Plumb rustc -Zhint-mostly-unused flag through as a profile option (#15643)

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`.

### How to test and review this PR?

The tests in the testsuite demonstrate both that the option works as
expected and that it gets ignored with a warning if not passing
`-Zprofile-hint-mostly-unused`.

This will remain a draft until
rust-lang/rust#135656 gets merged in rustc; once
that happens, the "nightly" jobs will pass in CI.

Author: epage

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/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());