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

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     no_index_update: bool = ("Do not update the registry index even if the cache is outdated"),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

@@ -101,6 +101,7 @@ Each new feature described below should explain how to use it.
     * [rustdoc-depinfo](#rustdoc-depinfo) --- Use dep-info files in rustdoc rebuild detection.
 * `Cargo.toml` extensions
     * [Profile `rustflags` option](#profile-rustflags-option) --- Passed directly to rustc.
+    * [Profile `hint-mostly-unused` option](#profile-hint-mostly-unused-option) --- Hint that a dependency is mostly unused, to optimize compilation time.
     * [codegen-backend](#codegen-backend) --- Select the codegen backend used by rustc.
     * [per-package-target](#per-package-target) --- Sets the `--target` to use for each individual package.
     * [artifact dependencies](#artifact-dependencies) --- Allow build artifacts to be included into other build artifacts and build them for different targets.
@@ -924,6 +925,25 @@ 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/config.rs

@@ -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());