Do not allow extern unsized_fn_param

compiler/rustc_hir_typeck/src/check.rs

@@ -83,7 +83,8 @@ pub(super) fn check_fn<'a, 'tcx>(
         }
 
         // Check that argument is Sized.
-        if !params_can_be_unsized {
+        let tail = fcx.tcx.struct_tail_with_normalize(param_ty, |ty| ty, || {});
+        if !params_can_be_unsized || matches!(tail.kind(), ty::Foreign(..)) {
             fcx.require_type_is_sized(
                 param_ty,
                 param.pat.span,

compiler/rustc_monomorphize/messages.ftl

@@ -30,6 +30,7 @@ monomorphize_type_length_limit = reached the type-length limit while instantiati
 monomorphize_unknown_cgu_collection_mode =
     unknown codegen-item collection mode '{$mode}', falling back to 'lazy' mode
 
+monomorphize_unsized_extern_fn_param = unsized arguments must not be `extern` types
 monomorphize_unused_generic_params = item has unused generic parameters
 
 monomorphize_written_to_path = the full type name has been written to '{$path}'

compiler/rustc_monomorphize/src/collector.rs

@@ -1041,6 +1041,12 @@ fn visit_fn_use<'tcx>(
                 _ => bug!("failed to resolve instance for {ty}"),
             }
         };
+        for &param_ty in ty.fn_sig(tcx).inputs().skip_binder() {
+            let tail = tcx.struct_tail_with_normalize(param_ty, |ty| ty, || {});
+            if matches!(tail.kind(), ty::Foreign(..)) {
+                tcx.dcx().emit_fatal(errors::UnsizedExternParam { span: source });
+            }
+        }
         visit_instance_use(tcx, instance, is_direct_call, source, output);
     }
 }

compiler/rustc_monomorphize/src/errors.rs

@@ -104,3 +104,10 @@ pub struct StartNotFound;
 pub struct UnknownCguCollectionMode<'a> {
     pub mode: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(monomorphize_unsized_extern_fn_param)]
+pub struct UnsizedExternParam {
+    #[primary_span]
+    pub span: Span,
+}

tests/ui/recursion/recursion.stderr

@@ -1,3 +1,18 @@
+error: reached the recursion limit finding the struct tail for `Nil`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]`
+
+error: reached the recursion limit finding the struct tail for `Nil`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]`
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+note: the above error was encountered while instantiating `fn test::<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Cons<Nil>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>`
+  --> $DIR/recursion.rs:18:11
+   |
+LL |     _ => {test (n-1, i+1, Cons {head:2*i+1, tail:first}, Cons{head:i*i, tail:second})}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error: reached the recursion limit while instantiating `test::<Cons<Cons<Cons<Cons<Cons<...>>>>>>`
   --> $DIR/recursion.rs:18:11
    |
@@ -11,5 +26,5 @@ LL | fn test<T:Dot> (n:isize, i:isize, first:T, second:T) ->isize {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: the full type name has been written to '$TEST_BUILD_DIR/recursion/recursion/recursion.long-type.txt'
 
-error: aborting due to 1 previous error
+error: aborting due to 3 previous errors
 

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

@@ -0,0 +1,16 @@
+// 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;
+}
+
+struct Wrapper<T: ?Sized>(T);
+fn f(_: impl ?Sized) {}
+fn g(x: Box<Wrapper<ExternType>>) {
+    f(*x); //~ ERROR unsized arguments must not be `extern` types
+}
+
+fn main() {}

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

@@ -0,0 +1,8 @@
+error: unsized arguments must not be `extern` types
+  --> $DIR/extern-parameter-issue-123887-generic-wrapped.rs:13:5
+   |
+LL |     f(*x);
+   |     ^^^^^
+
+error: aborting due to 1 previous error
+

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

@@ -0,0 +1,15 @@
+// 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(_: impl ?Sized) {}
+fn g(x: Box<ExternType>) {
+    f(*x); //~ ERROR unsized arguments must not be `extern` types
+}
+
+fn main() {}

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

@@ -0,0 +1,8 @@
+error: unsized arguments must not be `extern` types
+  --> $DIR/extern-parameter-issue-123887-generic.rs:12:5
+   |
+LL |     f(*x);
+   |     ^^^^^
+
+error: aborting due to 1 previous error
+

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

@@ -0,0 +1,13 @@
+// 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;
+}
+
+struct Wrapper<T: ?Sized>(T);
+fn f(_: Wrapper<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-wrapped.stderr

@@ -0,0 +1,20 @@
+error[E0277]: the size for values of type `ExternType` cannot be known at compilation time
+  --> $DIR/extern-parameter-issue-123887-wrapped.rs:11:6
+   |
+LL | fn f(_: Wrapper<ExternType>) {}
+   |      ^ doesn't have a size known at compile-time
+   |
+   = help: within `Wrapper<ExternType>`, the trait `Sized` is not implemented for `ExternType`, which is required by `Wrapper<ExternType>: Sized`
+note: required because it appears within the type `Wrapper<ExternType>`
+  --> $DIR/extern-parameter-issue-123887-wrapped.rs:10:8
+   |
+LL | struct Wrapper<T: ?Sized>(T);
+   |        ^^^^^^^
+help: function arguments must have a statically known size, borrowed types always have a known size
+   |
+LL | fn f(_: &Wrapper<ExternType>) {}
+   |         +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

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`.