Warn on elided lifetimes in associated constants

compiler/rustc_lint/src/context.rs

@@ -966,6 +966,14 @@ pub trait LintContext: Sized {
                         Applicability::MachineApplicable
                     );
                 }
+                BuiltinLintDiagnostics::AssociatedConstElidedLifetime { elided, span } => {
+                    db.span_suggestion_verbose(
+                        if elided { span.shrink_to_hi() } else { span },
+                        "use the `'static` lifetime",
+                        if elided { "'static " } else { "'static" },
+                        Applicability::MachineApplicable
+                    );
+                }
             }
             // Rewrap `db`, and pass control to the user.
             decorate(db)

compiler/rustc_lint_defs/src/builtin.rs

@@ -3376,6 +3376,7 @@ declare_lint_pass! {
         DEPRECATED_IN_FUTURE,
         DEPRECATED_WHERE_CLAUSE_LOCATION,
         DUPLICATE_MACRO_ATTRIBUTES,
+        ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
         ELIDED_LIFETIMES_IN_PATHS,
         EXPORTED_PRIVATE_DEPENDENCIES,
         FFI_UNWIND_CALLS,
@@ -4527,3 +4528,44 @@ declare_lint! {
         reference: "issue #114095 <https://github.com/rust-lang/rust/issues/114095>",
     };
 }
+
+declare_lint! {
+    /// The `elided_lifetimes_in_associated_constant` lint detects elided lifetimes
+    /// that were erroneously allowed in associated constants.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(elided_lifetimes_in_associated_constant)]
+    ///
+    /// struct Foo;
+    ///
+    /// impl Foo {
+    ///     const STR: &str = "hello, world";
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Previous version of Rust
+    ///
+    /// Implicit static-in-const behavior was decided [against] for associated
+    /// constants because of ambiguity. This, however, regressed and the compiler
+    /// erroneously treats elided lifetimes in associated constants as lifetime
+    /// parameters on the impl.
+    ///
+    /// This is a [future-incompatible] lint to transition this to a
+    /// hard error in the future.
+    ///
+    /// [against]: https://github.com/rust-lang/rust/issues/38831
+    /// [future-incompatible]: ../index.md#future-incompatible-lints
+    pub ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
+    Warn,
+    "elided lifetimes cannot be used in associated constants in impls",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseError,
+        reference: "issue #115010 <https://github.com/rust-lang/rust/issues/115010>",
+    };
+}

compiler/rustc_lint_defs/src/lib.rs

@@ -572,6 +572,10 @@ pub enum BuiltinLintDiagnostics {
         /// The span of the unnecessarily-qualified path to remove.
         removal_span: Span,
     },
+    AssociatedConstElidedLifetime {
+        elided: bool,
+        span: Span,
+    },
 }
 
 /// Lints that are buffered up early on in the `Session` before the

compiler/rustc_resolve/src/late.rs

@@ -311,6 +311,10 @@ 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.
+    AnonymousWarnToStatic(NodeId),
+
     /// Signal we cannot find which should be the anonymous lifetime.
     ElisionFailure,
 
@@ -1148,6 +1152,7 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                             }
                             LifetimeRibKind::AnonymousCreateParameter { .. }
                             | LifetimeRibKind::AnonymousReportError
+                            | LifetimeRibKind::AnonymousWarnToStatic(_)
                             | LifetimeRibKind::Elided(_)
                             | LifetimeRibKind::ElisionFailure
                             | LifetimeRibKind::ConcreteAnonConst(_)
@@ -1515,6 +1520,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                                     // lifetime would be illegal.
                                     LifetimeRibKind::Item
                                     | LifetimeRibKind::AnonymousReportError
+                                    | LifetimeRibKind::AnonymousWarnToStatic(_)
                                     | LifetimeRibKind::ElisionFailure => Some(LifetimeUseSet::Many),
                                     // An anonymous lifetime is legal here, and bound to the right
                                     // place, go ahead.
@@ -1576,7 +1582,8 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                 | LifetimeRibKind::Elided(_)
                 | LifetimeRibKind::Generics { .. }
                 | LifetimeRibKind::ElisionFailure
-                | LifetimeRibKind::AnonymousReportError => {}
+                | LifetimeRibKind::AnonymousReportError
+                | LifetimeRibKind::AnonymousWarnToStatic(_) => {}
             }
         }
 
@@ -1616,6 +1623,25 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     self.record_lifetime_res(lifetime.id, res, elision_candidate);
                     return;
                 }
+                LifetimeRibKind::AnonymousWarnToStatic(node_id) => {
+                    self.record_lifetime_res(lifetime.id, LifetimeRes::Static, elision_candidate);
+                    let msg = if elided {
+                        "`&` without an explicit lifetime name cannot be used here"
+                    } else {
+                        "`'_` cannot be used here"
+                    };
+                    self.r.lint_buffer.buffer_lint_with_diagnostic(
+                        lint::builtin::ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
+                        node_id,
+                        lifetime.ident.span,
+                        msg,
+                        lint::BuiltinLintDiagnostics::AssociatedConstElidedLifetime {
+                            elided,
+                            span: lifetime.ident.span,
+                        },
+                    );
+                    return;
+                }
                 LifetimeRibKind::AnonymousReportError => {
                     let (msg, note) = if elided {
                         (
@@ -1811,7 +1837,8 @@ 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::AnonymousCreateParameter { report_in_path: true, .. }
+                    | LifetimeRibKind::AnonymousWarnToStatic(_) => {
                         let sess = self.r.tcx.sess;
                         let mut err = rustc_errors::struct_span_err!(
                             sess,
@@ -2898,7 +2925,6 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
         match &item.kind {
             AssocItemKind::Const(box ast::ConstItem { generics, ty, expr, .. }) => {
                 debug!("resolve_implementation AssocItemKind::Const");
-
                 self.with_generic_param_rib(
                     &generics.params,
                     RibKind::AssocItem,
@@ -2908,28 +2934,33 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                         kind: LifetimeBinderKind::ConstItem,
                     },
                     |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::AnonymousWarnToStatic(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.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);
+                                }
+                            },
+                        );
                     },
                 );
             }

tests/ui/associated-consts/infer-placeholder-in-non-suggestable-pos.rs

@@ -5,6 +5,8 @@ trait Trait {
 impl Trait for () {
     const ASSOC: &dyn Fn(_) = 1i32;
     //~^ ERROR the placeholder `_` is not allowed within types on item signatures for associated constants
+    //~| WARN `&` without an explicit lifetime name cannot be used here
+    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 }
 
 fn main() {}

tests/ui/associated-consts/infer-placeholder-in-non-suggestable-pos.stderr

@@ -1,9 +1,23 @@
+warning: `&` without an explicit lifetime name cannot be used here
+  --> $DIR/infer-placeholder-in-non-suggestable-pos.rs:6:18
+   |
+LL |     const ASSOC: &dyn Fn(_) = 1i32;
+   |                  ^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #115010 <https://github.com/rust-lang/rust/issues/115010>
+   = note: `#[warn(elided_lifetimes_in_associated_constant)]` on by default
+help: use the `'static` lifetime
+   |
+LL |     const ASSOC: &'static dyn Fn(_) = 1i32;
+   |                   +++++++
+
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for associated constants
   --> $DIR/infer-placeholder-in-non-suggestable-pos.rs:6:26
    |
 LL |     const ASSOC: &dyn Fn(_) = 1i32;
    |                          ^ not allowed in type signatures
 
-error: aborting due to previous error
+error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0121`.

tests/ui/consts/assoc-const-elided-lifetime.rs

@@ -0,0 +1,19 @@
+#![deny(elided_lifetimes_in_associated_constant)]
+
+use std::marker::PhantomData;
+
+struct Foo<'a> {
+    x: PhantomData<&'a ()>,
+}
+
+impl<'a> Foo<'a> {
+    const FOO: Foo<'_> = Foo { x: PhantomData::<&()> };
+    //~^ ERROR `'_` cannot be used here
+    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
+    const BAR: &() = &();
+    //~^ ERROR `&` without an explicit lifetime name cannot be used here
+    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+}
+
+fn main() {}

tests/ui/consts/assoc-const-elided-lifetime.stderr

@@ -0,0 +1,33 @@
+error: `'_` cannot be used here
+  --> $DIR/assoc-const-elided-lifetime.rs:10:20
+   |
+LL |     const FOO: Foo<'_> = Foo { x: PhantomData::<&()> };
+   |                    ^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #115010 <https://github.com/rust-lang/rust/issues/115010>
+note: the lint level is defined here
+  --> $DIR/assoc-const-elided-lifetime.rs:1:9
+   |
+LL | #![deny(elided_lifetimes_in_associated_constant)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: use the `'static` lifetime
+   |
+LL |     const FOO: Foo<'static> = Foo { x: PhantomData::<&()> };
+   |                    ~~~~~~~
+
+error: `&` without an explicit lifetime name cannot be used here
+  --> $DIR/assoc-const-elided-lifetime.rs:14:16
+   |
+LL |     const BAR: &() = &();
+   |                ^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #115010 <https://github.com/rust-lang/rust/issues/115010>
+help: use the `'static` lifetime
+   |
+LL |     const BAR: &'static () = &();
+   |                 +++++++
+
+error: aborting due to 2 previous errors
+