Auto merge of #124228 - compiler-errors:lint-overcaptures, r=oli-obk

Warn against changes in opaque lifetime captures in 2024

Adds a (mostly[^1]) machine-applicable lint `IMPL_TRAIT_OVERCAPTURES` which detects cases where we will capture more lifetimes in edition 2024 than in edition <= 2021, which may lead to erroneous borrowck errors.

This lint is gated behind the `precise_capturing` feature gate and marked `Allow` for now.

[^1]: Except when there are APITs -- I may work on that soon

r? oli-obk

compiler/rustc_ast_lowering/src/lib.rs

@@ -1407,7 +1407,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         bounds,
                         fn_kind,
                         itctx,
-                        precise_capturing.as_deref().map(|(args, _)| args.as_slice()),
+                        precise_capturing.as_deref().map(|(args, span)| (args.as_slice(), *span)),
                     ),
                     ImplTraitContext::Universal => {
                         if let Some(&(_, span)) = precise_capturing.as_deref() {
@@ -1523,7 +1523,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         bounds: &GenericBounds,
         fn_kind: Option<FnDeclKind>,
         itctx: ImplTraitContext,
-        precise_capturing_args: Option<&[PreciseCapturingArg]>,
+        precise_capturing_args: Option<(&[PreciseCapturingArg], Span)>,
     ) -> hir::TyKind<'hir> {
         // Make sure we know that some funky desugaring has been going on here.
         // This is a first: there is code in other places like for loop
@@ -1533,7 +1533,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let opaque_ty_span = self.mark_span_with_reason(DesugaringKind::OpaqueTy, span, None);
 
         let captured_lifetimes_to_duplicate =
-            if let Some(precise_capturing) = precise_capturing_args {
+            if let Some((precise_capturing, _)) = precise_capturing_args {
                 // We'll actually validate these later on; all we need is the list of
                 // lifetimes to duplicate during this portion of lowering.
                 precise_capturing
@@ -1607,7 +1607,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         captured_lifetimes_to_duplicate: FxIndexSet<Lifetime>,
         span: Span,
         opaque_ty_span: Span,
-        precise_capturing_args: Option<&[PreciseCapturingArg]>,
+        precise_capturing_args: Option<(&[PreciseCapturingArg], Span)>,
         lower_item_bounds: impl FnOnce(&mut Self) -> &'hir [hir::GenericBound<'hir>],
     ) -> hir::TyKind<'hir> {
         let opaque_ty_def_id = self.create_def(
@@ -1698,8 +1698,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 this.with_remapping(captured_to_synthesized_mapping, |this| {
                     (
                         lower_item_bounds(this),
-                        precise_capturing_args.map(|precise_capturing| {
-                            this.lower_precise_capturing_args(precise_capturing)
+                        precise_capturing_args.map(|(precise_capturing, span)| {
+                            (
+                                this.lower_precise_capturing_args(precise_capturing),
+                                this.lower_span(span),
+                            )
                         }),
                     )
                 });

compiler/rustc_hir/src/hir.rs

@@ -2631,7 +2631,7 @@ pub struct OpaqueTy<'hir> {
     /// lowered as an associated type.
     pub in_trait: bool,
     /// List of arguments captured via `impl use<'a, P, ...> Trait` syntax.
-    pub precise_capturing_args: Option<&'hir [PreciseCapturingArg<'hir>]>,
+    pub precise_capturing_args: Option<(&'hir [PreciseCapturingArg<'hir>], Span)>,
 }
 
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
@@ -2641,6 +2641,15 @@ pub enum PreciseCapturingArg<'hir> {
     Param(PreciseCapturingNonLifetimeArg),
 }
 
+impl PreciseCapturingArg<'_> {
+    pub fn hir_id(self) -> HirId {
+        match self {
+            PreciseCapturingArg::Lifetime(lt) => lt.hir_id,
+            PreciseCapturingArg::Param(param) => param.hir_id,
+        }
+    }
+}
+
 /// We need to have a [`Node`] for the [`HirId`] that we attach the type/const param
 /// resolution to. Lifetimes don't have this problem, and for them, it's actually
 /// kind of detrimental to use a custom node type versus just using [`Lifetime`],

compiler/rustc_hir/src/intravisit.rs

@@ -533,7 +533,7 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item<'v>) -> V::
             try_visit!(visitor.visit_id(item.hir_id()));
             try_visit!(walk_generics(visitor, generics));
             walk_list!(visitor, visit_param_bound, bounds);
-            if let Some(precise_capturing_args) = precise_capturing_args {
+            if let Some((precise_capturing_args, _)) = precise_capturing_args {
                 for arg in precise_capturing_args {
                     try_visit!(visitor.visit_precise_capturing_arg(arg));
                 }

compiler/rustc_hir_analysis/src/check/check.rs

@@ -486,7 +486,7 @@ fn sanity_check_found_hidden_type<'tcx>(
 fn check_opaque_precise_captures<'tcx>(tcx: TyCtxt<'tcx>, opaque_def_id: LocalDefId) {
     let hir::OpaqueTy { precise_capturing_args, .. } =
         *tcx.hir_node_by_def_id(opaque_def_id).expect_item().expect_opaque_ty();
-    let Some(precise_capturing_args) = precise_capturing_args else {
+    let Some((precise_capturing_args, _)) = precise_capturing_args else {
         // No precise capturing args; nothing to validate
         return;
     };

compiler/rustc_lint/messages.ftl

@@ -269,6 +269,17 @@ lint_identifier_uncommon_codepoints = identifier contains {$codepoints_len ->
 
 lint_ignored_unless_crate_specified = {$level}({$name}) is ignored unless specified at crate level
 
+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
+    .suggestion = use the precise capturing `use<...>` syntax to make the captures explicit
+
+lint_impl_trait_redundant_captures = all possible in-scope parameters are already captured, so `use<...>` syntax is redundant
+    .suggestion = remove the `use<...>` syntax
+
 lint_improper_ctypes = `extern` {$desc} uses type `{$ty}`, which is not FFI-safe
     .label = not FFI-safe
     .note = the type is defined here