Do not write user type annotation for const param value path

Author: compiler-errors

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -220,6 +220,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) {
         debug!("fcx {}", self.tag());
 
+        // Don't write user type annotations for const param types, since we give them
+        // identity args just so that we can trivially substitute their `EarlyBinder`.
+        // We enforce that they match their type in MIR later on.
+        if matches!(self.tcx.def_kind(def_id), DefKind::ConstParam) {
+            return;
+        }
+
         if Self::can_contain_user_lifetime_bounds((args, user_self_ty)) {
             let canonicalized = self.canonicalize_user_type_annotation(ty::UserType::new(
                 ty::UserTypeKind::TypeOf(def_id, UserArgs { args, user_self_ty }),

tests/crashes/138048.rs

@@ -0,0 +1,10 @@
+struct Foo<'a>(&'a ());
+
+// We need a lifetime in scope or else we do not write a user type annotation as a fast-path.
+impl<'a> Foo<'a> {
+    fn bar<const V: u8>() {
+        let V;
+        //~^ ERROR constant parameters cannot be referenced in patterns
+    }
+}
+fn main() {}

tests/ui/const-generics/generic_const_parameter_types/bad-param-in-pat.rs

@@ -0,0 +1,11 @@
+error[E0158]: constant parameters cannot be referenced in patterns
+  --> $DIR/bad-param-in-pat.rs:6:13
+   |
+LL |     fn bar<const V: u8>() {
+   |            ----------- constant defined here
+LL |         let V;
+   |             ^ can't be used in patterns
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0158`.