ty: projections in transparent_newtype_field

This commit modifies `transparent_newtype_field` so that it handles
projections with generic parameters, where `normalize_erasing_regions`
would ICE.

Signed-off-by: David Wood <david@davidtw.co>

src/librustc_middle/ty/mod.rs

@@ -2387,11 +2387,15 @@ impl<'tcx> AdtDef {
         assert!(self.is_struct() && self.repr.transparent());
 
         for field in &self.non_enum_variant().fields {
-            let field_ty = tcx.normalize_erasing_regions(
-                param_env,
-                field.ty(tcx, InternalSubsts::identity_for_item(tcx, self.did)),
-            );
+            let field_ty = field.ty(tcx, InternalSubsts::identity_for_item(tcx, self.did));
 
+            // `normalize_erasing_regions` will fail for projections that contain generic
+            // parameters, so check these before normalizing.
+            if field_ty.has_projections() && field_ty.needs_subst() {
+                return Some(field);
+            }
+
+            let field_ty = tcx.normalize_erasing_regions(param_env, field_ty);
             if !field_ty.is_zst(tcx, self.did) {
                 return Some(field);
             }

src/test/ui/lint/lint-ctypes-73249-1.rs

@@ -0,0 +1,21 @@
+// check-pass
+#![deny(improper_ctypes)]
+
+pub trait Foo {
+    type Assoc: 'static;
+}
+
+impl Foo for () {
+    type Assoc = u32;
+}
+
+extern "C" {
+    pub fn lint_me(x: Bar<()>);
+}
+
+#[repr(transparent)]
+pub struct Bar<T: Foo> {
+    value: &'static <T as Foo>::Assoc,
+}
+
+fn main() {}

src/test/ui/lint/lint-ctypes-73249.rs

@@ -0,0 +1,21 @@
+// check-pass
+#![deny(improper_ctypes)]
+
+pub trait Foo {
+    type Assoc;
+}
+
+impl Foo for () {
+    type Assoc = u32;
+}
+
+extern "C" {
+    pub fn lint_me(x: Bar<()>);
+}
+
+#[repr(transparent)]
+pub struct Bar<T: Foo> {
+    value: <T as Foo>::Assoc,
+}
+
+fn main() {}