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Do not run per-module late lints if they can be all skipped #139597

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Do not run per-module late lints if they can be all skipped #139597

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637bf91
Fix doc comment
Kobzol Apr 9, 2025
6083850
Skip `late_lint_mod_inner` if all built-in lints can be skipped
Kobzol Apr 9, 2025
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11 changes: 10 additions & 1 deletion compiler/rustc_lint/src/late.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -356,7 +356,16 @@ pub fn late_lint_mod<'tcx, T: LateLintPass<'tcx> + 'tcx>(
let store = unerased_lint_store(tcx.sess);

if store.late_module_passes.is_empty() {
late_lint_mod_inner(tcx, module_def_id, context, builtin_lints);
// If all builtin lints can be skipped, there is no point in running `late_lint_mod_inner`
// at all. This happens often for dependencies built with `--cap-lints=allow`.
let dont_need_to_run = tcx.lints_that_dont_need_to_run(());
let can_skip_lints = builtin_lints
.get_lints()
.iter()
.all(|lint| dont_need_to_run.contains(&LintId::of(lint)));
if !can_skip_lints {
late_lint_mod_inner(tcx, module_def_id, context, builtin_lints);
}
} else {
let builtin_lints = Box::new(builtin_lints) as Box<dyn LateLintPass<'tcx>>;
let mut binding = store
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why not do the same thing here that we do in late_lint_crate and filter out passes that only contain stuff that doesn't need to be run?
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We could do that, but I think that would be slower. When we run everything, it is built in the compile-time.prepared structure, which merged all lints together. If we filtered individual lints, they need to be built using a runtime-prepared structure. For the Clippy lints that makes sense, but here I essentially only want to distinguish between "everything is disabled due to --cap-lints=allow" and "something might be disabled, but let's still run in the merged mode".
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i meant in the else branch (but github doesnt let me comment on the right line ☹️) we make a RuntimeCombinedLateLintPass but don't do any filtering out of dyn LateLintPasss that are just only allowed lints. e.g.

        let mut binding = store
            .late_module_passes
            .iter()
            .map(|mk_pass| (mk_pass)(tcx))
            .chain(std::iter::once(builtin_lints))
            .filter(|pass| { ... }) // new, filter out passes that only contain allowed lints
            .collect::<Vec<_>>();
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But builtin_lints is a single LintPass that combines all the builtin late lints together. In other words, in its e.g. check_mod implementation, it calls:

LINT_1.check_mod();
LINT_2.check_mod();

etc. So we cannot filter these lints from the outside. In theory, we could filter them inside these lint visitor functions, but that would mean doing that check for every call of each visitor function times the number of lints we have, which... doesn't sound fast 😅
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6 changes: 3 additions & 3 deletions compiler/rustc_lint/src/passes.rs
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Expand Up @@ -92,7 +92,7 @@ macro_rules! expand_combined_late_lint_pass_methods {
/// Combines multiple lints passes into a single lint pass, at compile time,
/// for maximum speed. Each `check_foo` method in `$methods` within this pass
/// simply calls `check_foo` once per `$pass`. Compare with
/// `LateLintPassObjects`, which is similar, but combines lint passes at
/// `RuntimeCombinedLateLintPass`, which is similar, but combines lint passes at
/// runtime.
#[macro_export]
macro_rules! declare_combined_late_lint_pass {
Expand Down Expand Up @@ -123,10 +123,10 @@ macro_rules! declare_combined_late_lint_pass {
#[allow(rustc::lint_pass_impl_without_macro)]
impl $crate::LintPass for $name {
fn name(&self) -> &'static str {
panic!()
stringify!($name)
}
fn get_lints(&self) -> LintVec {
panic!()
$name::get_lints()
}
}
)
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