/src/abi/call/m68k.rs
@@ -10,7 +10,7 @@ fn classify_ret<Ty>(ret: &mut ArgAbi<'_, Ty>) {
 
 fn classify_arg<Ty>(arg: &mut ArgAbi<'_, Ty>) {
     if arg.layout.is_aggregate() {
-        arg.make_indirect_byval();
+        arg.make_indirect_byval(None);
     } else {
         arg.extend_integer_width_to(32);
     }
diff --git a/compiler/rustc_target/src/abi/call/mod.rs b/compiler/rustc_target/src/abi/call/mod.rs
index a5ffaebea0b98..9e799282c4f31 100644
--- a/compiler/rustc_target/src/abi/call/mod.rs
+++ b/compiler/rustc_target/src/abi/call/mod.rs
@@ -500,9 +500,7 @@ impl<'a, Ty> ArgAbi<'a, Ty> {
             .set(ArgAttribute::NonNull)
             .set(ArgAttribute::NoUndef);
         attrs.pointee_size = layout.size;
-        // FIXME(eddyb) We should be doing this, but at least on
-        // i686-pc-windows-msvc, it results in wrong stack offsets.
-        // attrs.pointee_align = Some(layout.align.abi);
+        attrs.pointee_align = Some(layout.align.abi);
 
         let extra_attrs = layout.is_unsized().then_some(ArgAttributes::new());
 
@@ -519,11 +517,19 @@ impl<'a, Ty> ArgAbi<'a, Ty> {
         self.mode = Self::indirect_pass_mode(&self.layout);
     }
 
-    pub fn make_indirect_byval(&mut self) {
+    pub fn make_indirect_byval(&mut self, byval_align: Option<Align>) {
         self.make_indirect();
         match self.mode {
-            PassMode::Indirect { attrs: _, extra_attrs: _, ref mut on_stack } => {
+            PassMode::Indirect { ref mut attrs, extra_attrs: _, ref mut on_stack } => {
                 *on_stack = true;
+
+                // Some platforms, like 32-bit x86, change the alignment of the type when passing
+                // `byval`. Account for that.
+                if let Some(byval_align) = byval_align {
+                    // On all targets with byval align this is currently true, so let's assert it.
+                    debug_assert!(byval_align >= Align::from_bytes(4).unwrap());
+                    attrs.pointee_align = Some(byval_align);
+                }
             }
             _ => unreachable!(),
         }
@@ -660,7 +666,8 @@ impl<'a, Ty> FnAbi<'a, Ty> {
     {
         if abi == spec::abi::Abi::X86Interrupt {
             if let Some(arg) = self.args.first_mut() {
-                arg.make_indirect_byval();
+                // FIXME(pcwalton): This probably should use the x86 `byval` ABI...
+                arg.make_indirect_byval(None);
             }
             return Ok(());
         }
diff --git a/compiler/rustc_target/src/abi/call/wasm.rs b/compiler/rustc_target/src/abi/call/wasm.rs
index 44427ee5317c1..0eb2309ecb28b 100644
--- a/compiler/rustc_target/src/abi/call/wasm.rs
+++ b/compiler/rustc_target/src/abi/call/wasm.rs
@@ -36,7 +36,7 @@ where
 {
     arg.extend_integer_width_to(32);
     if arg.layout.is_aggregate() && !unwrap_trivial_aggregate(cx, arg) {
-        arg.make_indirect_byval();
+        arg.make_indirect_byval(None);
     }
 }
 
diff --git a/compiler/rustc_target/src/abi/call/x86.rs b/compiler/rustc_target/src/abi/call/x86.rs
index 7c26335dcf4cd..58c0717b7d1b3 100644
--- a/compiler/rustc_target/src/abi/call/x86.rs
+++ b/compiler/rustc_target/src/abi/call/x86.rs
@@ -1,5 +1,5 @@
 use crate::abi::call::{ArgAttribute, FnAbi, PassMode, Reg, RegKind};
-use crate::abi::{HasDataLayout, TyAbiInterface};
+use crate::abi::{Align, HasDataLayout, TyAbiInterface};
 use crate::spec::HasTargetSpec;
 
 #[derive(PartialEq)]
@@ -53,11 +53,38 @@ where
         if arg.is_ignore() {
             continue;
         }
-        if arg.layout.is_aggregate() {
-            arg.make_indirect_byval();
-        } else {
+        if !arg.layout.is_aggregate() {
             arg.extend_integer_width_to(32);
+            continue;
         }
+
+        // We need to compute the alignment of the `byval` argument. The rules can be found in
+        // `X86_32ABIInfo::getTypeStackAlignInBytes` in Clang's `TargetInfo.cpp`. Summarized here,
+        // they are:
+        //
+        // 1. If the natural alignment of the type is less than or equal to 4, the alignment is 4.
+        //
+        // 2. Otherwise, on Linux, the alignment of any vector type is the natural alignment.
+        // (This doesn't matter here because we ensure we have an aggregate with the check above.)
+        //
+        // 3. Otherwise, on Apple platforms, the alignment of anything that contains a vector type
+        // is 16.
+        //
+        // 4. If none of these conditions are true, the alignment is 4.
+        let t = cx.target_spec();
+        let align_4 = Align::from_bytes(4).unwrap();
+        let align_16 = Align::from_bytes(16).unwrap();
+        let byval_align = if arg.layout.align.abi < align_4 {
+            align_4
+        } else if t.is_like_osx && arg.layout.align.abi >= align_16 {
+            // FIXME(pcwalton): This is dubious--we should actually be looking inside the type to
+            // determine if it contains SIMD vector values--but I think it's fine?
+            align_16
+        } else {
+            align_4
+        };
+
+        arg.make_indirect_byval(Some(byval_align));
     }
 
     if flavor == Flavor::FastcallOrVectorcall {
diff --git a/compiler/rustc_target/src/abi/call/x86_64.rs b/compiler/rustc_target/src/abi/call/x86_64.rs
index c0c071a614f50..54abf23c15054 100644
--- a/compiler/rustc_target/src/abi/call/x86_64.rs
+++ b/compiler/rustc_target/src/abi/call/x86_64.rs
@@ -213,7 +213,7 @@ where
         match cls_or_mem {
             Err(Memory) => {
                 if is_arg {
-                    arg.make_indirect_byval();
+                    arg.make_indirect_byval(None);
                 } else {
                     // `sret` parameter thus one less integer register available
                     arg.make_indirect();
diff --git a/src/bootstrap/Cargo.lock b/src/bootstrap/Cargo.lock
index efe8ae3169fc2..4a0ba5925773e 100644
--- a/src/bootstrap/Cargo.lock
+++ b/src/bootstrap/Cargo.lock
@@ -36,6 +36,7 @@ dependencies = [
 name = "bootstrap"
 version = "0.0.0"
 dependencies = [
+ "build_helper",
  "cc",
  "cmake",
  "fd-lock",
@@ -70,6 +71,10 @@ dependencies = [
  "regex-automata",
 ]
 
+[[package]]
+name = "build_helper"
+version = "0.1.0"
+
 [[package]]
 name = "cc"
 version = "1.0.73"
diff --git a/src/bootstrap/Cargo.toml b/src/bootstrap/Cargo.toml
index fafe82a9c128a..22ceeca941e93 100644
--- a/src/bootstrap/Cargo.toml
+++ b/src/bootstrap/Cargo.toml
@@ -30,6 +30,7 @@ path = "bin/sccache-plus-cl.rs"
 test = false
 
 [dependencies]
+build_helper = { path = "../tools/build_helper" }
 cmake = "0.1.38"
 fd-lock = "3.0.8"
 filetime = "0.2"
diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 707e4169002d9..66bc0f023b6c9 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -97,18 +97,36 @@ impl RunConfig<'_> {
         self.builder.build.build
     }
 
-    /// Return a `-p=x -p=y` string suitable for passing to a cargo invocation.
+    /// Return a list of crate names selected by `run.paths`.
     pub fn cargo_crates_in_set(&self) -> Interned<Vec<String>> {
         let mut crates = Vec::new();
         for krate in &self.paths {
             let path = krate.assert_single_path();
             let crate_name = self.builder.crate_paths[&path.path];
-            crates.push(format!("-p={crate_name}"));
+            crates.push(crate_name.to_string());
         }
         INTERNER.intern_list(crates)
     }
 }
 
+/// A description of the crates in this set, suitable for passing to `builder.info`.
+///
+/// `crates` should be generated by [`RunConfig::cargo_crates_in_set`].
+pub fn crate_description(crates: &[impl AsRef<str>]) -> String {
+    if crates.is_empty() {
+        return "".into();
+    }
+
+    let mut descr = String::from(" {");
+    descr.push_str(crates[0].as_ref());
+    for krate in &crates[1..] {
+        descr.push_str(", ");
+        descr.push_str(krate.as_ref());
+    }
+    descr.push('}');
+    descr
+}
+
 struct StepDescription {
     default: bool,
     only_hosts: bool,
diff --git a/src/bootstrap/check.rs b/src/bootstrap/check.rs
index 2e1bd8d6d1f6d..32e5d414061ec 100644
--- a/src/bootstrap/check.rs
+++ b/src/bootstrap/check.rs
@@ -99,19 +99,13 @@ impl Step for Std {
             cargo_subcommand(builder.kind),
         );
         std_cargo(builder, target, compiler.stage, &mut cargo);
+        cargo.args(args(builder));
 
         builder.info(&format!(
-            "Checking stage{} std artifacts ({} -> {})",
+            "Checking stage{} library artifacts ({} -> {})",
             builder.top_stage, &compiler.host, target
         ));
-        run_cargo(
-            builder,
-            cargo,
-            args(builder),
-            &libstd_stamp(builder, compiler, target),
-            vec![],
-            true,
-        );
+        run_cargo(builder, cargo, &libstd_stamp(builder, compiler, target), vec![], true);
 
         // We skip populating the sysroot in non-zero stage because that'll lead
         // to rlib/rmeta conflicts if std gets built during this session.
@@ -155,19 +149,13 @@ impl Step for Std {
         for krate in builder.in_tree_crates("test", Some(target)) {
             cargo.arg("-p").arg(krate.name);
         }
+        cargo.args(args(builder));
 
         builder.info(&format!(
-            "Checking stage{} std test/bench/example targets ({} -> {})",
+            "Checking stage{} library test/bench/example targets ({} -> {})",
             builder.top_stage, &compiler.host, target
         ));
-        run_cargo(
-            builder,
-            cargo,
-            args(builder),
-            &libstd_test_stamp(builder, compiler, target),
-            vec![],
-            true,
-        );
+        run_cargo(builder, cargo, &libstd_test_stamp(builder, compiler, target), vec![], true);
     }
 }
 
@@ -231,19 +219,13 @@ impl Step for Rustc {
         for krate in builder.in_tree_crates("rustc-main", Some(target)) {
             cargo.arg("-p").arg(krate.name);
         }
+        cargo.args(args(builder));
 
         builder.info(&format!(
             "Checking stage{} compiler artifacts ({} -> {})",
             builder.top_stage, &compiler.host, target
         ));
-        run_cargo(
-            builder,
-            cargo,
-            args(builder),
-            &librustc_stamp(builder, compiler, target),
-            vec![],
-            true,
-        );
+        run_cargo(builder, cargo, &librustc_stamp(builder, compiler, target), vec![], true);
 
         let libdir = builder.sysroot_libdir(compiler, target);
         let hostdir = builder.sysroot_libdir(compiler, compiler.host);
@@ -290,6 +272,7 @@ impl Step for CodegenBackend {
             .arg("--manifest-path")
             .arg(builder.src.join(format!("compiler/rustc_codegen_{}/Cargo.toml", backend)));
         rustc_cargo_env(builder, &mut cargo, target);
+        cargo.args(args(builder));
 
         builder.info(&format!(
             "Checking stage{} {} artifacts ({} -> {})",
@@ -299,7 +282,6 @@ impl Step for CodegenBackend {
         run_cargo(
             builder,
             cargo,
-            args(builder),
             &codegen_backend_stamp(builder, compiler, target, backend),
             vec![],
             true,
@@ -355,11 +337,13 @@ impl Step for RustAnalyzer {
             cargo.arg("--benches");
         }
 
+        cargo.args(args(builder));
+
         builder.info(&format!(
             "Checking stage{} {} artifacts ({} -> {})",
             compiler.stage, "rust-analyzer", &compiler.host.triple, target.triple
         ));
-        run_cargo(builder, cargo, args(builder), &stamp(builder, compiler, target), vec![], true);
+        run_cargo(builder, cargo, &stamp(builder, compiler, target), vec![], true);
 
         /// Cargo's output path in a given stage, compiled by a particular
         /// compiler for the specified target.
@@ -413,6 +397,8 @@ macro_rules! tool_check_step {
                     cargo.arg("--all-targets");
                 }
 
+                cargo.args(args(builder));
+
                 // Enable internal lints for clippy and rustdoc
                 // NOTE: this doesn't enable lints for any other tools unless they explicitly add `#![warn(rustc::internal)]`
                 // See https://github.com/rust-lang/rust/pull/80573#issuecomment-754010776
@@ -428,7 +414,6 @@ macro_rules! tool_check_step {
                 run_cargo(
                     builder,
                     cargo,
-                    args(builder),
                     &stamp(builder, compiler, target),
                     vec![],
                     true,
diff --git a/src/bootstrap/clean.rs b/src/bootstrap/clean.rs
index 8e363ee1290ee..468efc1114c43 100644
--- a/src/bootstrap/clean.rs
+++ b/src/bootstrap/clean.rs
@@ -9,11 +9,10 @@ use std::fs;
 use std::io::{self, ErrorKind};
 use std::path::Path;
 
-use crate::builder::{Builder, RunConfig, ShouldRun, Step};
+use crate::builder::{crate_description, Builder, RunConfig, ShouldRun, Step};
 use crate::cache::Interned;
-use crate::config::TargetSelection;
 use crate::util::t;
-use crate::{Build, Mode, Subcommand};
+use crate::{Build, Compiler, Mode, Subcommand};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct CleanAll {}
@@ -40,7 +39,7 @@ macro_rules! clean_crate_tree {
     ( $( $name:ident, $mode:path, $root_crate:literal);+ $(;)? ) => { $(
         #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
         pub struct $name {
-            target: TargetSelection,
+            compiler: Compiler,
             crates: Interned<Vec<String>>,
         }
 
@@ -54,22 +53,21 @@ macro_rules! clean_crate_tree {
 
             fn make_run(run: RunConfig<'_>) {
                 let builder = run.builder;
-                if builder.top_stage != 0 {
-                    panic!("non-stage-0 clean not supported for individual crates");
-                }
-                builder.ensure(Self { crates: run.cargo_crates_in_set(), target: run.target });
+                let compiler = builder.compiler(builder.top_stage, run.target);
+                builder.ensure(Self { crates: run.cargo_crates_in_set(), compiler });
             }
 
             fn run(self, builder: &Builder<'_>) -> Self::Output {
-                let compiler = builder.compiler(0, self.target);
-                let mut cargo = builder.bare_cargo(compiler, $mode, self.target, "clean");
+                let compiler = self.compiler;
+                let target = compiler.host;
+                let mut cargo = builder.bare_cargo(compiler, $mode, target, "clean");
                 for krate in &*self.crates {
                     cargo.arg(krate);
                 }
 
                 builder.info(&format!(
-                    "Cleaning stage{} {} artifacts ({} -> {})",
-                    compiler.stage, stringify!($name).to_lowercase(), &compiler.host, self.target
+                    "Cleaning{} stage{} {} artifacts ({} -> {})",
+                    crate_description(&self.crates), compiler.stage, stringify!($name).to_lowercase(), &compiler.host, target,
                 ));
 
                 // NOTE: doesn't use `run_cargo` because we don't want to save a stamp file,
diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index 1030247b890c3..f9a04f2e91dbf 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -18,6 +18,7 @@ use std::str;
 
 use serde::Deserialize;
 
+use crate::builder::crate_description;
 use crate::builder::Cargo;
 use crate::builder::{Builder, Kind, RunConfig, ShouldRun, Step};
 use crate::cache::{Interned, INTERNER};
@@ -110,7 +111,10 @@ impl Step for Std {
         let compiler_to_use = builder.compiler_for(compiler.stage, compiler.host, target);
         if compiler_to_use != compiler {
             builder.ensure(Std::new(compiler_to_use, target));
-            builder.info(&format!("Uplifting stage1 std ({} -> {})", compiler_to_use.host, target));
+            builder.info(&format!(
+                "Uplifting stage1 library ({} -> {})",
+                compiler_to_use.host, target
+            ));
 
             // Even if we're not building std this stage, the new sysroot must
             // still contain the third party objects needed by various targets.
@@ -126,19 +130,18 @@ impl Step for Std {
 
         let mut cargo = builder.cargo(compiler, Mode::Std, SourceType::InTree, target, "build");
         std_cargo(builder, target, compiler.stage, &mut cargo);
+        for krate in &*self.crates {
+            cargo.arg("-p").arg(krate);
+        }
 
         builder.info(&format!(
-            "Building stage{} std artifacts ({} -> {})",
-            compiler.stage, &compiler.host, target
+            "Building{} stage{} library artifacts ({} -> {})",
+            crate_description(&self.crates),
+            compiler.stage,
+            &compiler.host,
+            target,
         ));
-        run_cargo(
-            builder,
-            cargo,
-            self.crates.to_vec(),
-            &libstd_stamp(builder, compiler, target),
-            target_deps,
-            false,
-        );
+        run_cargo(builder, cargo, &libstd_stamp(builder, compiler, target), target_deps, false);
 
         builder.ensure(StdLink::from_std(
             self,
@@ -425,7 +428,7 @@ impl Step for StdLink {
         let target_compiler = self.target_compiler;
         let target = self.target;
         builder.info(&format!(
-            "Copying stage{} std from stage{} ({} -> {} / {})",
+            "Copying stage{} library from stage{} ({} -> {} / {})",
             target_compiler.stage, compiler.stage, &compiler.host, target_compiler.host, target
         ));
         let libdir = builder.sysroot_libdir(target_compiler, target);
@@ -714,18 +717,18 @@ impl Step for Rustc {
             }
         }
 
+        for krate in &*self.crates {
+            cargo.arg("-p").arg(krate);
+        }
+
         builder.info(&format!(
-            "Building stage{} compiler artifacts ({} -> {})",
-            compiler.stage, &compiler.host, target
+            "Building{} stage{} compiler artifacts ({} -> {})",
+            crate_description(&self.crates),
+            compiler.stage,
+            &compiler.host,
+            target,
         ));
-        run_cargo(
-            builder,
-            cargo,
-            self.crates.to_vec(),
-            &librustc_stamp(builder, compiler, target),
-            vec![],
-            false,
-        );
+        run_cargo(builder, cargo, &librustc_stamp(builder, compiler, target), vec![], false);
 
         builder.ensure(RustcLink::from_rustc(
             self,
@@ -981,7 +984,7 @@ impl Step for CodegenBackend {
             "Building stage{} codegen backend {} ({} -> {})",
             compiler.stage, backend, &compiler.host, target
         ));
-        let files = run_cargo(builder, cargo, vec![], &tmp_stamp, vec![], false);
+        let files = run_cargo(builder, cargo, &tmp_stamp, vec![], false);
         if builder.config.dry_run() {
             return;
         }
@@ -1405,7 +1408,6 @@ pub fn add_to_sysroot(
 pub fn run_cargo(
     builder: &Builder<'_>,
     cargo: Cargo,
-    tail_args: Vec<String>,
     stamp: &Path,
     additional_target_deps: Vec<(PathBuf, DependencyType)>,
     is_check: bool,
@@ -1431,7 +1433,7 @@ pub fn run_cargo(
     // files we need to probe for later.
     let mut deps = Vec::new();
     let mut toplevel = Vec::new();
-    let ok = stream_cargo(builder, cargo, tail_args, &mut |msg| {
+    let ok = stream_cargo(builder, cargo, &mut |msg| {
         let (filenames, crate_types) = match msg {
             CargoMessage::CompilerArtifact {
                 filenames,
@@ -1546,7 +1548,6 @@ pub fn run_cargo(
 pub fn stream_cargo(
     builder: &Builder<'_>,
     cargo: Cargo,
-    tail_args: Vec<String>,
     cb: &mut dyn FnMut(CargoMessage<'_>),
 ) -> bool {
     let mut cargo = Command::from(cargo);
@@ -1566,10 +1567,6 @@ pub fn stream_cargo(
     }
     cargo.arg("--message-format").arg(message_format).stdout(Stdio::piped());
 
-    for arg in tail_args {
-        cargo.arg(arg);
-    }
-
     builder.verbose(&format!("running: {:?}", cargo));
     let mut child = match cargo.spawn() {
         Ok(child) => child,
diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index 3f43c68d2e082..9bad9046ecc2c 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -12,6 +12,7 @@ use std::fs;
 use std::io;
 use std::path::{Path, PathBuf};
 
+use crate::builder::crate_description;
 use crate::builder::{Builder, Compiler, Kind, RunConfig, ShouldRun, Step};
 use crate::cache::{Interned, INTERNER};
 use crate::compile;
@@ -558,7 +559,8 @@ fn doc_std(
     requested_crates: &[String],
 ) {
     builder.info(&format!(
-        "Documenting stage{} std ({}) in {} format",
+        "Documenting{} stage{} library ({}) in {} format",
+        crate_description(requested_crates),
         stage,
         target,
         format.as_str()
diff --git a/src/bootstrap/flags.rs b/src/bootstrap/flags.rs
index 3be7ad4075ed8..2c6d201d18fbe 100644
--- a/src/bootstrap/flags.rs
+++ b/src/bootstrap/flags.rs
@@ -352,32 +352,32 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`",
 
         // fn usage()
         let usage = |exit_code: i32, opts: &Options, verbose: bool, subcommand_help: &str| -> ! {
-            // We have an unfortunate situation here: some Steps use `builder.in_tree_crates` to determine their paths.
-            // To determine those crates, we need to run `cargo metadata`, which means we need all submodules to be checked out.
-            // That takes a while to run, so only do it when paths were explicitly requested, not on all CLI errors.
-            // `Build::new` won't load submodules for the `setup` command.
-            let cmd = if verbose {
-                println!("note: updating submodules before printing available paths");
-                "build"
-            } else {
-                "setup"
-            };
-            let config = Config::parse(&[cmd.to_string()]);
-            let build = Build::new(config);
-            let paths = Builder::get_help(&build, subcommand);
-
             println!("{}", opts.usage(subcommand_help));
-            if let Some(s) = paths {
-                if verbose {
+            if verbose {
+                // We have an unfortunate situation here: some Steps use `builder.in_tree_crates` to determine their paths.
+                // To determine those crates, we need to run `cargo metadata`, which means we need all submodules to be checked out.
+                // That takes a while to run, so only do it when paths were explicitly requested, not on all CLI errors.
+                // `Build::new` won't load submodules for the `setup` command.
+                let cmd = if verbose {
+                    println!("note: updating submodules before printing available paths");
+                    "build"
+                } else {
+                    "setup"
+                };
+                let config = Config::parse(&[cmd.to_string()]);
+                let build = Build::new(config);
+                let paths = Builder::get_help(&build, subcommand);
+
+                if let Some(s) = paths {
                     println!("{}", s);
                 } else {
-                    println!(
-                        "Run `./x.py {} -h -v` to see a list of available paths.",
-                        subcommand.as_str()
-                    );
+                    panic!("No paths available for subcommand `{}`", subcommand.as_str());
                 }
-            } else if verbose {
-                panic!("No paths available for subcommand `{}`", subcommand.as_str());
+            } else {
+                println!(
+                    "Run `./x.py {} -h -v` to see a list of available paths.",
+                    subcommand.as_str()
+                );
             }
             crate::detail_exit(exit_code);
         };
diff --git a/src/bootstrap/format.rs b/src/bootstrap/format.rs
index 1d57c6ecbbb8d..cca45f4a634e8 100644
--- a/src/bootstrap/format.rs
+++ b/src/bootstrap/format.rs
@@ -2,6 +2,7 @@
 
 use crate::builder::Builder;
 use crate::util::{output, program_out_of_date, t};
+use build_helper::git::get_rust_lang_rust_remote;
 use ignore::WalkBuilder;
 use std::collections::VecDeque;
 use std::path::{Path, PathBuf};
@@ -79,56 +80,22 @@ fn update_rustfmt_version(build: &Builder<'_>) {
 ///
 /// Returns `None` if all files should be formatted.
 fn get_modified_rs_files(build: &Builder<'_>) -> Option<Vec<String>> {
-    let Ok(remote) = get_rust_lang_rust_remote() else {return None;};
+    let Ok(remote) = get_rust_lang_rust_remote() else { return None; };
     if !verify_rustfmt_version(build) {
         return None;
     }
+
+    let merge_base =
+        output(build.config.git().arg("merge-base").arg(&format!("{remote}/master")).arg("HEAD"));
     Some(
-        output(
-            build
-                .config
-                .git()
-                .arg("diff-index")
-                .arg("--name-only")
-                .arg("--merge-base")
-                .arg(&format!("{remote}/master")),
-        )
-        .lines()
-        .map(|s| s.trim().to_owned())
-        .filter(|f| Path::new(f).extension().map_or(false, |ext| ext == "rs"))
-        .collect(),
+        output(build.config.git().arg("diff-index").arg("--name-only").arg(merge_base.trim()))
+            .lines()
+            .map(|s| s.trim().to_owned())
+            .filter(|f| Path::new(f).extension().map_or(false, |ext| ext == "rs"))
+            .collect(),
     )
 }
 
-/// Finds the remote for rust-lang/rust.
-/// For example for these remotes it will return `upstream`.
-/// ```text
-/// origin  https://github.com/Nilstrieb/rust.git (fetch)
-/// origin  https://github.com/Nilstrieb/rust.git (push)
-/// upstream        https://github.com/rust-lang/rust (fetch)
-/// upstream        https://github.com/rust-lang/rust (push)
-/// ```
-fn get_rust_lang_rust_remote() -> Result<String, String> {
-    let mut git = Command::new("git");
-    git.args(["config", "--local", "--get-regex", "remote\\..*\\.url"]);
-
-    let output = git.output().map_err(|err| format!("{err:?}"))?;
-    if !output.status.success() {
-        return Err("failed to execute git config command".to_owned());
-    }
-
-    let stdout = String::from_utf8(output.stdout).map_err(|err| format!("{err:?}"))?;
-
-    let rust_lang_remote = stdout
-        .lines()
-        .find(|remote| remote.contains("rust-lang"))
-        .ok_or_else(|| "rust-lang/rust remote not found".to_owned())?;
-
-    let remote_name =
-        rust_lang_remote.split('.').nth(1).ok_or_else(|| "remote name not found".to_owned())?;
-    Ok(remote_name.into())
-}
-
 #[derive(serde::Deserialize)]
 struct RustfmtConfig {
     ignore: Vec<String>,
diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 5ea41d10bc817..d44b96cfb991e 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -113,6 +113,7 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 use std::str;
 
+use build_helper::ci::CiEnv;
 use channel::GitInfo;
 use config::{DryRun, Target};
 use filetime::FileTime;
@@ -121,7 +122,7 @@ use once_cell::sync::OnceCell;
 use crate::builder::Kind;
 use crate::config::{LlvmLibunwind, TargetSelection};
 use crate::util::{
-    exe, libdir, mtime, output, run, run_suppressed, symlink_dir, try_run_suppressed, CiEnv,
+    exe, libdir, mtime, output, run, run_suppressed, symlink_dir, try_run_suppressed,
 };
 
 mod bolt;
diff --git a/src/bootstrap/native.rs b/src/bootstrap/native.rs
index 4e503dfe864e2..bdc81b07b8d54 100644
--- a/src/bootstrap/native.rs
+++ b/src/bootstrap/native.rs
@@ -24,6 +24,8 @@ use crate::util::get_clang_cl_resource_dir;
 use crate::util::{self, exe, output, t, up_to_date};
 use crate::{CLang, GitRepo};
 
+use build_helper::ci::CiEnv;
+
 #[derive(Clone)]
 pub struct LlvmResult {
     /// Path to llvm-config binary.
@@ -217,7 +219,7 @@ pub(crate) fn is_ci_llvm_available(config: &Config, asserts: bool) -> bool {
         return false;
     }
 
-    if crate::util::CiEnv::is_ci() {
+    if CiEnv::is_ci() {
         // We assume we have access to git, so it's okay to unconditionally pass
         // `true` here.
         let llvm_sha = detect_llvm_sha(config, true);
diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index c8b4134391e5f..d5bec268a4567 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -11,6 +11,7 @@ use std::iter;
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 
+use crate::builder::crate_description;
 use crate::builder::{Builder, Compiler, Kind, RunConfig, ShouldRun, Step};
 use crate::cache::Interned;
 use crate::compile;
@@ -2154,8 +2155,12 @@ impl Step for Crate {
         }
 
         builder.info(&format!(
-            "{} {:?} stage{} ({} -> {})",
-            test_kind, self.crates, compiler.stage, &compiler.host, target
+            "{}{} stage{} ({} -> {})",
+            test_kind,
+            crate_description(&self.crates),
+            compiler.stage,
+            &compiler.host,
+            target
         ));
         let _time = util::timeit(&builder);
         try_run(builder, &mut cargo.into());
diff --git a/src/bootstrap/tool.rs b/src/bootstrap/tool.rs
index 24b033cc0dc5e..63026bd44d475 100644
--- a/src/bootstrap/tool.rs
+++ b/src/bootstrap/tool.rs
@@ -72,7 +72,7 @@ impl Step for ToolBuild {
 
         builder.info(&format!("Building stage{} tool {} ({})", compiler.stage, tool, target));
         let mut duplicates = Vec::new();
-        let is_expected = compile::stream_cargo(builder, cargo, vec![], &mut |msg| {
+        let is_expected = compile::stream_cargo(builder, cargo, &mut |msg| {
             // Only care about big things like the RLS/Cargo for now
             match tool {
                 "rls" | "cargo" | "clippy-driver" | "miri" | "rustfmt" => {}
diff --git a/src/bootstrap/util.rs b/src/bootstrap/util.rs
index 58220783228b2..8ce9a9ce38cc5 100644
--- a/src/bootstrap/util.rs
+++ b/src/bootstrap/util.rs
@@ -255,35 +255,6 @@ pub enum CiEnv {
     GitHubActions,
 }
 
-impl CiEnv {
-    /// Obtains the current CI environment.
-    pub fn current() -> CiEnv {
-        if env::var("TF_BUILD").map_or(false, |e| e == "True") {
-            CiEnv::AzurePipelines
-        } else if env::var("GITHUB_ACTIONS").map_or(false, |e| e == "true") {
-            CiEnv::GitHubActions
-        } else {
-            CiEnv::None
-        }
-    }
-
-    pub fn is_ci() -> bool {
-        Self::current() != CiEnv::None
-    }
-
-    /// If in a CI environment, forces the command to run with colors.
-    pub fn force_coloring_in_ci(self, cmd: &mut Command) {
-        if self != CiEnv::None {
-            // Due to use of stamp/docker, the output stream of rustbuild is not
-            // a TTY in CI, so coloring is by-default turned off.
-            // The explicit `TERM=xterm` environment is needed for
-            // `--color always` to actually work. This env var was lost when
-            // compiling through the Makefile. Very strange.
-            cmd.env("TERM", "xterm").args(&["--color", "always"]);
-        }
-    }
-}
-
 pub fn forcing_clang_based_tests() -> bool {
     if let Some(var) = env::var_os("RUSTBUILD_FORCE_CLANG_BASED_TESTS") {
         match &var.to_string_lossy().to_lowercase()[..] {
diff --git a/src/ci/github-actions/ci.yml b/src/ci/github-actions/ci.yml
index d33396dcc80fe..d3b07419486b2 100644
--- a/src/ci/github-actions/ci.yml
+++ b/src/ci/github-actions/ci.yml
@@ -103,6 +103,12 @@ x--expand-yaml-anchors--remove:
         with:
           fetch-depth: 2
 
+      - name: checkout the `master` branch for tidy
+        uses: actions/checkout@v3
+        with:
+          ref: master
+          fetch-depth: 1
+
       # Rust Log Analyzer can't currently detect the PR number of a GitHub
       # Actions build on its own, so a hint in the log message is needed to
       # point it in the right direction.
diff --git a/src/test/codegen/align-byval.rs b/src/test/codegen/align-byval.rs
new file mode 100644
index 0000000000000..35b3b1adc2cdc
--- /dev/null
+++ b/src/test/codegen/align-byval.rs
@@ -0,0 +1,56 @@
+// ignore-x86
+// ignore-aarch64
+// ignore-aarch64_be
+// ignore-arm
+// ignore-armeb
+// ignore-avr
+// ignore-bpfel
+// ignore-bpfeb
+// ignore-hexagon
+// ignore-mips
+// ignore-mips64
+// ignore-msp430
+// ignore-powerpc64
+// ignore-powerpc64le
+// ignore-powerpc
+// ignore-r600
+// ignore-amdgcn
+// ignore-sparc
+// ignore-sparcv9
+// ignore-sparcel
+// ignore-s390x
+// ignore-tce
+// ignore-thumb
+// ignore-thumbeb
+// ignore-xcore
+// ignore-nvptx
+// ignore-nvptx64
+// ignore-le32
+// ignore-le64
+// ignore-amdil
+// ignore-amdil64
+// ignore-hsail
+// ignore-hsail64
+// ignore-spir
+// ignore-spir64
+// ignore-kalimba
+// ignore-shave
+//
+// Tests that `byval` alignment is properly specified (#80127).
+// The only targets that use `byval` are m68k, wasm, x86-64, and x86. Note that
+// x86 has special rules (see #103830), and it's therefore ignored here.
+
+#[repr(C)]
+#[repr(align(16))]
+struct Foo {
+    a: [i32; 16],
+}
+
+extern "C" {
+    // CHECK: declare void @f({{.*}}byval(%Foo) align 16{{.*}})
+    fn f(foo: Foo);
+}
+
+pub fn main() {
+    unsafe { f(Foo { a: [1; 16] }) }
+}
diff --git a/src/test/codegen/function-arguments-noopt.rs b/src/test/codegen/function-arguments-noopt.rs
index ff76405a4ea32..5b41ab7f8dc0e 100644
--- a/src/test/codegen/function-arguments-noopt.rs
+++ b/src/test/codegen/function-arguments-noopt.rs
@@ -36,7 +36,7 @@ pub fn borrow_call(x: &i32, f: fn(&i32) -> &i32) -> &i32 {
   f(x)
 }
 
-// CHECK: void @struct_({{%S\*|ptr}} sret(%S){{( %0)?}}, {{%S\*|ptr}} %x)
+// CHECK: void @struct_({{%S\*|ptr}} sret(%S) align 4{{( %0)?}}, {{%S\*|ptr}} align 4 %x)
 #[no_mangle]
 pub fn struct_(x: S) -> S {
   x
@@ -45,7 +45,7 @@ pub fn struct_(x: S) -> S {
 // CHECK-LABEL: @struct_call
 #[no_mangle]
 pub fn struct_call(x: S, f: fn(S) -> S) -> S {
-  // CHECK: call void %f({{%S\*|ptr}} sret(%S){{( %0)?}}, {{%S\*|ptr}} %{{.+}})
+  // CHECK: call void %f({{%S\*|ptr}} sret(%S) align 4{{( %0)?}}, {{%S\*|ptr}} align 4 %{{.+}})
   f(x)
 }
 
diff --git a/src/test/codegen/function-arguments.rs b/src/test/codegen/function-arguments.rs
index 44fee952307f2..fd413bcf1d57f 100644
--- a/src/test/codegen/function-arguments.rs
+++ b/src/test/codegen/function-arguments.rs
@@ -127,7 +127,7 @@ pub fn mutable_notunpin_borrow(_: &mut NotUnpin) {
 pub fn notunpin_borrow(_: &NotUnpin) {
 }
 
-// CHECK: @indirect_struct({{%S\*|ptr}} noalias nocapture noundef readonly dereferenceable(32) %_1)
+// CHECK: @indirect_struct({{%S\*|ptr}} noalias nocapture noundef readonly align 4 dereferenceable(32) %_1)
 #[no_mangle]
 pub fn indirect_struct(_: S) {
 }
@@ -151,7 +151,7 @@ pub fn _box(x: Box<i32>) -> Box<i32> {
   x
 }
 
-// CHECK: @struct_return({{%S\*|ptr}} noalias nocapture noundef sret(%S) dereferenceable(32){{( %0)?}})
+// CHECK: @struct_return({{%S\*|ptr}} noalias nocapture noundef sret(%S) align 4 dereferenceable(32){{( %0)?}})
 #[no_mangle]
 pub fn struct_return() -> S {
   S {
diff --git a/src/test/codegen/issue-86106.rs b/src/test/codegen/issue-86106.rs
new file mode 100644
index 0000000000000..e647dbe2514db
--- /dev/null
+++ b/src/test/codegen/issue-86106.rs
@@ -0,0 +1,55 @@
+// min-llvm-version: 15.0
+// compile-flags: -C opt-level=3
+
+// The below two functions ensure that both `String::new()` and `"".to_string()`
+// produce the identical code.
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @string_new = unnamed_addr alias void (ptr), ptr @empty_to_string
+#[no_mangle]
+pub fn string_new() -> String {
+    String::new()
+}
+
+// CHECK-LABEL: define void @empty_to_string
+#[no_mangle]
+pub fn empty_to_string() -> String {
+    // CHECK-NOT: load i8
+    // CHECK: store i{{32|64}}
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store ptr
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store i{{32|64}}
+    // CHECK-NEXT: ret void
+    "".to_string()
+}
+
+// The below two functions ensure that both `vec![]` and `vec![].clone()`
+// produce the identical code.
+
+// CHECK-LABEL: @empty_vec
+#[no_mangle]
+pub fn empty_vec() -> Vec<u8> {
+    // CHECK: store i{{32|64}}
+    // CHECK-NOT: load i8
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store ptr
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store i{{32|64}}
+    // CHECK-NEXT: ret void
+    vec![]
+}
+
+// CHECK-LABEL: @empty_vec_clone
+#[no_mangle]
+pub fn empty_vec_clone() -> Vec<u8> {
+    // CHECK: store i{{32|64}}
+    // CHECK-NOT: load i8
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store ptr
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store i{{32|64}}
+    // CHECK-NEXT: ret void
+    vec![].clone()
+}
diff --git a/src/test/run-make-fulldeps/extern-fn-explicit-align/Makefile b/src/test/run-make-fulldeps/extern-fn-explicit-align/Makefile
new file mode 100644
index 0000000000000..4f5d026f213f2
--- /dev/null
+++ b/src/test/run-make-fulldeps/extern-fn-explicit-align/Makefile
@@ -0,0 +1,5 @@
+include ../tools.mk
+
+all: $(call NATIVE_STATICLIB,test)
+	$(RUSTC) test.rs
+	$(call RUN,test) || exit 1
diff --git a/src/test/run-make-fulldeps/extern-fn-explicit-align/test.c b/src/test/run-make-fulldeps/extern-fn-explicit-align/test.c
new file mode 100644
index 0000000000000..a015fc9aaf606
--- /dev/null
+++ b/src/test/run-make-fulldeps/extern-fn-explicit-align/test.c
@@ -0,0 +1,35 @@
+#include <assert.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <string.h>
+
+struct TwoU64s
+{
+    uint64_t a;
+    uint64_t b;
+} __attribute__((aligned(16)));
+
+struct BoolAndU32
+{
+    bool a;
+    uint32_t b;
+};
+
+int32_t many_args(
+    void *a,
+    void *b,
+    const char *c,
+    uint64_t d,
+    bool e,
+    struct BoolAndU32 f,
+    void *g,
+    struct TwoU64s h,
+    void *i,
+    void *j,
+    void *k,
+    void *l,
+    const char *m)
+{
+    assert(strcmp(m, "Hello world") == 0);
+    return 0;
+}
diff --git a/src/test/run-make-fulldeps/extern-fn-explicit-align/test.rs b/src/test/run-make-fulldeps/extern-fn-explicit-align/test.rs
new file mode 100644
index 0000000000000..ba6cc87bd1857
--- /dev/null
+++ b/src/test/run-make-fulldeps/extern-fn-explicit-align/test.rs
@@ -0,0 +1,61 @@
+// Issue #80127: Passing structs via FFI should work with explicit alignment.
+
+use std::ffi::CString;
+use std::ptr::null_mut;
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+#[repr(C)]
+#[repr(align(16))]
+pub struct TwoU64s {
+    pub a: u64,
+    pub b: u64,
+}
+
+#[repr(C)]
+#[derive(Debug, Copy, Clone)]
+pub struct BoolAndU32 {
+    pub a: bool,
+    pub b: u32,
+}
+
+#[link(name = "test", kind = "static")]
+extern "C" {
+    fn many_args(
+        a: *mut (),
+        b: *mut (),
+        c: *const i8,
+        d: u64,
+        e: bool,
+        f: BoolAndU32,
+        g: *mut (),
+        h: TwoU64s,
+        i: *mut (),
+        j: *mut (),
+        k: *mut (),
+        l: *mut (),
+        m: *const i8,
+    ) -> i32;
+}
+
+fn main() {
+    let two_u64s = TwoU64s { a: 1, b: 2 };
+    let bool_and_u32 = BoolAndU32 { a: true, b: 3 };
+    let string = CString::new("Hello world").unwrap();
+    unsafe {
+        many_args(
+            null_mut(),
+            null_mut(),
+            null_mut(),
+            4,
+            true,
+            bool_and_u32,
+            null_mut(),
+            two_u64s,
+            null_mut(),
+            null_mut(),
+            null_mut(),
+            null_mut(),
+            string.as_ptr(),
+        );
+    }
+}
diff --git a/src/test/rustdoc-gui/scrape-examples-layout.goml b/src/test/rustdoc-gui/scrape-examples-layout.goml
index fde9a0ab0bc30..95102528ec11d 100644
--- a/src/test/rustdoc-gui/scrape-examples-layout.goml
+++ b/src/test/rustdoc-gui/scrape-examples-layout.goml
@@ -33,3 +33,17 @@ assert-property: (
     ".more-scraped-examples .scraped-example:nth-child(6) .code-wrapper .src-line-numbers",
     {"clientWidth": |clientWidth|}
 )
+
+// Check that for both mobile and desktop sizes, the buttons in scraped examples are displayed
+// correctly.
+
+store-value: (offset_y, 4)
+
+// First with desktop
+assert-position: (".scraped-example .code-wrapper", {"y": 255})
+assert-position: (".scraped-example .code-wrapper .prev", {"y": 255 + |offset_y|})
+
+// Then with mobile
+size: (600, 600)
+assert-position: (".scraped-example .code-wrapper", {"y": 314})
+assert-position: (".scraped-example .code-wrapper .prev", {"y": 314 + |offset_y|})
diff --git a/src/tools/build_helper/Cargo.toml b/src/tools/build_helper/Cargo.toml
new file mode 100644
index 0000000000000..99f6fea2ecf99
--- /dev/null
+++ b/src/tools/build_helper/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "build_helper"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/src/tools/build_helper/src/ci.rs b/src/tools/build_helper/src/ci.rs
new file mode 100644
index 0000000000000..9f113c72b9338
--- /dev/null
+++ b/src/tools/build_helper/src/ci.rs
@@ -0,0 +1,40 @@
+use std::process::Command;
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum CiEnv {
+    /// Not a CI environment.
+    None,
+    /// The Azure Pipelines environment, for Linux (including Docker), Windows, and macOS builds.
+    AzurePipelines,
+    /// The GitHub Actions environment, for Linux (including Docker), Windows and macOS builds.
+    GitHubActions,
+}
+
+impl CiEnv {
+    /// Obtains the current CI environment.
+    pub fn current() -> CiEnv {
+        if std::env::var("TF_BUILD").map_or(false, |e| e == "True") {
+            CiEnv::AzurePipelines
+        } else if std::env::var("GITHUB_ACTIONS").map_or(false, |e| e == "true") {
+            CiEnv::GitHubActions
+        } else {
+            CiEnv::None
+        }
+    }
+
+    pub fn is_ci() -> bool {
+        Self::current() != CiEnv::None
+    }
+
+    /// If in a CI environment, forces the command to run with colors.
+    pub fn force_coloring_in_ci(self, cmd: &mut Command) {
+        if self != CiEnv::None {
+            // Due to use of stamp/docker, the output stream of rustbuild is not
+            // a TTY in CI, so coloring is by-default turned off.
+            // The explicit `TERM=xterm` environment is needed for
+            // `--color always` to actually work. This env var was lost when
+            // compiling through the Makefile. Very strange.
+            cmd.env("TERM", "xterm").args(&["--color", "always"]);
+        }
+    }
+}
diff --git a/src/tools/build_helper/src/git.rs b/src/tools/build_helper/src/git.rs
new file mode 100644
index 0000000000000..e9638206e6729
--- /dev/null
+++ b/src/tools/build_helper/src/git.rs
@@ -0,0 +1,30 @@
+use std::process::Command;
+
+/// Finds the remote for rust-lang/rust.
+/// For example for these remotes it will return `upstream`.
+/// ```text
+/// origin  https://github.com/Nilstrieb/rust.git (fetch)
+/// origin  https://github.com/Nilstrieb/rust.git (push)
+/// upstream        https://github.com/rust-lang/rust (fetch)
+/// upstream        https://github.com/rust-lang/rust (push)
+/// ```
+pub fn get_rust_lang_rust_remote() -> Result<String, String> {
+    let mut git = Command::new("git");
+    git.args(["config", "--local", "--get-regex", "remote\\..*\\.url"]);
+
+    let output = git.output().map_err(|err| format!("{err:?}"))?;
+    if !output.status.success() {
+        return Err("failed to execute git config command".to_owned());
+    }
+
+    let stdout = String::from_utf8(output.stdout).map_err(|err| format!("{err:?}"))?;
+
+    let rust_lang_remote = stdout
+        .lines()
+        .find(|remote| remote.contains("rust-lang"))
+        .ok_or_else(|| "rust-lang/rust remote not found".to_owned())?;
+
+    let remote_name =
+        rust_lang_remote.split('.').nth(1).ok_or_else(|| "remote name not found".to_owned())?;
+    Ok(remote_name.into())
+}
diff --git a/src/tools/build_helper/src/lib.rs b/src/tools/build_helper/src/lib.rs
new file mode 100644
index 0000000000000..d3d2323db853a
--- /dev/null
+++ b/src/tools/build_helper/src/lib.rs
@@ -0,0 +1,2 @@
+pub mod ci;
+pub mod git;
diff --git a/src/tools/tidy/Cargo.toml b/src/tools/tidy/Cargo.toml
index 97d038da702d5..bf291dd681475 100644
--- a/src/tools/tidy/Cargo.toml
+++ b/src/tools/tidy/Cargo.toml
@@ -5,12 +5,14 @@ edition = "2021"
 autobins = false
 
 [dependencies]
+build_helper = { path = "../build_helper" }
 cargo_metadata = "0.14"
 regex = "1"
 miropt-test-tools = { path = "../miropt-test-tools" }
 lazy_static = "1"
 walkdir = "2"
 ignore = "0.4.18"
+termcolor = "1.1.3"
 
 [[bin]]
 name = "rust-tidy"
diff --git a/src/tools/tidy/src/error_codes_check.rs b/src/tools/tidy/src/error_codes_check.rs
index fd870b0997ce0..40a46c630d70a 100644
--- a/src/tools/tidy/src/error_codes_check.rs
+++ b/src/tools/tidy/src/error_codes_check.rs
@@ -80,15 +80,6 @@ fn check_if_error_code_is_test_in_explanation(f: &str, err_code: &str) -> bool {
     ignore_found
 }
 
-macro_rules! some_or_continue {
-    ($e:expr) => {
-        match $e {
-            Some(e) => e,
-            None => continue,
-        }
-    };
-}
-
 fn extract_error_codes(
     f: &str,
     error_codes: &mut HashMap<String, ErrorCodeStatus>,
@@ -122,10 +113,16 @@ fn extract_error_codes(
                     Some((file_name, _)) => file_name,
                 },
             };
-            let path = some_or_continue!(path.parent())
+
+            let Some(parent) = path.parent() else {
+                continue;
+            };
+
+            let path = parent
                 .join(md_file_name)
                 .canonicalize()
                 .expect("failed to canonicalize error explanation file path");
+
             match read_to_string(&path) {
                 Ok(content) => {
                     let has_test = check_if_error_code_is_test_in_explanation(&content, &err_code);
@@ -215,8 +212,6 @@ pub fn check(paths: &[&Path], bad: &mut bool) {
     // * #[error = "E0111"]
     let regex = Regex::new(r#"[(,"\s](E\d{4})[,)"]"#).unwrap();
 
-    println!("Checking which error codes lack tests...");
-
     for path in paths {
         walk(path, &mut filter_dirs, &mut |entry, contents| {
             let file_name = entry.file_name();
@@ -245,20 +240,15 @@ pub fn check(paths: &[&Path], bad: &mut bool) {
         });
     }
     if found_explanations == 0 {
-        eprintln!("No error code explanation was tested!");
-        *bad = true;
+        tidy_error!(bad, "No error code explanation was tested!");
     }
     if found_tests == 0 {
-        eprintln!("No error code was found in compilation errors!");
-        *bad = true;
+        tidy_error!(bad, "No error code was found in compilation errors!");
     }
     if explanations.is_empty() {
-        eprintln!("No error code explanation was found!");
-        *bad = true;
+        tidy_error!(bad, "No error code explanation was found!");
     }
     if errors.is_empty() {
-        println!("Found {} error codes", error_codes.len());
-
         for (err_code, error_status) in &error_codes {
             if !error_status.has_test && !EXEMPTED_FROM_TEST.contains(&err_code.as_str()) {
                 errors.push(format!("Error code {err_code} needs to have at least one UI test!"));
@@ -310,11 +300,6 @@ pub fn check(paths: &[&Path], bad: &mut bool) {
     }
     errors.sort();
     for err in &errors {
-        eprintln!("{err}");
-    }
-    println!("Found {} error(s) in error codes", errors.len());
-    if !errors.is_empty() {
-        *bad = true;
+        tidy_error!(bad, "{err}");
     }
-    println!("Done!");
 }
diff --git a/src/tools/tidy/src/lib.rs b/src/tools/tidy/src/lib.rs
index 698e4850bea9b..54f11c48b545e 100644
--- a/src/tools/tidy/src/lib.rs
+++ b/src/tools/tidy/src/lib.rs
@@ -3,6 +3,10 @@
 //! This library contains the tidy lints and exposes it
 //! to be used by tools.
 
+use std::fmt::Display;
+
+use termcolor::WriteColor;
+
 /// A helper macro to `unwrap` a result except also print out details like:
 ///
 /// * The expression that failed
@@ -26,18 +30,27 @@ macro_rules! t {
 }
 
 macro_rules! tidy_error {
-    ($bad:expr, $fmt:expr) => ({
-        *$bad = true;
-        eprint!("tidy error: ");
-        eprintln!($fmt);
-    });
-    ($bad:expr, $fmt:expr, $($arg:tt)*) => ({
-        *$bad = true;
-        eprint!("tidy error: ");
-        eprintln!($fmt, $($arg)*);
+    ($bad:expr, $($fmt:tt)*) => ({
+        $crate::tidy_error($bad, format_args!($($fmt)*)).expect("failed to output error");
     });
 }
 
+fn tidy_error(bad: &mut bool, args: impl Display) -> std::io::Result<()> {
+    use std::io::Write;
+    use termcolor::{Color, ColorChoice, ColorSpec, StandardStream};
+
+    *bad = true;
+
+    let mut stderr = StandardStream::stdout(ColorChoice::Auto);
+    stderr.set_color(ColorSpec::new().set_fg(Some(Color::Red)))?;
+
+    write!(&mut stderr, "tidy error")?;
+    stderr.set_color(&ColorSpec::new())?;
+
+    writeln!(&mut stderr, ": {args}")?;
+    Ok(())
+}
+
 pub mod alphabetical;
 pub mod bins;
 pub mod debug_artifacts;
@@ -48,6 +61,7 @@ pub mod errors;
 pub mod extdeps;
 pub mod features;
 pub mod mir_opt_tests;
+pub mod no_merge;
 pub mod pal;
 pub mod primitive_docs;
 pub mod style;
diff --git a/src/tools/tidy/src/main.rs b/src/tools/tidy/src/main.rs
index 6714c63ee62a1..65f0fe9743f80 100644
--- a/src/tools/tidy/src/main.rs
+++ b/src/tools/tidy/src/main.rs
@@ -107,6 +107,8 @@ fn main() {
         check!(alphabetical, &compiler_path);
         check!(alphabetical, &library_path);
 
+        check!(no_merge, ());
+
         let collected = {
             drain_handles(&mut handles);
 
diff --git a/src/tools/tidy/src/no_merge.rs b/src/tools/tidy/src/no_merge.rs
new file mode 100644
index 0000000000000..362d6f6f4d77f
--- /dev/null
+++ b/src/tools/tidy/src/no_merge.rs
@@ -0,0 +1,72 @@
+//! This check makes sure that no accidental merge commits are introduced to the repository.
+//! It forbids all merge commits that are not caused by rollups/bors or subtree syncs.
+
+use std::process::Command;
+
+use build_helper::ci::CiEnv;
+use build_helper::git::get_rust_lang_rust_remote;
+
+macro_rules! try_unwrap_in_ci {
+    ($expr:expr) => {
+        match $expr {
+            Ok(value) => value,
+            Err(err) if CiEnv::is_ci() => {
+                panic!("Encountered error while testing Git status: {:?}", err)
+            }
+            Err(_) => return,
+        }
+    };
+}
+
+pub fn check(_: (), bad: &mut bool) {
+    let remote = try_unwrap_in_ci!(get_rust_lang_rust_remote());
+    let merge_commits = try_unwrap_in_ci!(find_merge_commits(&remote));
+
+    let mut bad_merge_commits = merge_commits.lines().filter(|commit| {
+        !(
+            // Bors is the ruler of merge commits.
+            commit.starts_with("Auto merge of") || commit.starts_with("Rollup merge of")
+        )
+    });
+
+    if let Some(merge) = bad_merge_commits.next() {
+        tidy_error!(
+            bad,
+            "found a merge commit in the history: `{merge}`.
+To resolve the issue, see this: https://rustc-dev-guide.rust-lang.org/git.html#i-made-a-merge-commit-by-accident.
+If you're doing a subtree sync, add your tool to the list in the code that emitted this error."
+        );
+    }
+}
+
+/// Runs `git log --merges --format=%s $REMOTE/master..HEAD` and returns all commits
+fn find_merge_commits(remote: &str) -> Result<String, String> {
+    let mut git = Command::new("git");
+    git.args([
+        "log",
+        "--merges",
+        "--format=%s",
+        &format!("{remote}/master..HEAD"),
+        // Ignore subtree syncs. Add your new subtrees here.
+        ":!src/tools/miri",
+        ":!src/tools/rust-analyzer",
+        ":!compiler/rustc_smir",
+        ":!library/portable-simd",
+        ":!compiler/rustc_codegen_gcc",
+        ":!src/tools/rustfmt",
+        ":!compiler/rustc_codegen_cranelift",
+        ":!src/tools/clippy",
+    ]);
+
+    let output = git.output().map_err(|err| format!("{err:?}"))?;
+    if !output.status.success() {
+        return Err(format!(
+            "failed to execute git log command: {}",
+            String::from_utf8_lossy(&output.stderr)
+        ));
+    }
+
+    let stdout = String::from_utf8(output.stdout).map_err(|err| format!("{err:?}"))?;
+
+    Ok(stdout)
+}