Distinguish crates with the same name in type errors

Previously, errors for crates with the same name would only distinguish them by the span of the source:
```
note: `HashMap<_, _, _, _>` is defined in crate `hashbrown`
   --> /Users/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.12.3/src/map.rs:188:1
note: `HashMap<u32, u32>` is defined in crate `hashbrown`
   --> /Users/jyn/.local/lib/cargo/registry/src/index.crates.io-6f17d22bba15001f/hashbrown-0.12.3/src/map.rs:188:1
```

When the same version of the crate is loaded twice, this isn't particularly helpful. Additionally
show where the .rlib was loaded from (in this case one was loaded from the sysroot and the other
from a cargo target dir).

Author: jyn514

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1824,6 +1824,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             let expected_defid = expected_adt.did();
 
                             diagnostic.note(format!("{found_name} and {expected_name} have similar names, but are actually distinct types"));
+                            let have_same_crate_name = self.tcx.crate_name(found_defid.krate) == self.tcx.crate_name(expected_defid.krate);
                             for (defid, name) in
                                 [(found_defid, found_name), (expected_defid, expected_name)]
                             {
@@ -1843,7 +1844,15 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                                     format!("{name} is defined in the current crate")
                                 } else {
                                     let crate_name = self.tcx.crate_name(defid.krate);
-                                    format!("{name} is defined in crate `{crate_name}`")
+                                    diagnostic.span_note(def_span, format!("{name} is defined in crate `{crate_name}`"));
+
+                                    // If these are named the same, give a hint about why the compiler thinks they're different.
+                                    if have_same_crate_name {
+                                        let crate_paths = self.tcx.crate_extern_paths(defid.krate);
+                                        diagnostic.note(format!("`{crate_name}` was loaded from {}", crate_paths[0].display()));
+                                    }
+
+                                    continue;
                                 };
                                 diagnostic.span_note(def_span, msg);
                             }

tests/run-make/type-mismatch-sysroot-crate/Makefile

@@ -0,0 +1,21 @@
+# Test that we emit useful diagnostics when the same crate is loaded from the sysroot and --extern.
+# This can't be a run-make test because it needs the aux-file to itself have a dependency passed with --extern
+# (and just bare `--extern hashbrown` errors too early, while compiling `uses_hashbrown.rs`).
+
+include ../tools.mk
+# s/hashbrown-[0-9.]\+/VERSION/;
+define SED_REGEX
+s#[^ ]*registry/src/index.crates.io[^ ]*/src#CARGO_REGISTRY/hashbrown-VERSION/src#;
+s#[^ ]*/build/[^ ]*/lib/rustlib/[^ ]*#BUILD_DIR/HOST/stageN/lib/rustlib/TARGET/lib/libhashbrown.rlib#;
+s#[^ ]*/build/[^ ]*/test/run-make/#BUILD_DIR/HOST/test/run-make/#
+s#[^ ]*tests/run-make/#TEST_DIR/#
+endef
+export SED_REGEX
+
+all:
+	$(RUSTC) hashbrown.rs --crate-type lib
+	$(RUSTC) uses-hashbrown.rs --extern hashbrown=$(TMPDIR)/libhashbrown.rlib --crate-type lib
+	$(RUSTC) mismatch.rs --extern uses_hashbrown=$(TMPDIR)/libuses_hashbrown.rlib 2>$(TMPDIR)/stderr.txt || true
+	sed -e "$$SED_REGEX" < $(TMPDIR)/stderr.txt > $(TMPDIR)/normalized.txt
+	$(RUSTC_TEST_OP) $(TMPDIR)/normalized.txt expected.txt
+	$(CGREP) "loaded from" < $(TMPDIR)/stderr.txt

tests/run-make/type-mismatch-sysroot-crate/expected.txt

@@ -0,0 +1,31 @@
+error[E0308]: mismatched types
+   --> mismatch.rs:5:25
+    |
+5   |     uses_hashbrown::foo(hashbrown::HashMap::default())
+    |     ------------------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `HashMap<_, _>`, found `HashMap<_, _, _, _>`
+    |     |
+    |     arguments to this function are incorrect
+    |
+    = note: `hashbrown::HashMap<_, _, _, _>` and `hashbrown::HashMap<_, _>` have similar names, but are actually distinct types
+note: `hashbrown::HashMap<_, _, _, _>` is defined in crate `hashbrown`
+   --> CARGO_REGISTRY/hashbrown-VERSION/src/map.rs:190:1
+    |
+190 | pub struct HashMap<K, V, S = DefaultHashBuilder, A: Allocator = Global> {
+    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    = note: `hashbrown` was loaded from BUILD_DIR/HOST/stageN/lib/rustlib/TARGET/lib/libhashbrown.rlib
+note: `hashbrown::HashMap<_, _>` is defined in crate `hashbrown`
+   --> TEST_DIR/type-mismatch-sysroot-crate/hashbrown.rs:3:1
+    |
+3   | pub struct HashMap<T, U>(PhantomData<(T, U)>);
+    | ^^^^^^^^^^^^^^^^^^^^^^^^
+    = note: `hashbrown` was loaded from BUILD_DIR/HOST/test/run-make/type-mismatch-sysroot-crate/type-mismatch-sysroot-crate/libhashbrown.rlib
+    = note: perhaps two different versions of crate `hashbrown` are being used?
+note: function defined here
+   --> TEST_DIR/type-mismatch-sysroot-crate/uses-hashbrown.rs:2:8
+    |
+2   | pub fn foo<T, U>(_: hashbrown::HashMap<T, U>) {}
+    |        ^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/run-make/type-mismatch-sysroot-crate/hashbrown.rs

@@ -0,0 +1,8 @@
+use std::marker::PhantomData;
+
+pub struct HashMap<T, U>(PhantomData<(T, U)>);
+impl<T, U> Default for HashMap<T, U> {
+    fn default() -> Self {
+        Self(PhantomData)
+    }
+}

tests/run-make/type-mismatch-sysroot-crate/mismatch.rs

@@ -0,0 +1,6 @@
+#![feature(rustc_private)]
+extern crate hashbrown;
+
+fn main() {
+    uses_hashbrown::foo(hashbrown::HashMap::default())
+}

tests/run-make/type-mismatch-sysroot-crate/uses-hashbrown.rs

@@ -0,0 +1,2 @@
+// compile-flags:--extern hashbrown
+pub fn foo<T, U>(_: hashbrown::HashMap<T, U>) {}