Deny const auto traits

compiler/rustc_ast_passes/messages.ftl

@@ -89,6 +89,9 @@ ast_passes_const_and_coroutine = functions cannot be both `const` and `{$corouti
     .coroutine = `{$coroutine_kind}` because of this
     .label = {""}
 
+ast_passes_const_auto_trait = auto traits cannot be const
+    .help = remove the `const` keyword
+
 ast_passes_const_bound_trait_object = const trait bounds are not allowed in trait object types
 
 ast_passes_const_without_body =

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -820,6 +820,12 @@ impl<'a> AstValidator<'a> {
         self.dcx().emit_err(errors::ModuleNonAscii { span: ident.span, name: ident.name });
     }
 
+    fn deny_const_auto_traits(&self, constness: Const) {
+        if let Const::Yes(span) = constness {
+            self.dcx().emit_err(errors::ConstAutoTrait { span });
+        }
+    }
+
     fn deny_generic_params(&self, generics: &Generics, ident_span: Span) {
         if !generics.params.is_empty() {
             self.dcx()
@@ -1257,6 +1263,8 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             }) => {
                 self.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
                 if *is_auto == IsAuto::Yes {
+                    // For why we reject `const auto trait`, see rust-lang/rust#149285.
+                    self.deny_const_auto_traits(*constness);
                     // Auto traits cannot have generics, super traits nor contain items.
                     self.deny_generic_params(generics, ident.span);
                     self.deny_super_traits(bounds, ident.span);

compiler/rustc_ast_passes/src/errors.rs

@@ -429,6 +429,14 @@ pub(crate) struct AutoTraitItems {
     pub ident: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_passes_const_auto_trait)]
+#[help]
+pub(crate) struct ConstAutoTrait {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_passes_generic_before_constraints)]
 pub(crate) struct ArgsBeforeConstraint {

tests/ui/traits/const-traits/const-auto-trait.rs

@@ -0,0 +1,6 @@
+#![feature(auto_traits, const_trait_impl)]
+
+const auto trait Marker {}
+//~^ ERROR: auto traits cannot be const
+
+fn main() {}

tests/ui/traits/const-traits/const-auto-trait.stderr

@@ -0,0 +1,10 @@
+error: auto traits cannot be const
+  --> $DIR/const-auto-trait.rs:3:1
+   |
+LL | const auto trait Marker {}
+   | ^^^^^
+   |
+   = help: remove the `const` keyword
+
+error: aborting due to 1 previous error
+

tests/ui/traits/const-traits/parse-const-unsafe-trait.rs

@@ -1,5 +1,6 @@
 // Test that `const unsafe trait` and `const unsafe auto trait` works.
 
+//@ compile-flags: -Zparse-crate-root-only
 //@ check-pass
 
 #![feature(const_trait_impl)]