feat: Add lint for global use of hint-mostly-unused

Muscraft · Muscraft · commit 4f29e256d87e · 2025-09-22T15:23:07.000-06:00
diff --git a/src/cargo/core/workspace.rs b/src/cargo/core/workspace.rs
@@ -25,7 +25,9 @@ use crate::util::context::FeatureUnification;
 use crate::util::edit_distance;
 use crate::util::errors::{CargoResult, ManifestError};
 use crate::util::interning::InternedString;
-use crate::util::lints::{analyze_cargo_lints_table, check_im_a_teapot};
+use crate::util::lints::{
+    analyze_cargo_lints_table, blanket_hint_mostly_unused, check_im_a_teapot,
+};
 use crate::util::toml::{InheritableFields, read_manifest};
 use crate::util::{
     Filesystem, GlobalContext, IntoUrl, context::CargoResolverConfig, context::ConfigRelativePath,
@@ -1287,9 +1289,9 @@ impl<'gctx> Workspace<'gctx> {
     }
 
     pub fn emit_ws_lints(&self) -> CargoResult<()> {
-        let error_count = 0;
+        let mut error_count = 0;
 
-        let _cargo_lints = match self.root_maybe() {
+        let cargo_lints = match self.root_maybe() {
             MaybePackage::Package(pkg) => {
                 let toml = pkg.manifest().normalized_toml();
                 if let Some(ws) = &toml.workspace {
@@ -1314,6 +1316,19 @@ impl<'gctx> Workspace<'gctx> {
             // Calls to lint functions go in here
         }
 
+        // This is a short term hack to allow `blanket_hint_mostly_unused`
+        // to run without requiring `-Zcargo-lints`, which should hopefully
+        // improve the testing expierience while we are collecting feedback
+        if self.gctx.cli_unstable().profile_hint_mostly_unused {
+            blanket_hint_mostly_unused(
+                self.root_maybe(),
+                self.root_manifest(),
+                &cargo_lints,
+                &mut error_count,
+                self.gctx,
+            )?;
+        }
+
         if error_count > 0 {
             Err(crate::util::errors::AlreadyPrintedError::new(anyhow!(
                 "encountered {error_count} errors(s) while running lints"
diff --git a/src/cargo/util/lints.rs b/src/cargo/util/lints.rs
@@ -1,14 +1,14 @@
-use crate::core::{Edition, Feature, Features, Manifest, Package};
+use crate::core::{Edition, Feature, Features, Manifest, MaybePackage, Package};
 use crate::{CargoResult, GlobalContext};
-use annotate_snippets::{AnnotationKind, Group, Level, Snippet};
-use cargo_util_schemas::manifest::{TomlLintLevel, TomlToolLints};
+use annotate_snippets::{AnnotationKind, Group, Level, Patch, Snippet};
+use cargo_util_schemas::manifest::{ProfilePackageSpec, TomlLintLevel, TomlToolLints};
 use pathdiff::diff_paths;
 use std::fmt::Display;
 use std::ops::Range;
 use std::path::Path;
 
 const LINT_GROUPS: &[LintGroup] = &[TEST_DUMMY_UNSTABLE];
-pub const LINTS: &[Lint] = &[IM_A_TEAPOT, UNKNOWN_LINTS];
+pub const LINTS: &[Lint] = &[BLANKET_HINT_MOSTLY_UNUSED, IM_A_TEAPOT, UNKNOWN_LINTS];
 
 pub fn analyze_cargo_lints_table(
     pkg: &Package,
@@ -473,6 +473,153 @@ pub fn check_im_a_teapot(
     Ok(())
 }
 
+const BLANKET_HINT_MOSTLY_UNUSED: Lint = Lint {
+    name: "blanket_hint_mostly_unused",
+    desc: "blanket_hint_mostly_unused lint",
+    groups: &[],
+    default_level: LintLevel::Warn,
+    edition_lint_opts: None,
+    feature_gate: None,
+    docs: Some(
+        r#"
+### What it does
+Checks if `hint-mostly-unused` being applied to all dependencies.
+
+### Why it is bad
+`hint-mostly-unused` indicates that most of a crate's API surface will go
+unused by anything depending on it; this hint can speed up the build by
+attempting to minimize compilation time for items that aren't used at all.
+Misapplication to crates that don't fit that criteria will slow down the build
+rather than speeding it up. It should be selectively applied to dependencies
+that meet these criteria. Applying it globally is always a misapplication and
+will likely slow down the build.
+
+### Example
+```toml
+[profile.dev.package."*"]
+hint-mostly-unused = true
+```
+
+Should instead be:
+```toml
+[profile.dev.package.huge-mostly-unused-dependency]
+hint-mostly-unused = true
+```
+"#,
+    ),
+};
+
+pub fn blanket_hint_mostly_unused(
+    maybe_pkg: &MaybePackage,
+    path: &Path,
+    pkg_lints: &TomlToolLints,
+    error_count: &mut usize,
+    gctx: &GlobalContext,
+) -> CargoResult<()> {
+    let (lint_level, reason) = BLANKET_HINT_MOSTLY_UNUSED.level(
+        pkg_lints,
+        maybe_pkg.edition(),
+        maybe_pkg.unstable_features(),
+    );
+
+    if lint_level == LintLevel::Allow {
+        return Ok(());
+    }
+
+    let level = lint_level.to_diagnostic_level();
+    let manifest_path = rel_cwd_manifest_path(path, gctx);
+    let mut paths = Vec::new();
+
+    if let Some(profiles) = maybe_pkg.profiles() {
+        for (profile_name, top_level_profile) in &profiles.0 {
+            if let Some(true) = top_level_profile.hint_mostly_unused {
+                paths.push((
+                    vec!["profile", profile_name.as_str(), "hint-mostly-unused"],
+                    true,
+                ));
+            }
+
+            if let Some(build_override) = &top_level_profile.build_override
+                && let Some(true) = build_override.hint_mostly_unused
+            {
+                paths.push((
+                    vec![
+                        "profile",
+                        profile_name.as_str(),
+                        "build-override",
+                        "hint-mostly-unused",
+                    ],
+                    false,
+                ));
+            }
+
+            if let Some(packages) = &top_level_profile.package
+                && let Some(profile) = packages.get(&ProfilePackageSpec::All)
+                && let Some(true) = profile.hint_mostly_unused
+            {
+                paths.push((
+                    vec![
+                        "profile",
+                        profile_name.as_str(),
+                        "package",
+                        "*",
+                        "hint-mostly-unused",
+                    ],
+                    false,
+                ));
+            }
+        }
+    }
+
+    for (i, (path, show_per_pkg_suggestion)) in paths.iter().enumerate() {
+        if lint_level.is_error() {
+            *error_count += 1;
+        }
+        let title = "`hint-mostly-unused` is being blanket applied to all dependencies";
+        if let (Some(span), Some(table_span)) = (
+            get_key_value_span(maybe_pkg.document(), &path),
+            get_key_value_span(maybe_pkg.document(), &path[..path.len() - 1]),
+        ) {
+            let mut report = Vec::new();
+            let mut group = level.clone().primary_title(title).element(
+                Snippet::source(maybe_pkg.contents())
+                    .path(&manifest_path)
+                    .annotation(AnnotationKind::Primary.span(span.key.start..span.value.end))
+                    .annotation(
+                        AnnotationKind::Visible.span(table_span.key.start..table_span.key.end),
+                    ),
+            );
+
+            if i == 0 {
+                group =
+                    group
+                        .element(Level::NOTE.message(
+                            BLANKET_HINT_MOSTLY_UNUSED.emitted_source(lint_level, reason),
+                        ));
+            }
+            report.push(group);
+
+            if *show_per_pkg_suggestion {
+                report.push(
+                    Level::HELP
+                        .secondary_title("try enabling `hint-mostly-unused` for just one package")
+                        .element(
+                            Snippet::source(maybe_pkg.contents())
+                                .path(&manifest_path)
+                                .patch(Patch::new(
+                                    table_span.key.end..table_span.key.end,
+                                    ".package.<pkg_name>",
+                                )),
+                        ),
+                );
+            }
+            gctx.shell().print_report(&report, lint_level.force())?;
+        }
+    }
+
+    Ok(())
+}
+
 const UNKNOWN_LINTS: Lint = Lint {
     name: "unknown_lints",
     desc: "unknown lint",
diff --git a/src/doc/src/reference/lints.md b/src/doc/src/reference/lints.md
@@ -5,8 +5,37 @@ Note: [Cargo's linting system is unstable](unstable.md#lintscargo) and can only
 ## Warn-by-default
 
 These lints are all set to the 'warn' level by default.
+- [`blanket_hint_mostly_unused`](#blanket_hint_mostly_unused)
 - [`unknown_lints`](#unknown_lints)
 
+## `blanket_hint_mostly_unused`
+Set to `warn` by default
+
+### What it does
+Checks if `hint-mostly-unused` being applied to all dependencies.
+
+### Why it is bad
+`hint-mostly-unused` indicates that most of a crate's API surface will go
+unused by anything depending on it; this hint can speed up the build by
+attempting to minimize compilation time for items that aren't used at all.
+Misapplication to crates that don't fit that criteria will slow down the build
+rather than speeding it up. It should be selectively applied to dependencies
+that meet these criteria. Applying it globally is always a misapplication and
+will likely slow down the build.
+
+### Example
+```toml
+[profile.dev.package."*"]
+hint-mostly-unused = true
+```
+
+Should instead be:
+```toml
+[profile.dev.package.huge-mostly-unused-dependency]
+hint-mostly-unused = true
+```
+
+
 ## `unknown_lints`
 Set to `warn` by default
 
diff --git a/tests/testsuite/lints/blanket_hint_mostly_unused.rs b/tests/testsuite/lints/blanket_hint_mostly_unused.rs
@@ -22,6 +22,18 @@ hint-mostly-unused = true
     p.cargo("check -Zprofile-hint-mostly-unused -v")
         .masquerade_as_nightly_cargo(&["profile-hint-mostly-unused", "cargo-lints"])
         .with_stderr_data(str![[r#"
+[WARNING] `hint-mostly-unused` is being blanket applied to all dependencies
+ --> Cargo.toml:8:1
+  |
+7 | [profile.dev]
+8 | hint-mostly-unused = true
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = [NOTE] `cargo::blanket_hint_mostly_unused` is set to `warn` by default
+[HELP] try enabling `hint-mostly-unused` for just one package
+  |
+7 | [profile.dev.package.<pkg_name>]
+  |             +++++++++++++++++++
 [CHECKING] foo v0.0.1 ([ROOT]/foo)
 [RUNNING] `rustc --crate-name foo [..]`
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
@@ -50,6 +62,14 @@ hint-mostly-unused = true
     p.cargo("check -Zprofile-hint-mostly-unused -v")
         .masquerade_as_nightly_cargo(&["profile-hint-mostly-unused", "cargo-lints"])
         .with_stderr_data(str![[r#"
+[WARNING] `hint-mostly-unused` is being blanket applied to all dependencies
+ --> Cargo.toml:8:1
+  |
+7 | [profile.dev.package."*"]
+8 | hint-mostly-unused = true
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = [NOTE] `cargo::blanket_hint_mostly_unused` is set to `warn` by default
 [CHECKING] foo v0.0.1 ([ROOT]/foo)
 [RUNNING] `rustc --crate-name foo [..]`
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
@@ -78,6 +98,14 @@ hint-mostly-unused = true
     p.cargo("check -Zprofile-hint-mostly-unused -v")
         .masquerade_as_nightly_cargo(&["profile-hint-mostly-unused", "cargo-lints"])
         .with_stderr_data(str![[r#"
+[WARNING] `hint-mostly-unused` is being blanket applied to all dependencies
+ --> Cargo.toml:8:1
+  |
+7 | [profile.dev.build-override]
+8 | hint-mostly-unused = true
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = [NOTE] `cargo::blanket_hint_mostly_unused` is set to `warn` by default
 [CHECKING] foo v0.0.1 ([ROOT]/foo)
 [RUNNING] `rustc --crate-name foo [..]`
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
@@ -115,6 +143,14 @@ authors = []
     p.cargo("check -Zprofile-hint-mostly-unused -v")
         .masquerade_as_nightly_cargo(&["profile-hint-mostly-unused", "cargo-lints"])
         .with_stderr_data(str![[r#"
+[WARNING] `hint-mostly-unused` is being blanket applied to all dependencies
+ --> Cargo.toml:6:1
+  |
+5 | [profile.dev.package."*"]
+6 | hint-mostly-unused = true
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = [NOTE] `cargo::blanket_hint_mostly_unused` is set to `warn` by default
 [CHECKING] foo v0.0.1 ([ROOT]/foo/foo)
 [RUNNING] `rustc --crate-name foo [..]`
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
