lint/ctypes: only try normalize

Now that this lint runs on any external-ABI fn-ptr, normalization won't
always succeed, so use `try_normalize_erasing_regions` instead.

Signed-off-by: David Wood <david@davidtw.co>
(cherry picked from commit 09434a2575)

compiler/rustc_lint/src/types.rs

@@ -963,12 +963,12 @@ fn check_field_type_for_ffi(
         substs: SubstsRef<'tcx>,
     ) -> FfiResult<'tcx> {
         let field_ty = field.ty(self.cx.tcx, substs);
-        if field_ty.has_opaque_types() {
-            self.check_type_for_ffi(cache, field_ty)
-        } else {
-            let field_ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, field_ty);
-            self.check_type_for_ffi(cache, field_ty)
-        }
+        let field_ty = self
+            .cx
+            .tcx
+            .try_normalize_erasing_regions(self.cx.param_env, field_ty)
+            .unwrap_or(field_ty);
+        self.check_type_for_ffi(cache, field_ty)
     }
 
     /// Checks if the given `VariantDef`'s field types are "ffi-safe".
@@ -1315,7 +1315,8 @@ fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         if let Some(ty) = self
             .cx
             .tcx
-            .normalize_erasing_regions(self.cx.param_env, ty)
+            .try_normalize_erasing_regions(self.cx.param_env, ty)
+            .unwrap_or(ty)
             .visit_with(&mut ProhibitOpaqueTypes)
             .break_value()
         {
@@ -1333,16 +1334,12 @@ fn check_type_for_ffi_and_report_errors(
         is_static: bool,
         is_return_type: bool,
     ) {
-        // We have to check for opaque types before `normalize_erasing_regions`,
-        // which will replace opaque types with their underlying concrete type.
         if self.check_for_opaque_ty(sp, ty) {
             // We've already emitted an error due to an opaque type.
             return;
         }
 
-        // it is only OK to use this function because extern fns cannot have
-        // any generic types right now:
-        let ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, ty);
+        let ty = self.cx.tcx.try_normalize_erasing_regions(self.cx.param_env, ty).unwrap_or(ty);
 
         // C doesn't really support passing arrays by value - the only way to pass an array by value
         // is through a struct. So, first test that the top level isn't an array, and then

tests/ui/lint/lint-ctypes-113900.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+// Extending `improper_ctypes` to check external-ABI fn-ptrs means that it can encounter
+// projections which cannot be normalized - unsurprisingly, this shouldn't crash the compiler.
+
+trait Bar {
+    type Assoc;
+}
+
+type Foo<T> = extern "C" fn() -> <T as Bar>::Assoc;
+
+fn main() {}