Rollup merge of #125258 - compiler-errors:static-if-no-lt, r=nnethercote

Resolve elided lifetimes in assoc const to static if no other lifetimes are in scope

Implements the change to elided lifetime resolution in *associated consts* subject to FCP here: https://github.com/rust-lang/rust/issues/125190#issue-2301532282

Specifically, walk the enclosing lifetime ribs in an associated const, and if we find no other lifetimes, then resolve to `'static`.

Also make it work for traits, but don't lint -- just give a hard error in that case.

1 files changed, 89 insertions, 53 deletions

diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index fa711d932b6..c1e83c59f98 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -310,9 +310,10 @@ enum LifetimeRibKind {
     /// error on default object bounds (e.g., `Box<dyn Foo>`).
     AnonymousReportError,
 
-    /// Resolves elided lifetimes to `'static`, but gives a warning that this behavior
-    /// is a bug and will be reverted soon.
-    AnonymousWarn(NodeId),
+    /// Resolves elided lifetimes to `'static` if there are no other lifetimes in scope,
+    /// otherwise give a warning that the previous behavior of introducing a new early-bound
+    /// lifetime is a bug and will be removed (if `emit_lint` is enabled).
+    StaticIfNoLifetimeInScope { lint_id: NodeId, emit_lint: bool },
 
     /// Signal we cannot find which should be the anonymous lifetime.
     ElisionFailure,
@@ -1212,7 +1213,7 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                             }
                             LifetimeRibKind::AnonymousCreateParameter { .. }
                             | LifetimeRibKind::AnonymousReportError
-                            | LifetimeRibKind::AnonymousWarn(_)
+                            | LifetimeRibKind::StaticIfNoLifetimeInScope { .. }
                             | LifetimeRibKind::Elided(_)
                             | LifetimeRibKind::ElisionFailure
                             | LifetimeRibKind::ConcreteAnonConst(_)
@@ -1580,7 +1581,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                                     // lifetime would be illegal.
                                     LifetimeRibKind::Item
                                     | LifetimeRibKind::AnonymousReportError
-                                    | LifetimeRibKind::AnonymousWarn(_)
+                                    | LifetimeRibKind::StaticIfNoLifetimeInScope { .. }
                                     | LifetimeRibKind::ElisionFailure => Some(LifetimeUseSet::Many),
                                     // An anonymous lifetime is legal here, and bound to the right
                                     // place, go ahead.
@@ -1643,7 +1644,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                 | LifetimeRibKind::Generics { .. }
                 | LifetimeRibKind::ElisionFailure
                 | LifetimeRibKind::AnonymousReportError
-                | LifetimeRibKind::AnonymousWarn(_) => {}
+                | LifetimeRibKind::StaticIfNoLifetimeInScope { .. } => {}
             }
         }
 
@@ -1677,16 +1678,36 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     self.record_lifetime_res(lifetime.id, res, elision_candidate);
                     return;
                 }
-                LifetimeRibKind::AnonymousWarn(node_id) => {
-                    self.r.lint_buffer.buffer_lint(
-                        lint::builtin::ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
-                        node_id,
-                        lifetime.ident.span,
-                        lint::BuiltinLintDiag::AssociatedConstElidedLifetime {
-                            elided,
-                            span: lifetime.ident.span,
-                        },
-                    );
+                LifetimeRibKind::StaticIfNoLifetimeInScope { lint_id: node_id, emit_lint } => {
+                    let mut lifetimes_in_scope = vec![];
+                    for rib in &self.lifetime_ribs[..i] {
+                        lifetimes_in_scope.extend(rib.bindings.iter().map(|(ident, _)| ident.span));
+                        // Consider any anonymous lifetimes, too
+                        if let LifetimeRibKind::AnonymousCreateParameter { binder, .. } = rib.kind
+                            && let Some(extra) = self.r.extra_lifetime_params_map.get(&binder)
+                        {
+                            lifetimes_in_scope.extend(extra.iter().map(|(ident, _, _)| ident.span));
+                        }
+                    }
+                    if lifetimes_in_scope.is_empty() {
+                        self.record_lifetime_res(
+                            lifetime.id,
+                            LifetimeRes::Static,
+                            elision_candidate,
+                        );
+                        return;
+                    } else if emit_lint {
+                        self.r.lint_buffer.buffer_lint(
+                            lint::builtin::ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
+                            node_id,
+                            lifetime.ident.span,
+                            lint::BuiltinLintDiag::AssociatedConstElidedLifetime {
+                                elided,
+                                span: lifetime.ident.span,
+                                lifetimes_in_scope: lifetimes_in_scope.into(),
+                            },
+                        );
+                    }
                 }
                 LifetimeRibKind::AnonymousReportError => {
                     if elided {
@@ -1904,7 +1925,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     //     impl Foo for std::cell::Ref<u32> // note lack of '_
                     //     async fn foo(_: std::cell::Ref<u32>) { ... }
                     LifetimeRibKind::AnonymousCreateParameter { report_in_path: true, .. }
-                    | LifetimeRibKind::AnonymousWarn(_) => {
+                    | LifetimeRibKind::StaticIfNoLifetimeInScope { .. } => {
                         let sess = self.r.tcx.sess;
                         let subdiag = rustc_errors::elided_lifetime_in_path_suggestion(
                             sess.source_map(),
@@ -2838,19 +2859,27 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                             kind: LifetimeBinderKind::ConstItem,
                         },
                         |this| {
-                            this.visit_generics(generics);
-                            this.visit_ty(ty);
-
-                            // Only impose the restrictions of `ConstRibKind` for an
-                            // actual constant expression in a provided default.
-                            if let Some(expr) = expr {
-                                // We allow arbitrary const expressions inside of associated consts,
-                                // even if they are potentially not const evaluatable.
-                                //
-                                // Type parameters can already be used and as associated consts are
-                                // not used as part of the type system, this is far less surprising.
-                                this.resolve_const_body(expr, None);
-                            }
+                            this.with_lifetime_rib(
+                                LifetimeRibKind::StaticIfNoLifetimeInScope {
+                                    lint_id: item.id,
+                                    emit_lint: false,
+                                },
+                                |this| {
+                                    this.visit_generics(generics);
+                                    this.visit_ty(ty);
+
+                                    // Only impose the restrictions of `ConstRibKind` for an
+                                    // actual constant expression in a provided default.
+                                    if let Some(expr) = expr {
+                                        // We allow arbitrary const expressions inside of associated consts,
+                                        // even if they are potentially not const evaluatable.
+                                        //
+                                        // Type parameters can already be used and as associated consts are
+                                        // not used as part of the type system, this is far less surprising.
+                                        this.resolve_const_body(expr, None);
+                                    }
+                                },
+                            )
                         },
                     );
                 }
@@ -3030,30 +3059,37 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                         kind: LifetimeBinderKind::ConstItem,
                     },
                     |this| {
-                        this.with_lifetime_rib(LifetimeRibKind::AnonymousWarn(item.id), |this| {
-                            // If this is a trait impl, ensure the const
-                            // exists in trait
-                            this.check_trait_item(
-                                item.id,
-                                item.ident,
-                                &item.kind,
-                                ValueNS,
-                                item.span,
-                                seen_trait_items,
-                                |i, s, c| ConstNotMemberOfTrait(i, s, c),
-                            );
+                        this.with_lifetime_rib(
+                            LifetimeRibKind::StaticIfNoLifetimeInScope {
+                                lint_id: item.id,
+                                // In impls, it's not a hard error yet due to backcompat.
+                                emit_lint: true,
+                            },
+                            |this| {
+                                // If this is a trait impl, ensure the const
+                                // exists in trait
+                                this.check_trait_item(
+                                    item.id,
+                                    item.ident,
+                                    &item.kind,
+                                    ValueNS,
+                                    item.span,
+                                    seen_trait_items,
+                                    |i, s, c| ConstNotMemberOfTrait(i, s, c),
+                                );
 
-                            this.visit_generics(generics);
-                            this.visit_ty(ty);
-                            if let Some(expr) = expr {
-                                // We allow arbitrary const expressions inside of associated consts,
-                                // even if they are potentially not const evaluatable.
-                                //
-                                // Type parameters can already be used and as associated consts are
-                                // not used as part of the type system, this is far less surprising.
-                                this.resolve_const_body(expr, None);
-                            }
-                        });
+                                this.visit_generics(generics);
+                                this.visit_ty(ty);
+                                if let Some(expr) = expr {
+                                    // We allow arbitrary const expressions inside of associated consts,
+                                    // even if they are potentially not const evaluatable.
+                                    //
+                                    // Type parameters can already be used and as associated consts are
+                                    // not used as part of the type system, this is far less surprising.
+                                    this.resolve_const_body(expr, None);
+                                }
+                            },
+                        );
                     },
                 );
             }