Implement initial IMPL_TRAIT_OVERCAPTURES lint

compiler/rustc_lint/messages.ftl

@@ -9,6 +9,13 @@ lint_array_into_iter =
     .use_explicit_into_iter_suggestion =
         or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value
 
+lint_impl_trait_overcaptures = `{$self_ty}` will capture more lifetimes than possibly intended in edition 2024
+    .note = specifically, {$num_captured ->
+        [one] this lifetime is
+        *[other] these lifetimes are
+     } in scope but not mentioned in the type's bounds
+    .note2 = all lifetimes in scope will be captured by `impl Trait`s in edition 2024
+
 lint_async_fn_in_trait = use of `async fn` in public traits is discouraged as auto trait bounds cannot be specified
     .note = you can suppress this lint if you plan to use the trait only in your own code, or do not care about auto traits like `Send` on the `Future`
     .suggestion = you can alternatively desugar to a normal `fn` that returns `impl Future` and add any desired bounds such as `Send`, but these cannot be relaxed without a breaking API change

compiler/rustc_lint/src/impl_trait_overcaptures.rs

@@ -0,0 +1,236 @@
+use rustc_data_structures::{fx::FxIndexSet, unord::UnordSet};
+use rustc_errors::LintDiagnostic;
+use rustc_hir as hir;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::intravisit;
+use rustc_middle::ty::{
+    self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor,
+};
+use rustc_span::Span;
+
+use crate::fluent_generated as fluent;
+use crate::{LateContext, LateLintPass};
+
+declare_lint! {
+    /// UwU
+    pub IMPL_TRAIT_OVERCAPTURES,
+    Warn,
+    "will capture more lifetimes than possibly intended in edition 2024",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2024),
+        reference: "<https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>",
+    };
+}
+
+declare_lint_pass!(
+    /// Lint for use of `async fn` in the definition of a publicly-reachable
+    /// trait.
+    ImplTraitOvercaptures => [IMPL_TRAIT_OVERCAPTURES]
+);
+
+impl<'tcx> LateLintPass<'tcx> for ImplTraitOvercaptures {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        _: intravisit::FnKind<'tcx>,
+        _: &'tcx hir::FnDecl<'tcx>,
+        _: &'tcx hir::Body<'tcx>,
+        _: Span,
+        parent_def_id: LocalDefId,
+    ) {
+        match cx.tcx.def_kind(parent_def_id) {
+            DefKind::AssocFn => {
+                // RPITITs already capture all lifetimes in scope, so skip them.
+                if matches!(
+                    cx.tcx.def_kind(cx.tcx.local_parent(parent_def_id)),
+                    DefKind::Trait | DefKind::Impl { of_trait: true }
+                ) {
+                    return;
+                }
+            }
+            DefKind::Fn => {
+                // All freee functions need to check for overcaptures.
+            }
+            DefKind::Closure => return,
+            kind => {
+                unreachable!(
+                    "expected function item, found {}",
+                    kind.descr(parent_def_id.to_def_id())
+                )
+            }
+        }
+
+        let sig = cx.tcx.fn_sig(parent_def_id).instantiate_identity();
+
+        let mut in_scope_parameters = FxIndexSet::default();
+        let mut current_def_id = Some(parent_def_id.to_def_id());
+        while let Some(def_id) = current_def_id {
+            let generics = cx.tcx.generics_of(def_id);
+            for param in &generics.params {
+                in_scope_parameters.insert(param.def_id);
+            }
+            current_def_id = generics.parent;
+        }
+
+        sig.visit_with(&mut VisitOpaqueTypes {
+            tcx: cx.tcx,
+            parent_def_id,
+            in_scope_parameters,
+            seen: Default::default(),
+        });
+    }
+}
+
+struct VisitOpaqueTypes<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    parent_def_id: LocalDefId,
+    in_scope_parameters: FxIndexSet<DefId>,
+    seen: FxIndexSet<LocalDefId>,
+}
+
+impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for VisitOpaqueTypes<'tcx> {
+    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
+        &mut self,
+        t: &ty::Binder<'tcx, T>,
+    ) -> Self::Result {
+        let mut added = vec![];
+        for arg in t.bound_vars() {
+            let arg: ty::BoundVariableKind = arg;
+            match arg {
+                ty::BoundVariableKind::Region(ty::BoundRegionKind::BrNamed(def_id, ..)) => {
+                    added.push(def_id);
+                    let unique = self.in_scope_parameters.insert(def_id);
+                    assert!(unique);
+                }
+                ty::BoundVariableKind::Ty(_) => {
+                    todo!("we don't support late-bound type params in `impl Trait`")
+                }
+                ty::BoundVariableKind::Region(..) => {
+                    unreachable!("all AST-derived bound regions should have a name")
+                }
+                ty::BoundVariableKind::Const => {
+                    unreachable!("non-lifetime binder consts are not allowed")
+                }
+            }
+        }
+
+        t.super_visit_with(self);
+
+        for arg in added.into_iter().rev() {
+            self.in_scope_parameters.shift_remove(&arg);
+        }
+    }
+
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
+        if !t.has_opaque_types() {
+            return;
+        }
+
+        if let ty::Alias(ty::Opaque, opaque_ty) = *t.kind()
+            && let Some(opaque_def_id) = opaque_ty.def_id.as_local()
+            && self.seen.insert(opaque_def_id)
+            && let opaque =
+                self.tcx.hir_node_by_def_id(opaque_def_id).expect_item().expect_opaque_ty()
+            && let hir::OpaqueTyOrigin::FnReturn(parent_def_id) = opaque.origin
+            && parent_def_id == self.parent_def_id
+            && opaque.precise_capturing_args.is_none()
+        {
+            let mut captured = UnordSet::default();
+            let variances = self.tcx.variances_of(opaque_def_id);
+            let mut current_def_id = Some(opaque_def_id.to_def_id());
+            while let Some(def_id) = current_def_id {
+                let generics = self.tcx.generics_of(def_id);
+                for param in &generics.params {
+                    if variances[param.index as usize] != ty::Invariant {
+                        continue;
+                    }
+                    captured.insert(extract_def_id_from_arg(
+                        self.tcx,
+                        generics,
+                        opaque_ty.args[param.index as usize],
+                    ));
+                }
+                current_def_id = generics.parent;
+            }
+
+            let uncaptured_spans: Vec<_> = self
+                .in_scope_parameters
+                .iter()
+                .filter(|def_id| !captured.contains(def_id))
+                .map(|def_id| self.tcx.def_span(def_id))
+                .collect();
+
+            if !uncaptured_spans.is_empty() {
+                self.tcx.emit_node_lint(
+                    IMPL_TRAIT_OVERCAPTURES,
+                    self.tcx.local_def_id_to_hir_id(opaque_def_id),
+                    ImplTraitOvercapturesLint {
+                        opaque_span: self.tcx.def_span(opaque_def_id),
+                        self_ty: t,
+                        num_captured: uncaptured_spans.len(),
+                        uncaptured_spans,
+                    },
+                );
+            }
+
+            for clause in
+                self.tcx.item_bounds(opaque_ty.def_id).iter_instantiated(self.tcx, opaque_ty.args)
+            {
+                clause.visit_with(self)
+            }
+        }
+
+        t.super_visit_with(self);
+    }
+}
+
+struct ImplTraitOvercapturesLint<'tcx> {
+    opaque_span: Span,
+    uncaptured_spans: Vec<Span>,
+    self_ty: Ty<'tcx>,
+    num_captured: usize,
+}
+
+impl<'a> LintDiagnostic<'a, ()> for ImplTraitOvercapturesLint<'_> {
+    fn decorate_lint<'b>(self, diag: &'b mut rustc_errors::Diag<'a, ()>) {
+        diag.arg("self_ty", self.self_ty.to_string())
+            .arg("num_captured", self.num_captured)
+            .span(self.opaque_span)
+            .span_note(self.uncaptured_spans, fluent::lint_note)
+            .note(fluent::lint_note2);
+    }
+
+    fn msg(&self) -> rustc_errors::DiagMessage {
+        fluent::lint_impl_trait_overcaptures
+    }
+}
+
+fn extract_def_id_from_arg<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    generics: &'tcx ty::Generics,
+    arg: ty::GenericArg<'tcx>,
+) -> DefId {
+    match arg.unpack() {
+        ty::GenericArgKind::Lifetime(re) => match *re {
+            ty::ReEarlyParam(ebr) => generics.region_param(ebr, tcx).def_id,
+            ty::ReBound(
+                _,
+                ty::BoundRegion { kind: ty::BoundRegionKind::BrNamed(def_id, ..), .. },
+            ) => def_id,
+            _ => unreachable!(),
+        },
+        ty::GenericArgKind::Type(ty) => {
+            let ty::Param(param_ty) = *ty.kind() else {
+                bug!();
+            };
+            generics.type_param(param_ty, tcx).def_id
+        }
+        ty::GenericArgKind::Const(ct) => {
+            let ty::ConstKind::Param(param_ct) = ct.kind() else {
+                bug!();
+            };
+            generics.const_param(param_ct, tcx).def_id
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -55,6 +55,7 @@
 mod for_loops_over_fallibles;
 mod foreign_modules;
 pub mod hidden_unicode_codepoints;
+mod impl_trait_overcaptures;
 mod internal;
 mod invalid_from_utf8;
 mod late;
@@ -94,6 +95,7 @@
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
 use for_loops_over_fallibles::*;
 use hidden_unicode_codepoints::*;
+use impl_trait_overcaptures::ImplTraitOvercaptures;
 use internal::*;
 use invalid_from_utf8::*;
 use let_underscore::*;
@@ -228,6 +230,7 @@ fn lint_mod(tcx: TyCtxt<'_>, module_def_id: LocalModDefId) {
             MissingDoc: MissingDoc,
             AsyncFnInTrait: AsyncFnInTrait,
             NonLocalDefinitions: NonLocalDefinitions::default(),
+            ImplTraitOvercaptures: ImplTraitOvercaptures,
         ]
     ]
 );