fix: use host-tuple for host target subsitution (#16003)

### What does this PR try to resolve?

The "host" string is ambiguous

* We have `-Zhost-config` that config `[host]` table applies to
artifacts running on host, such as build scripts and proc macros.
* `host` sounds like the default behavior, whereas `--target host` is in
the cross-compilation mode: `target/<triple>/debug`.
* We might want to reserve `host` for future use

This should address both concerns in
<#13051 (comment)>:

* "host-tuple" is aligned with `rustc --print host-tuple`, and doesn't
sound like a default behavior.
* Given "host" is not used, we reserved the future possibility to reset
to the default behavior

Author: weihanglo

src/cargo/core/compiler/compile_kind.rs

@@ -87,10 +87,10 @@ impl CompileKind {
             let deduplicated_targets = targets
                 .iter()
                 .map(|value| {
-                    // This neatly substitutes the manually-specified `host` target directive
+                    // This neatly substitutes the manually-specified `host-tuple` target directive
                     // with the compiling machine's target triple.
 
-                    if value.as_str() == "host" {
+                    if value.as_str() == "host-tuple" {
                         let host_triple = env!("RUST_HOST_TARGET");
                         Ok(CompileKind::Target(CompileTarget::new(host_triple)?))
                     } else {

src/cargo/util/command_prelude.rs

@@ -1263,10 +1263,12 @@ fn get_target_triples() -> Vec<clap_complete::CompletionCandidate> {
         }
     }
 
-    // Allow tab-completion for `host` as the desired target.
-    candidates.push(clap_complete::CompletionCandidate::new("host").help(Some(
-        concat!("alias for: ", env!("RUST_HOST_TARGET")).into(),
-    )));
+    // Allow tab-completion for `host-tuple` as the desired target.
+    candidates.push(
+        clap_complete::CompletionCandidate::new("host-tuple").help(Some(
+            concat!("alias for: ", env!("RUST_HOST_TARGET")).into(),
+        )),
+    );
 
     candidates
 }

src/doc/man/generated_txt/cargo-bench.txt

@@ -228,8 +228,8 @@ OPTIONS
 
            o  Any supported target in rustc --print target-list.
 
-           o  "host", which will internally be substituted by the host’s
-              target. This can be particularly useful if you’re
+           o  "host-tuple", which will internally be substituted by the
+              host’s target. This can be particularly useful if you’re
               cross-compiling some crates, and don’t want to specify your
               host’s machine as a target (for instance, an xtask in a shared
               project that may be worked on by many hosts).

src/doc/man/generated_txt/cargo-build.txt

@@ -145,8 +145,8 @@ OPTIONS
 
            o  Any supported target in rustc --print target-list.
 
-           o  "host", which will internally be substituted by the host’s
-              target. This can be particularly useful if you’re
+           o  "host-tuple", which will internally be substituted by the
+              host’s target. This can be particularly useful if you’re
               cross-compiling some crates, and don’t want to specify your
               host’s machine as a target (for instance, an xtask in a shared
               project that may be worked on by many hosts).

src/doc/man/generated_txt/cargo-check.txt

@@ -142,8 +142,8 @@ OPTIONS
 
            o  Any supported target in rustc --print target-list.
 
-           o  "host", which will internally be substituted by the host’s
-              target. This can be particularly useful if you’re
+           o  "host-tuple", which will internally be substituted by the
+              host’s target. This can be particularly useful if you’re
               cross-compiling some crates, and don’t want to specify your
               host’s machine as a target (for instance, an xtask in a shared
               project that may be worked on by many hosts).

src/doc/man/generated_txt/cargo-clean.txt

@@ -53,8 +53,8 @@ OPTIONS
 
            o  Any supported target in rustc --print target-list.
 
-           o  "host", which will internally be substituted by the host’s
-              target. This can be particularly useful if you’re
+           o  "host-tuple", which will internally be substituted by the
+              host’s target. This can be particularly useful if you’re
               cross-compiling some crates, and don’t want to specify your
               host’s machine as a target (for instance, an xtask in a shared
               project that may be worked on by many hosts).

src/doc/man/generated_txt/cargo-doc.txt

@@ -124,8 +124,8 @@ OPTIONS
 
            o  Any supported target in rustc --print target-list.
 
-           o  "host", which will internally be substituted by the host’s
-              target. This can be particularly useful if you’re
+           o  "host-tuple", which will internally be substituted by the
+              host’s target. This can be particularly useful if you’re
               cross-compiling some crates, and don’t want to specify your
               host’s machine as a target (for instance, an xtask in a shared
               project that may be worked on by many hosts).

src/doc/man/generated_txt/cargo-fetch.txt

@@ -33,8 +33,8 @@ OPTIONS
 
            o  Any supported target in rustc --print target-list.
 
-           o  "host", which will internally be substituted by the host’s
-              target. This can be particularly useful if you’re
+           o  "host-tuple", which will internally be substituted by the
+              host’s target. This can be particularly useful if you’re
               cross-compiling some crates, and don’t want to specify your
               host’s machine as a target (for instance, an xtask in a shared
               project that may be worked on by many hosts).

src/doc/man/generated_txt/cargo-fix.txt

@@ -216,8 +216,8 @@ OPTIONS
 
            o  Any supported target in rustc --print target-list.
 
-           o  "host", which will internally be substituted by the host’s
-              target. This can be particularly useful if you’re
+           o  "host-tuple", which will internally be substituted by the
+              host’s target. This can be particularly useful if you’re
               cross-compiling some crates, and don’t want to specify your
               host’s machine as a target (for instance, an xtask in a shared
               project that may be worked on by many hosts).

src/doc/man/generated_txt/cargo-install.txt

@@ -194,8 +194,8 @@ OPTIONS
 
            o  Any supported target in rustc --print target-list.
 
-           o  "host", which will internally be substituted by the host’s
-              target. This can be particularly useful if you’re
+           o  "host-tuple", which will internally be substituted by the
+              host’s target. This can be particularly useful if you’re
               cross-compiling some crates, and don’t want to specify your
               host’s machine as a target (for instance, an xtask in a shared
               project that may be worked on by many hosts).