lint: treat unsafe binders in improper_ctypes instead of ICE

This commit replaces the previous task for unsafe_binders with a proper
diagnostic updates the wording for consistency and switches the message to the
fluent error system.

The diagnostic now reports the full unsafe binder type unsafe<a> &a ()
instead of just the inner &() matching the highlighted span. the ui test
stderr has been updated accordingly.

Author: SATVIKsynopsis

compiler/rustc_lint/messages.ftl

@@ -360,7 +360,6 @@ lint_implicit_unsafe_autorefs = implicit autoref creates a reference to the dere
 lint_improper_ctypes = `extern` {$desc} uses type `{$ty}`, which is not FFI-safe
     .label = not FFI-safe
     .note = the type is defined here
-
 lint_improper_ctypes_array_help = consider passing a pointer to the array
 
 lint_improper_ctypes_array_reason = passing raw arrays by value is not FFI-safe
@@ -416,6 +415,8 @@ lint_improper_ctypes_union_layout_help = consider adding a `#[repr(C)]` or `#[re
 lint_improper_ctypes_union_layout_reason = this union has unspecified layout
 lint_improper_ctypes_union_non_exhaustive = this union is non-exhaustive
 
+lint_improper_ctypes_unsafe_binder = unsafe binders are incompatible with foreign function interfaces
+
 lint_int_to_ptr_transmutes = transmuting an integer to a pointer creates a pointer without provenance
     .note = this is dangerous because dereferencing the resulting pointer is undefined behavior
     .note_exposed_provenance = exposed provenance semantics can be used to create a pointer based on some previously exposed provenance

compiler/rustc_lint/src/types/improper_ctypes.rs

@@ -669,15 +669,8 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 FfiSafe
             }
 
-            ty::UnsafeBinder(binder) => {
-                let ty = binder.skip_binder(); // extract the inner type
-
-                FfiUnsafe {
-                    ty,
-                    reason: "types containing `unsafe` binders are not yet fully supported in FFI"
-                        .into(),
-                    help: None,
-                }
+            ty::UnsafeBinder(_binder) => {
+                FfiUnsafe { ty, reason: fluent::lint_improper_ctypes_unsafe_binder, help: None }
             }
 
             ty::Param(..)

tests/ui/lint/improper-ctypes/unsafe-binder-basic.rs

@@ -4,7 +4,7 @@
 
 extern "C" {
     fn exit_2(x: unsafe<'a> &'a ());
-    //~^ ERROR `extern` block uses type `&()`, which is not FFI-safe
+    //~^ ERROR `extern` block uses type `unsafe<'a> &'a ()`, which is not FFI-safe
 }
 
 fn main() {}

tests/ui/lint/improper-ctypes/unsafe-binder-basic.stderr

@@ -1,14 +1,15 @@
-error: `extern` block uses type `&()`, which is not FFI-safe
+error: `extern` block uses type `unsafe<'a> &'a ()`, which is not FFI-safe
   --> $DIR/unsafe-binder-basic.rs:6:18
    |
 LL |     fn exit_2(x: unsafe<'a> &'a ());
    |                  ^^^^^^^^^^^^^^^^^ not FFI-safe
    |
-   = note: types containing `unsafe` binders are not yet fully supported in FFI
+   = note: unsafe binders are incompatible with foreign function interfaces
 note: the lint level is defined here
   --> $DIR/unsafe-binder-basic.rs:3:9
    |
 LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
+