Do not write user type annotation for const param value path

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 }),

compiler/rustc_mir_build/src/thir/cx/mod.rs

@@ -207,7 +207,7 @@ impl<'tcx> ThirBuildCx<'tcx> {
         &self,
         hir_id: HirId,
     ) -> Option<ty::CanonicalUserType<'tcx>> {
-        crate::thir::util::user_args_applied_to_ty_of_hir_id(self.typeck_results, hir_id)
+        crate::thir::util::user_args_applied_to_ty_of_hir_id(self.tcx, self.typeck_results, hir_id)
     }
 }
 

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -539,7 +539,7 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
         &self,
         hir_id: hir::HirId,
     ) -> Option<ty::CanonicalUserType<'tcx>> {
-        crate::thir::util::user_args_applied_to_ty_of_hir_id(self.typeck_results, hir_id)
+        crate::thir::util::user_args_applied_to_ty_of_hir_id(self.tcx, self.typeck_results, hir_id)
     }
 
     /// Takes a HIR Path. If the path is a constant, evaluates it and feeds

compiler/rustc_mir_build/src/thir/util.rs

@@ -1,12 +1,16 @@
+use std::assert_matches::assert_matches;
+
 use rustc_hir as hir;
+use rustc_hir::def::DefKind;
 use rustc_middle::bug;
-use rustc_middle::ty::{self, CanonicalUserType};
+use rustc_middle::ty::{self, CanonicalUserType, TyCtxt};
 use tracing::debug;
 
 /// Looks up the type associated with this hir-id and applies the
 /// user-given generic parameters; the hir-id must map to a suitable
 /// type.
 pub(crate) fn user_args_applied_to_ty_of_hir_id<'tcx>(
+    tcx: TyCtxt<'tcx>,
     typeck_results: &ty::TypeckResults<'tcx>,
     hir_id: hir::HirId,
 ) -> Option<CanonicalUserType<'tcx>> {
@@ -16,7 +20,23 @@ pub(crate) fn user_args_applied_to_ty_of_hir_id<'tcx>(
     let ty = typeck_results.node_type(hir_id);
     match ty.kind() {
         ty::Adt(adt_def, ..) => {
+            // This "fixes" user type annotations for tupled ctor patterns for ADTs.
+            // That's because `type_of(ctor_did)` returns a FnDef, but we actually
+            // want to be annotating the type of the ADT itself. It's a bit goofy,
+            // but it's easier to adjust this here rather than in the path lowering
+            // code for patterns in HIR.
             if let ty::UserTypeKind::TypeOf(did, _) = &mut user_ty.value.kind {
+                // This is either already set up correctly (struct, union, enum, or variant),
+                // or needs adjusting (ctor). Make sure we don't start adjusting other
+                // user annotations like consts or fn calls.
+                assert_matches!(
+                    tcx.def_kind(*did),
+                    DefKind::Ctor(..)
+                        | DefKind::Struct
+                        | DefKind::Enum
+                        | DefKind::Union
+                        | DefKind::Variant
+                );
                 *did = adt_def.did();
             }
             Some(user_ty)

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

@@ -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`.