Revert #131669 due to ICEs

Revert <#131669> due to ICE
reports:

- <#134059> (real-world)
- <#134060> (fuzzing)

The changes can be re-landed with those cases addressed.

This reverts commit 703bb98, reversing
changes made to f415c07.

Author: jieyouxu

Diff for: compiler/rustc_lint/messages.ftl

@@ -359,6 +359,7 @@ lint_improper_ctypes_128bit = 128-bit integers don't currently have a known stab
 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
+lint_improper_ctypes_box = box cannot be represented as a single pointer
 
 lint_improper_ctypes_char_help = consider using `u32` or `libc::wchar_t` instead
 
@@ -376,9 +377,7 @@ lint_improper_ctypes_enum_repr_help =
 lint_improper_ctypes_enum_repr_reason = enum has no representation hint
 lint_improper_ctypes_fnptr_help = consider using an `extern fn(...) -> ...` function pointer instead
 
-lint_improper_ctypes_fnptr_indirect_reason = the function pointer to `{$ty}` is FFI-unsafe due to `{$inner_ty}`
 lint_improper_ctypes_fnptr_reason = this function pointer has Rust-specific calling convention
-
 lint_improper_ctypes_non_exhaustive = this enum is non-exhaustive
 lint_improper_ctypes_non_exhaustive_variant = this enum has non-exhaustive variants
 
@@ -389,11 +388,7 @@ lint_improper_ctypes_opaque = opaque types have no C equivalent
 lint_improper_ctypes_pat_help = consider using the base type instead
 
 lint_improper_ctypes_pat_reason = pattern types have no C equivalent
-
-lint_improper_ctypes_sized_ptr_to_unsafe_type =
-    this reference (`{$ty}`) is ABI-compatible with a C pointer, but `{$inner_ty}` itself does not have a C layout
-
-lint_improper_ctypes_slice_help = consider using a raw pointer to the slice's first element (and a length) instead
+lint_improper_ctypes_slice_help = consider using a raw pointer instead
 
 lint_improper_ctypes_slice_reason = slices have no C equivalent
 lint_improper_ctypes_str_help = consider using `*const u8` and a length instead
@@ -419,10 +414,6 @@ 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_unsized_box = this box for an unsized type contains metadata, which makes it incompatible with a C pointer
-lint_improper_ctypes_unsized_ptr = this pointer to an unsized type contains metadata, which makes it incompatible with a C pointer
-lint_improper_ctypes_unsized_ref = this reference to an unsized type contains metadata, which makes it incompatible with a C pointer
-
 lint_incomplete_include =
     include macro expected single expression in source
 

Diff for: compiler/rustc_lint/src/lints.rs

@@ -1851,44 +1851,13 @@ pub(crate) struct UnpredictableFunctionPointerComparisonsSuggestion<'a> {
     pub right: Span,
 }
 
-pub(crate) struct ImproperCTypesLayer<'a> {
-    pub ty: Ty<'a>,
-    pub inner_ty: Option<Ty<'a>>,
-    pub note: DiagMessage,
-    pub span_note: Option<Span>,
-    pub help: Option<DiagMessage>,
-}
-
-impl<'a> Subdiagnostic for ImproperCTypesLayer<'a> {
-    fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
-        self,
-        diag: &mut Diag<'_, G>,
-        f: &F,
-    ) {
-        diag.arg("ty", self.ty);
-        if let Some(ty) = self.inner_ty {
-            diag.arg("inner_ty", ty);
-        }
-
-        if let Some(help) = self.help {
-            let msg = f(diag, help.into());
-            diag.help(msg);
-        }
-
-        let msg = f(diag, self.note.into());
-        diag.note(msg);
-        if let Some(note) = self.span_note {
-            let msg = f(diag, fluent::lint_note.into());
-            diag.span_note(note, msg);
-        };
-    }
-}
-
 pub(crate) struct ImproperCTypes<'a> {
     pub ty: Ty<'a>,
     pub desc: &'a str,
     pub label: Span,
-    pub reasons: Vec<ImproperCTypesLayer<'a>>,
+    pub help: Option<DiagMessage>,
+    pub note: DiagMessage,
+    pub span_note: Option<Span>,
 }
 
 // Used because of the complexity of Option<DiagMessage>, DiagMessage, and Option<Span>
@@ -1898,8 +1867,12 @@ impl<'a> LintDiagnostic<'a, ()> for ImproperCTypes<'_> {
         diag.arg("ty", self.ty);
         diag.arg("desc", self.desc);
         diag.span_label(self.label, fluent::lint_label);
-        for reason in self.reasons.into_iter() {
-            diag.subdiagnostic(reason);
+        if let Some(help) = self.help {
+            diag.help(help);
+        }
+        diag.note(self.note);
+        if let Some(note) = self.span_note {
+            diag.span_note(note, fluent::lint_note);
         }
     }
 }