Plumb rustc -Zhint-mostly-unused flag through as a profile option

The rustc `-Zhint-mostly-unused` flag tells rustc that most of a crate
will go unused. This is useful for speeding up compilation of large
dependencies from which you only use a few items. Plumb that option
through as a profile option, to allow specifying it for specific
dependencies:

```toml
[profile.dev.package.huge-mostly-unused-dependency]
hint-mostly-unused = true
```

To enable this feature, pass `-Zprofile-hint-mostly-unused`. However,
since this option is a hint, using it without passing
`-Zprofile-hint-mostly-unused` will only warn and ignore the profile
option. Versions of Cargo prior to the introduction of this feature will
give an "unused manifest key" warning, but will otherwise function
without erroring. This allows using the hint in a crate's `Cargo.toml`
without mandating the use of a newer Cargo to build it.

Add a test verifying that the profile option gets ignored with a warning
without passing `-Zprofile-hint-mostly-unused`, and another test
verifying that it gets handled when passing
`-Zprofile-hint-mostly-unused`.

Author: joshtriplett

crates/cargo-util-schemas/manifest.schema.json

@@ -1385,6 +1385,14 @@
             }
           ],
           "default": null
+        },
+        "hint-mostly-unused": {
+          "description": "Unstable feature `hint-mostly-unused`",
+          "type": [
+            "boolean",
+            "null"
+          ],
+          "default": null
         }
       }
     },

crates/cargo-util-schemas/src/manifest/mod.rs

@@ -907,6 +907,8 @@ pub struct TomlProfile {
     pub build_override: Option<Box<TomlProfile>>,
     /// Unstable feature `-Ztrim-paths`.
     pub trim_paths: Option<TomlTrimPaths>,
+    /// Unstable feature `hint-mostly-unused`
+    pub hint_mostly_unused: Option<bool>,
 }
 
 impl TomlProfile {
@@ -998,6 +1000,10 @@ impl TomlProfile {
         if let Some(v) = &profile.trim_paths {
             self.trim_paths = Some(v.clone())
         }
+
+        if let Some(v) = profile.hint_mostly_unused {
+            self.hint_mostly_unused = Some(v);
+        }
     }
 }
 

src/cargo/core/compiler/mod.rs

@@ -1135,6 +1135,7 @@ fn build_base_args(
         strip,
         rustflags: profile_rustflags,
         trim_paths,
+        hint_mostly_unused,
         ..
     } = unit.profile.clone();
     let test = unit.mode.is_any_test();
@@ -1325,6 +1326,16 @@ fn build_base_args(
         opt(cmd, "-C", "incremental=", Some(dir));
     }
 
+    if hint_mostly_unused {
+        if bcx.gctx.cli_unstable().profile_hint_mostly_unused {
+            cmd.arg("-Zhint-mostly-unused");
+        } else {
+            bcx.gctx
+                .shell()
+                .warn("ignoring 'hint-mostly-unused' profile option, pass `-Zprofile-hint-mostly-unused` to enable it")?;
+        }
+    }
+
     let strip = strip.into_inner();
     if strip != StripInner::None {
         cmd.arg("-C").arg(format!("strip={}", strip));

src/cargo/core/features.rs

@@ -845,6 +845,7 @@ unstable_cli_options!(
     no_index_update: bool = ("Do not update the registry index even if the cache is outdated"),
     package_workspace: bool = ("Handle intra-workspace dependencies when packaging"),
     panic_abort_tests: bool = ("Enable support to run tests with -Cpanic=abort"),
+    profile_hint_mostly_unused: bool = ("Enable the `hint-mostly-unused` setting in profiles to mark a crate as mostly unused."),
     profile_rustflags: bool = ("Enable the `rustflags` option in profiles in .cargo/config.toml file"),
     public_dependency: bool = ("Respect a dependency's `public` field in Cargo.toml to control public/private dependencies"),
     publish_timeout: bool = ("Enable the `publish.timeout` key in .cargo/config.toml file"),
@@ -1366,6 +1367,7 @@ impl CliUnstable {
             "package-workspace" => self.package_workspace = parse_empty(k, v)?,
             "panic-abort-tests" => self.panic_abort_tests = parse_empty(k, v)?,
             "public-dependency" => self.public_dependency = parse_empty(k, v)?,
+            "profile-hint-mostly-unused" => self.profile_hint_mostly_unused = parse_empty(k, v)?,
             "profile-rustflags" => self.profile_rustflags = parse_empty(k, v)?,
             "trim-paths" => self.trim_paths = parse_empty(k, v)?,
             "publish-timeout" => self.publish_timeout = parse_empty(k, v)?,

src/cargo/core/profiles.rs

@@ -577,6 +577,9 @@ fn merge_profile(profile: &mut Profile, toml: &TomlProfile) {
     if let Some(trim_paths) = &toml.trim_paths {
         profile.trim_paths = Some(trim_paths.clone());
     }
+    if let Some(hint_mostly_unused) = toml.hint_mostly_unused {
+        profile.hint_mostly_unused = hint_mostly_unused;
+    }
     profile.strip = match toml.strip {
         Some(StringOrBool::Bool(true)) => Strip::Resolved(StripInner::Named("symbols".into())),
         Some(StringOrBool::Bool(false)) => Strip::Resolved(StripInner::None),
@@ -626,6 +629,8 @@ pub struct Profile {
     // remove when `-Ztrim-paths` is stablized
     #[serde(skip_serializing_if = "Option::is_none")]
     pub trim_paths: Option<TomlTrimPaths>,
+    #[serde(skip_serializing_if = "std::ops::Not::not")]
+    pub hint_mostly_unused: bool,
 }
 
 impl Default for Profile {
@@ -647,6 +652,7 @@ impl Default for Profile {
             strip: Strip::Deferred(StripInner::None),
             rustflags: vec![],
             trim_paths: None,
+            hint_mostly_unused: false,
         }
     }
 }
@@ -676,6 +682,7 @@ compact_debug! {
                 strip
                 rustflags
                 trim_paths
+                hint_mostly_unused
             )]
         }
     }

src/doc/src/reference/unstable.md

@@ -924,6 +924,25 @@ profile-rustflags = true
 rustflags = [ "-C", "..." ]
 ```
 
+## Profile `hint-mostly-unused` option
+* Tracking Issue: [#15644](https://github.com/rust-lang/cargo/issues/15644)
+
+This feature provides a new option in the `[profile]` section to enable the
+rustc `hint-mostly-unused` option. This is primarily useful to enable for
+specific dependencies:
+
+```toml
+[profile.dev.package.huge-mostly-unused-dependency]
+hint-mostly-unused = true
+```
+
+To enable this feature, pass `-Zprofile-hint-mostly-unused`. However, since
+this option is a hint, using it without passing `-Zprofile-hint-mostly-unused`
+will only warn and ignore the profile option. Versions of Cargo prior to the
+introduction of this feature will give an "unused manifest key" warning, but
+will otherwise function without erroring. This allows using the hint in a
+crate's `Cargo.toml` without mandating the use of a newer Cargo to build it.
+
 ## rustdoc-map
 * Tracking Issue: [#8296](https://github.com/rust-lang/cargo/issues/8296)
 

tests/testsuite/cargo/z_help/stdout.term.svg

@@ -1666,6 +1666,7 @@ fn all_profile_options() {
         build_override: None,
         rustflags: None,
         trim_paths: None,
+        hint_mostly_unused: None,
     };
     let mut overrides = BTreeMap::new();
     let key = cargo_toml::ProfilePackageSpec::Spec(PackageIdSpec::parse("foo").unwrap());