Migrate trait_fn_const

Author: finalchild

compiler/rustc_ast_passes/src/errors.rs

@@ -82,3 +82,11 @@ pub struct TraitFnAsync {
     #[label]
     pub span: Span,
 }
+
+#[derive(SessionDiagnostic)]
+#[error(ast_passes::trait_fn_const, code = "E0379")]
+pub struct TraitFnConst {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}

compiler/rustc_error_messages/locales/en-US/ast_passes.ftl

@@ -27,3 +27,7 @@ ast_passes_trait_fn_async =
     .label = `async` because of this
     .note = `async` trait functions are not currently supported
     .note2 = consider using the `async-trait` crate: https://crates.io/crates/async-trait
+
+ast_passes_trait_fn_const =
+    functions in traits cannot be declared const
+    .label = functions in traits cannot be const