Forbid const fn within const impls

compiler/rustc_ast_passes/messages.ftl

@@ -204,6 +204,11 @@ ast_passes_generic_before_constraints = generic arguments must come before the f
 
 ast_passes_generic_default_trailing = generic parameters with a default must be trailing
 
+ast_passes_impl_fn_const =
+    redundant `const` fn marker in const impl
+    .parent_constness = this declares all associated functions implicitly const
+    .label = remove the `const`
+
 ast_passes_incompatible_features = `{$f1}` and `{$f2}` are incompatible, using them at the same time is not allowed
     .help = remove one of these features
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -242,6 +242,20 @@ impl<'a> AstValidator<'a> {
         }
     }
 
+    fn check_impl_fn_not_const(&self, constness: Const, parent_constness: Const) {
+        let Const::Yes(span) = constness else {
+            return;
+        };
+
+        let span = self.sess.source_map().span_extend_while_whitespace(span);
+
+        let Const::Yes(parent_constness) = parent_constness else {
+            return;
+        };
+
+        self.dcx().emit_err(errors::ImplFnConst { span, parent_constness });
+    }
+
     fn check_trait_fn_not_const(&self, constness: Const, parent: &TraitOrImpl) {
         let Const::Yes(span) = constness else {
             return;
@@ -1689,14 +1703,23 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             );
         }
 
-        if let Some(parent @ (TraitOrImpl::Trait { .. } | TraitOrImpl::TraitImpl { .. })) =
-            &self.outer_trait_or_trait_impl
-        {
-            self.visibility_not_permitted(&item.vis, errors::VisibilityNotPermittedNote::TraitImpl);
-            if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
-                self.check_trait_fn_not_const(sig.header.constness, parent);
-                self.check_async_fn_in_const_trait_or_impl(sig, parent);
+        match &self.outer_trait_or_trait_impl {
+            Some(parent @ (TraitOrImpl::Trait { .. } | TraitOrImpl::TraitImpl { .. })) => {
+                self.visibility_not_permitted(
+                    &item.vis,
+                    errors::VisibilityNotPermittedNote::TraitImpl,
+                );
+                if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
+                    self.check_trait_fn_not_const(sig.header.constness, parent);
+                    self.check_async_fn_in_const_trait_or_impl(sig, parent);
+                }
             }
+            Some(TraitOrImpl::Impl { constness }) => {
+                if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
+                    self.check_impl_fn_not_const(sig.header.constness, *constness);
+                }
+            }
+            None => {}
         }
 
         if let AssocItemKind::Const(ci) = &item.kind {

compiler/rustc_ast_passes/src/errors.rs

@@ -35,6 +35,15 @@ pub(crate) enum VisibilityNotPermittedNote {
     #[note(ast_passes_individual_foreign_items)]
     IndividualForeignItems,
 }
+#[derive(Diagnostic)]
+#[diag(ast_passes_impl_fn_const)]
+pub(crate) struct ImplFnConst {
+    #[primary_span]
+    #[suggestion(ast_passes_label, code = "", applicability = "machine-applicable")]
+    pub span: Span,
+    #[label(ast_passes_parent_constness)]
+    pub parent_constness: Span,
+}
 
 #[derive(Diagnostic)]
 #[diag(ast_passes_trait_fn_const, code = E0379)]

tests/ui/traits/const-traits/const-impl-inherent-double-const.fixed

@@ -0,0 +1,18 @@
+//@run-rustfix
+
+#![feature(const_trait_impl)]
+struct Foo;
+
+const impl Foo {
+    fn bar() {}
+    fn baz() {}
+    //~^ ERROR: redundant `const`
+}
+
+const _: () = {
+    Foo::bar();
+    Foo::baz();
+};
+
+fn main() {
+}

tests/ui/traits/const-traits/const-impl-inherent-double-const.rs

@@ -0,0 +1,18 @@
+//@run-rustfix
+
+#![feature(const_trait_impl)]
+struct Foo;
+
+const impl Foo {
+    fn bar() {}
+    const fn baz() {}
+    //~^ ERROR: redundant `const`
+}
+
+const _: () = {
+    Foo::bar();
+    Foo::baz();
+};
+
+fn main() {
+}

tests/ui/traits/const-traits/const-impl-inherent-double-const.stderr

@@ -0,0 +1,11 @@
+error: redundant `const` fn marker in const impl
+  --> $DIR/const-impl-inherent-double-const.rs:8:5
+   |
+LL | const impl Foo {
+   | ----- this declares all associated functions implicitly const
+LL |     fn bar() {}
+LL |     const fn baz() {}
+   |     ^^^^^^ help: remove the `const`
+
+error: aborting due to 1 previous error
+

tests/ui/traits/const-traits/span-bug-issue-121418.rs

@@ -7,6 +7,7 @@ impl const dyn T {
     pub const fn new() -> std::sync::Mutex<dyn T> {}
     //~^ ERROR mismatched types
     //~| ERROR cannot be known at compilation time
+    //~| ERROR redundant `const` fn marker in const impl
 }
 
 fn main() {}

tests/ui/traits/const-traits/span-bug-issue-121418.stderr

@@ -1,3 +1,11 @@
+error: redundant `const` fn marker in const impl
+  --> $DIR/span-bug-issue-121418.rs:7:9
+   |
+LL | impl const dyn T {
+   |      ----- this declares all associated functions implicitly const
+LL |     pub const fn new() -> std::sync::Mutex<dyn T> {}
+   |         ^^^^^^ help: remove the `const`
+
 error[E0277]: the size for values of type `(dyn T + 'static)` cannot be known at compilation time
   --> $DIR/span-bug-issue-121418.rs:7:27
    |
@@ -20,7 +28,7 @@ LL |     pub const fn new() -> std::sync::Mutex<dyn T> {}
    = note: expected struct `std::sync::Mutex<(dyn T + 'static)>`
            found unit type `()`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0277, E0308.
 For more information about an error, try `rustc --explain E0277`.