Do not allow extern unsized_fn_param

Author: eggyal

compiler/rustc_hir_typeck/src/check.rs

@@ -83,7 +83,7 @@ pub(super) fn check_fn<'a, 'tcx>(
         }
 
         // Check that argument is Sized.
-        if !params_can_be_unsized {
+        if !params_can_be_unsized || matches!(param_ty.kind(), ty::Foreign(..)) {
             fcx.require_type_is_sized(
                 param_ty,
                 param.pat.span,

tests/ui/unsized-locals/extern-parameter-issue-123887.rs

@@ -0,0 +1,12 @@
+// https://github.com/rust-lang/rust/issues/123887
+// Do not ICE on unsized extern parameter
+//@ compile-flags: -Clink-dead-code --emit=link
+#![feature(extern_types, unsized_fn_params)]
+
+extern "C" {
+    type ExternType;
+}
+
+fn f(_: ExternType) {} //~ ERROR the size for values of type `ExternType` cannot be known at compilation time
+
+fn main() {}

tests/ui/unsized-locals/extern-parameter-issue-123887.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the size for values of type `ExternType` cannot be known at compilation time
+  --> $DIR/extern-parameter-issue-123887.rs:10:6
+   |
+LL | fn f(_: ExternType) {}
+   |      ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `ExternType`
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn f(_: &ExternType) {}
+   |         +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.