Auto merge of #73990 - jumbatm:clashing-extern-decl, r=nagisa

Fix incorrect clashing_extern_declarations warnings.

Fixes #73735, fixes #73872.

Fix clashing_extern_declarations warning for `#[repr(transparent)]` structs and safely-FFI-convertible enums, and not warning for clashes of struct members of different types, but the same size.

r? @nagisa

6 files changed, 456 insertions, 161 deletions

diff --git a/src/librustc_lint/builtin.rs b/src/librustc_lint/builtin.rs
index a45817beea1..06e7c2b6f36 100644
--- a/src/librustc_lint/builtin.rs
+++ b/src/librustc_lint/builtin.rs
@@ -20,7 +20,9 @@
 //! If you define a new `LateLintPass`, you will also need to add it to the
 //! `late_lint_methods!` invocation in `lib.rs`.
 
-use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
+use crate::{
+    types::CItemKind, EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext,
+};
 use rustc_ast::ast::{self, Expr};
 use rustc_ast::attr::{self, HasAttrs};
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
@@ -36,14 +38,14 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::{ForeignItemKind, GenericParamKind, PatKind};
 use rustc_hir::{HirId, HirIdSet, Node};
 use rustc_middle::lint::LintDiagnosticBuilder;
-use rustc_middle::ty::subst::GenericArgKind;
+use rustc_middle::ty::subst::{GenericArgKind, Subst};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_session::lint::FutureIncompatibleInfo;
 use rustc_span::edition::Edition;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, Span};
-use rustc_target::abi::VariantIdx;
+use rustc_target::abi::{LayoutOf, VariantIdx};
 use rustc_trait_selection::traits::misc::can_type_implement_copy;
 
 use crate::nonstandard_style::{method_context, MethodLateContext};
@@ -2144,7 +2146,13 @@ impl ClashingExternDeclarations {
     /// Checks whether two types are structurally the same enough that the declarations shouldn't
     /// clash. We need this so we don't emit a lint when two modules both declare an extern struct,
     /// with the same members (as the declarations shouldn't clash).
-    fn structurally_same_type<'tcx>(cx: &LateContext<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
+    fn structurally_same_type<'tcx>(
+        cx: &LateContext<'tcx>,
+        a: Ty<'tcx>,
+        b: Ty<'tcx>,
+        ckind: CItemKind,
+    ) -> bool {
+        debug!("structurally_same_type(cx, a = {:?}, b = {:?})", a, b);
         let tcx = cx.tcx;
         if a == b || rustc_middle::ty::TyS::same_type(a, b) {
             // All nominally-same types are structurally same, too.
@@ -2155,47 +2163,77 @@ impl ClashingExternDeclarations {
             let a_kind = &a.kind;
             let b_kind = &b.kind;
 
+            let compare_layouts = |a, b| -> bool {
+                let a_layout = &cx.layout_of(a).unwrap().layout.abi;
+                let b_layout = &cx.layout_of(b).unwrap().layout.abi;
+                debug!("{:?} == {:?} = {}", a_layout, b_layout, a_layout == b_layout);
+                a_layout == b_layout
+            };
+
+            #[allow(rustc::usage_of_ty_tykind)]
+            let is_primitive_or_pointer =
+                |kind: &ty::TyKind<'_>| kind.is_primitive() || matches!(kind, RawPtr(..));
+
             match (a_kind, b_kind) {
-                (Adt(..), Adt(..)) => {
-                    // Adts are pretty straightforward: just compare the layouts.
-                    use rustc_target::abi::LayoutOf;
-                    let a_layout = cx.layout_of(a).unwrap().layout;
-                    let b_layout = cx.layout_of(b).unwrap().layout;
-                    a_layout == b_layout
+                (Adt(_, a_substs), Adt(_, b_substs)) => {
+                    let a = a.subst(cx.tcx, a_substs);
+                    let b = b.subst(cx.tcx, b_substs);
+                    debug!("Comparing {:?} and {:?}", a, b);
+
+                    if let (Adt(a_def, ..), Adt(b_def, ..)) = (&a.kind, &b.kind) {
+                        // Grab a flattened representation of all fields.
+                        let a_fields = a_def.variants.iter().flat_map(|v| v.fields.iter());
+                        let b_fields = b_def.variants.iter().flat_map(|v| v.fields.iter());
+                        compare_layouts(a, b)
+                            && a_fields.eq_by(
+                                b_fields,
+                                |&ty::FieldDef { did: a_did, .. },
+                                 &ty::FieldDef { did: b_did, .. }| {
+                                    Self::structurally_same_type(
+                                        cx,
+                                        tcx.type_of(a_did),
+                                        tcx.type_of(b_did),
+                                        ckind,
+                                    )
+                                },
+                            )
+                    } else {
+                        unreachable!()
+                    }
                 }
                 (Array(a_ty, a_const), Array(b_ty, b_const)) => {
                     // For arrays, we also check the constness of the type.
                     a_const.val == b_const.val
-                        && Self::structurally_same_type(cx, a_const.ty, b_const.ty)
-                        && Self::structurally_same_type(cx, a_ty, b_ty)
+                        && Self::structurally_same_type(cx, a_const.ty, b_const.ty, ckind)
+                        && Self::structurally_same_type(cx, a_ty, b_ty, ckind)
                 }
-                (Slice(a_ty), Slice(b_ty)) => Self::structurally_same_type(cx, a_ty, b_ty),
+                (Slice(a_ty), Slice(b_ty)) => Self::structurally_same_type(cx, a_ty, b_ty, ckind),
                 (RawPtr(a_tymut), RawPtr(b_tymut)) => {
                     a_tymut.mutbl == a_tymut.mutbl
-                        && Self::structurally_same_type(cx, &a_tymut.ty, &b_tymut.ty)
+                        && Self::structurally_same_type(cx, &a_tymut.ty, &b_tymut.ty, ckind)
                 }
                 (Ref(_a_region, a_ty, a_mut), Ref(_b_region, b_ty, b_mut)) => {
                     // For structural sameness, we don't need the region to be same.
-                    a_mut == b_mut && Self::structurally_same_type(cx, a_ty, b_ty)
+                    a_mut == b_mut && Self::structurally_same_type(cx, a_ty, b_ty, ckind)
                 }
                 (FnDef(..), FnDef(..)) => {
-                    // As we don't compare regions, skip_binder is fine.
                     let a_poly_sig = a.fn_sig(tcx);
                     let b_poly_sig = b.fn_sig(tcx);
 
+                    // As we don't compare regions, skip_binder is fine.
                     let a_sig = a_poly_sig.skip_binder();
                     let b_sig = b_poly_sig.skip_binder();
 
                     (a_sig.abi, a_sig.unsafety, a_sig.c_variadic)
                         == (b_sig.abi, b_sig.unsafety, b_sig.c_variadic)
                         && a_sig.inputs().iter().eq_by(b_sig.inputs().iter(), |a, b| {
-                            Self::structurally_same_type(cx, a, b)
+                            Self::structurally_same_type(cx, a, b, ckind)
                         })
-                        && Self::structurally_same_type(cx, a_sig.output(), b_sig.output())
+                        && Self::structurally_same_type(cx, a_sig.output(), b_sig.output(), ckind)
                 }
                 (Tuple(a_substs), Tuple(b_substs)) => {
                     a_substs.types().eq_by(b_substs.types(), |a_ty, b_ty| {
-                        Self::structurally_same_type(cx, a_ty, b_ty)
+                        Self::structurally_same_type(cx, a_ty, b_ty, ckind)
                     })
                 }
                 // For these, it's not quite as easy to define structural-sameness quite so easily.
@@ -2208,9 +2246,27 @@ impl ClashingExternDeclarations {
                 | (GeneratorWitness(..), GeneratorWitness(..))
                 | (Projection(..), Projection(..))
                 | (Opaque(..), Opaque(..)) => false,
+
                 // These definitely should have been caught above.
                 (Bool, Bool) | (Char, Char) | (Never, Never) | (Str, Str) => unreachable!(),
-                _ => false,
+
+                // An Adt and a primitive type. This can be FFI-safe is the ADT is an enum with a
+                // non-null field.
+                (Adt(..), other_kind) | (other_kind, Adt(..))
+                    if is_primitive_or_pointer(other_kind) =>
+                {
+                    let (primitive, adt) =
+                        if is_primitive_or_pointer(&a.kind) { (a, b) } else { (b, a) };
+                    if let Some(ty) = crate::types::repr_nullable_ptr(cx, adt, ckind) {
+                        ty == primitive
+                    } else {
+                        compare_layouts(a, b)
+                    }
+                }
+                // Otherwise, just compare the layouts. This may fail to lint for some
+                // incompatible types, but at the very least, will stop reads into
+                // uninitialised memory.
+                _ => compare_layouts(a, b),
             }
         }
     }
@@ -2231,7 +2287,12 @@ impl<'tcx> LateLintPass<'tcx> for ClashingExternDeclarations {
                     existing_hid, existing_decl_ty, this_fi.hir_id, this_decl_ty
                 );
                 // Check that the declarations match.
-                if !Self::structurally_same_type(cx, existing_decl_ty, this_decl_ty) {
+                if !Self::structurally_same_type(
+                    cx,
+                    existing_decl_ty,
+                    this_decl_ty,
+                    CItemKind::Declaration,
+                ) {
                     let orig_fi = tcx.hir().expect_foreign_item(existing_hid);
                     let orig = Self::name_of_extern_decl(tcx, orig_fi);
 
diff --git a/src/librustc_lint/types.rs b/src/librustc_lint/types.rs
index 1affc9457b8..de750010ed1 100644
--- a/src/librustc_lint/types.rs
+++ b/src/librustc_lint/types.rs
@@ -15,8 +15,9 @@ use rustc_middle::ty::{self, AdtKind, Ty, TypeFoldable};
 use rustc_span::source_map;
 use rustc_span::symbol::sym;
 use rustc_span::{Span, DUMMY_SP};
+use rustc_target::abi::Abi;
 use rustc_target::abi::{Integer, LayoutOf, TagEncoding, VariantIdx, Variants};
-use rustc_target::spec::abi::Abi;
+use rustc_target::spec::abi::Abi as SpecAbi;
 
 use log::debug;
 use std::cmp;
@@ -509,14 +510,15 @@ declare_lint! {
 
 declare_lint_pass!(ImproperCTypesDefinitions => [IMPROPER_CTYPES_DEFINITIONS]);
 
-enum ImproperCTypesMode {
-    Declarations,
-    Definitions,
+#[derive(Clone, Copy)]
+crate enum CItemKind {
+    Declaration,
+    Definition,
 }
 
 struct ImproperCTypesVisitor<'a, 'tcx> {
     cx: &'a LateContext<'tcx>,
-    mode: ImproperCTypesMode,
+    mode: CItemKind,
 }
 
 enum FfiResult<'tcx> {
@@ -525,54 +527,94 @@ enum FfiResult<'tcx> {
     FfiUnsafe { ty: Ty<'tcx>, reason: String, help: Option<String> },
 }
 
-impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
-    /// Is type known to be non-null?
-    fn ty_is_known_nonnull(&self, ty: Ty<'tcx>) -> bool {
-        match ty.kind {
-            ty::FnPtr(_) => true,
-            ty::Ref(..) => true,
-            ty::Adt(def, _)
-                if def.is_box() && matches!(self.mode, ImproperCTypesMode::Definitions) =>
-            {
-                true
-            }
-            ty::Adt(def, substs) if def.repr.transparent() && !def.is_union() => {
-                let guaranteed_nonnull_optimization = self
-                    .cx
-                    .tcx
-                    .get_attrs(def.did)
-                    .iter()
-                    .any(|a| a.check_name(sym::rustc_nonnull_optimization_guaranteed));
-
-                if guaranteed_nonnull_optimization {
-                    return true;
-                }
+/// Is type known to be non-null?
+fn ty_is_known_nonnull<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, mode: CItemKind) -> bool {
+    let tcx = cx.tcx;
+    match ty.kind {
+        ty::FnPtr(_) => true,
+        ty::Ref(..) => true,
+        ty::Adt(def, _) if def.is_box() && matches!(mode, CItemKind::Definition) => true,
+        ty::Adt(def, substs) if def.repr.transparent() && !def.is_union() => {
+            let guaranteed_nonnull_optimization = tcx
+                .get_attrs(def.did)
+                .iter()
+                .any(|a| a.check_name(sym::rustc_nonnull_optimization_guaranteed));
 
-                for variant in &def.variants {
-                    if let Some(field) = variant.transparent_newtype_field(self.cx.tcx) {
-                        if self.ty_is_known_nonnull(field.ty(self.cx.tcx, substs)) {
-                            return true;
-                        }
+            if guaranteed_nonnull_optimization {
+                return true;
+            }
+            for variant in &def.variants {
+                if let Some(field) = variant.transparent_newtype_field(tcx) {
+                    if ty_is_known_nonnull(cx, field.ty(tcx, substs), mode) {
+                        return true;
                     }
                 }
-
-                false
             }
-            _ => false,
+
+            false
         }
+        _ => false,
     }
+}
+/// Given a non-null scalar (or transparent) type `ty`, return the nullable version of that type.
+/// If the type passed in was not scalar, returns None.
+fn get_nullable_type<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
+    let tcx = cx.tcx;
+    Some(match ty.kind {
+        ty::Adt(field_def, field_substs) => {
+            let inner_field_ty = {
+                let first_non_zst_ty =
+                    field_def.variants.iter().filter_map(|v| v.transparent_newtype_field(tcx));
+                debug_assert_eq!(
+                    first_non_zst_ty.clone().count(),
+                    1,
+                    "Wrong number of fields for transparent type"
+                );
+                first_non_zst_ty
+                    .last()
+                    .expect("No non-zst fields in transparent type.")
+                    .ty(tcx, field_substs)
+            };
+            return get_nullable_type(cx, inner_field_ty);
+        }
+        ty::Int(ty) => tcx.mk_mach_int(ty),
+        ty::Uint(ty) => tcx.mk_mach_uint(ty),
+        ty::RawPtr(ty_mut) => tcx.mk_ptr(ty_mut),
+        // As these types are always non-null, the nullable equivalent of
+        // Option<T> of these types are their raw pointer counterparts.
+        ty::Ref(_region, ty, mutbl) => tcx.mk_ptr(ty::TypeAndMut { ty, mutbl }),
+        ty::FnPtr(..) => {
+            // There is no nullable equivalent for Rust's function pointers -- you
+            // must use an Option<fn(..) -> _> to represent it.
+            ty
+        }
 
-    /// Check if this enum can be safely exported based on the "nullable pointer optimization".
-    /// Currently restricted to function pointers, boxes, references, `core::num::NonZero*`,
-    /// `core::ptr::NonNull`, and `#[repr(transparent)]` newtypes.
-    fn is_repr_nullable_ptr(
-        &self,
-        ty: Ty<'tcx>,
-        ty_def: &'tcx ty::AdtDef,
-        substs: SubstsRef<'tcx>,
-    ) -> bool {
+        // We should only ever reach this case if ty_is_known_nonnull is extended
+        // to other types.
+        ref unhandled => {
+            debug!(
+                "get_nullable_type: Unhandled scalar kind: {:?} while checking {:?}",
+                unhandled, ty
+            );
+            return None;
+        }
+    })
+}
+
+/// Check if this enum can be safely exported based on the "nullable pointer optimization". If it
+/// can, return the the type that `ty` can be safely converted to, otherwise return `None`.
+/// Currently restricted to function pointers, boxes, references, `core::num::NonZero*`,
+/// `core::ptr::NonNull`, and `#[repr(transparent)]` newtypes.
+/// FIXME: This duplicates code in codegen.
+crate fn repr_nullable_ptr<'tcx>(
+    cx: &LateContext<'tcx>,
+    ty: Ty<'tcx>,
+    ckind: CItemKind,
+) -> Option<Ty<'tcx>> {
+    debug!("is_repr_nullable_ptr(cx, ty = {:?})", ty);
+    if let ty::Adt(ty_def, substs) = ty.kind {
         if ty_def.variants.len() != 2 {
-            return false;
+            return None;
         }
 
         let get_variant_fields = |index| &ty_def.variants[VariantIdx::new(index)].fields;
@@ -582,30 +624,42 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         } else if variant_fields[1].is_empty() {
             &variant_fields[0]
         } else {
-            return false;
+            return None;
         };
 
         if fields.len() != 1 {
-            return false;
+            return None;
         }
 
-        let field_ty = fields[0].ty(self.cx.tcx, substs);
-        if !self.ty_is_known_nonnull(field_ty) {
-            return false;
+        let field_ty = fields[0].ty(cx.tcx, substs);
+        if !ty_is_known_nonnull(cx, field_ty, ckind) {
+            return None;
         }
 
-        // At this point, the field's type is known to be nonnull and the parent enum is
-        // Option-like. If the computed size for the field and the enum are different, the non-null
-        // optimization isn't being applied (and we've got a problem somewhere).
-        let compute_size_skeleton =
-            |t| SizeSkeleton::compute(t, self.cx.tcx, self.cx.param_env).unwrap();
+        // At this point, the field's type is known to be nonnull and the parent enum is Option-like.
+        // If the computed size for the field and the enum are different, the nonnull optimization isn't
+        // being applied (and we've got a problem somewhere).
+        let compute_size_skeleton = |t| SizeSkeleton::compute(t, cx.tcx, cx.param_env).unwrap();
         if !compute_size_skeleton(ty).same_size(compute_size_skeleton(field_ty)) {
             bug!("improper_ctypes: Option nonnull optimization not applied?");
         }
 
-        true
+        // Return the nullable type this Option-like enum can be safely represented with.
+        let field_ty_abi = &cx.layout_of(field_ty).unwrap().abi;
+        if let Abi::Scalar(field_ty_scalar) = field_ty_abi {
+            match (field_ty_scalar.valid_range.start(), field_ty_scalar.valid_range.end()) {
+                (0, _) => unreachable!("Non-null optimisation extended to a non-zero value."),
+                (1, _) => {
+                    return Some(get_nullable_type(cx, field_ty).unwrap());
+                }
+                (start, end) => unreachable!("Unhandled start and end range: ({}, {})", start, end),
+            };
+        }
     }
+    None
+}
 
+impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
     /// Check if the type is array and emit an unsafe type lint.
     fn check_for_array_ty(&mut self, sp: Span, ty: Ty<'tcx>) -> bool {
         if let ty::Array(..) = ty.kind {
@@ -686,7 +740,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
     fn check_type_for_ffi(&self, cache: &mut FxHashSet<Ty<'tcx>>, ty: Ty<'tcx>) -> FfiResult<'tcx> {
         use FfiResult::*;
 
-        let cx = self.cx.tcx;
+        let tcx = self.cx.tcx;
 
         // Protect against infinite recursion, for example
         // `struct S(*mut S);`.
@@ -697,9 +751,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         }
 
         match ty.kind {
-            ty::Adt(def, _)
-                if def.is_box() && matches!(self.mode, ImproperCTypesMode::Definitions) =>
-            {
+            ty::Adt(def, _) if def.is_box() && matches!(self.mode, CItemKind::Definition) => {
                 FfiSafe
             }
 
@@ -753,7 +805,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                         // discriminant.
                         if !def.repr.c() && !def.repr.transparent() && def.repr.int.is_none() {
                             // Special-case types like `Option<extern fn()>`.
-                            if !self.is_repr_nullable_ptr(ty, def, substs) {
+                            if repr_nullable_ptr(self.cx, ty, self.mode).is_none() {
                                 return FfiUnsafe {
                                     ty,
                                     reason: "enum has no representation hint".into(),
@@ -836,7 +888,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
             ty::RawPtr(ty::TypeAndMut { ty, .. }) | ty::Ref(_, ty, _)
                 if {
-                    matches!(self.mode, ImproperCTypesMode::Definitions)
+                    matches!(self.mode, CItemKind::Definition)
                         && ty.is_sized(self.cx.tcx.at(DUMMY_SP), self.cx.param_env)
                 } =>
             {
@@ -862,7 +914,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                     };
                 }
 
-                let sig = cx.erase_late_bound_regions(&sig);
+                let sig = tcx.erase_late_bound_regions(&sig);
                 if !sig.output().is_unit() {
                     let r = self.check_type_for_ffi(cache, sig.output());
                     match r {
@@ -894,9 +946,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
             // `extern "C" fn` functions can have type parameters, which may or may not be FFI-safe,
             //  so they are currently ignored for the purposes of this lint.
-            ty::Param(..) | ty::Projection(..)
-                if matches!(self.mode, ImproperCTypesMode::Definitions) =>
-            {
+            ty::Param(..) | ty::Projection(..) if matches!(self.mode, CItemKind::Definition) => {
                 FfiSafe
             }
 
@@ -921,14 +971,14 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         help: Option<&str>,
     ) {
         let lint = match self.mode {
-            ImproperCTypesMode::Declarations => IMPROPER_CTYPES,
-            ImproperCTypesMode::Definitions => IMPROPER_CTYPES_DEFINITIONS,
+            CItemKind::Declaration => IMPROPER_CTYPES,
+            CItemKind::Definition => IMPROPER_CTYPES_DEFINITIONS,
         };
 
         self.cx.struct_span_lint(lint, sp, |lint| {
             let item_description = match self.mode {
-                ImproperCTypesMode::Declarations => "block",
-                ImproperCTypesMode::Definitions => "fn",
+                CItemKind::Declaration => "block",
+                CItemKind::Definition => "fn",
             };
             let mut diag = lint.build(&format!(
                 "`extern` {} uses type `{}`, which is not FFI-safe",
@@ -1052,8 +1102,12 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         self.check_type_for_ffi_and_report_errors(span, ty, true, false);
     }
 
-    fn is_internal_abi(&self, abi: Abi) -> bool {
-        if let Abi::Rust | Abi::RustCall | Abi::RustIntrinsic | Abi::PlatformIntrinsic = abi {
+    fn is_internal_abi(&self, abi: SpecAbi) -> bool {
+        if let SpecAbi::Rust
+        | SpecAbi::RustCall
+        | SpecAbi::RustIntrinsic
+        | SpecAbi::PlatformIntrinsic = abi
+        {
             true
         } else {
             false
@@ -1063,7 +1117,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
 impl<'tcx> LateLintPass<'tcx> for ImproperCTypesDeclarations {
     fn check_foreign_item(&mut self, cx: &LateContext<'_>, it: &hir::ForeignItem<'_>) {
-        let mut vis = ImproperCTypesVisitor { cx, mode: ImproperCTypesMode::Declarations };
+        let mut vis = ImproperCTypesVisitor { cx, mode: CItemKind::Declaration };
         let abi = cx.tcx.hir().get_foreign_abi(it.hir_id);
 
         if !vis.is_internal_abi(abi) {
@@ -1098,7 +1152,7 @@ impl<'tcx> LateLintPass<'tcx> for ImproperCTypesDefinitions {
             _ => return,
         };
 
-        let mut vis = ImproperCTypesVisitor { cx, mode: ImproperCTypesMode::Definitions };
+        let mut vis = ImproperCTypesVisitor { cx, mode: CItemKind::Definition };
         if !vis.is_internal_abi(abi) {
             vis.check_foreign_fn(hir_id, decl);
         }
diff --git a/src/librustc_middle/ty/sty.rs b/src/librustc_middle/ty/sty.rs
index df8fa4d73dd..310ab4f7235 100644
--- a/src/librustc_middle/ty/sty.rs
+++ b/src/librustc_middle/ty/sty.rs
@@ -202,6 +202,16 @@ pub enum TyKind<'tcx> {
     Error(DelaySpanBugEmitted),
 }
 
+impl TyKind<'tcx> {
+    #[inline]
+    pub fn is_primitive(&self) -> bool {
+        match self {
+            Bool | Char | Int(_) | Uint(_) | Float(_) => true,
+            _ => false,
+        }
+    }
+}
+
 /// A type that is not publicly constructable. This prevents people from making `TyKind::Error`
 /// except through `tcx.err*()`.
 #[derive(Copy, Clone, Debug, Eq, Hash, PartialEq, PartialOrd, Ord)]
@@ -1766,10 +1776,7 @@ impl<'tcx> TyS<'tcx> {
 
     #[inline]
     pub fn is_primitive(&self) -> bool {
-        match self.kind {
-            Bool | Char | Int(_) | Uint(_) | Float(_) => true,
-            _ => false,
-        }
+        self.kind.is_primitive()
     }
 
     #[inline]
diff --git a/src/test/ui/lint/auxiliary/external_extern_fn.rs b/src/test/ui/lint/auxiliary/external_extern_fn.rs
index b2caebc6fee..c2a8cadc6b5 100644
--- a/src/test/ui/lint/auxiliary/external_extern_fn.rs
+++ b/src/test/ui/lint/auxiliary/external_extern_fn.rs
@@ -1,3 +1,3 @@
-extern {
+extern "C" {
     pub fn extern_fn(x: u8);
 }
diff --git a/src/test/ui/lint/clashing-extern-fn.rs b/src/test/ui/lint/clashing-extern-fn.rs
index 544614100ba..d6ac7ccccc7 100644
--- a/src/test/ui/lint/clashing-extern-fn.rs
+++ b/src/test/ui/lint/clashing-extern-fn.rs
@@ -3,52 +3,40 @@
 #![crate_type = "lib"]
 #![warn(clashing_extern_declarations)]
 
-extern crate external_extern_fn;
-
-extern "C" {
-    fn clash(x: u8);
-    fn no_clash(x: u8);
-}
-
-fn redeclared_different_signature() {
-    extern "C" {
-        fn clash(x: u64); //~ WARN `clash` redeclared with a different signature
+mod redeclared_different_signature {
+    mod a {
+        extern "C" {
+            fn clash(x: u8);
+        }
     }
-
-    unsafe {
-        clash(123);
-        no_clash(123);
+    mod b {
+        extern "C" {
+            fn clash(x: u64); //~ WARN `clash` redeclared with a different signature
+        }
     }
 }
 
-fn redeclared_same_signature() {
-    extern "C" {
-        fn no_clash(x: u8);
+mod redeclared_same_signature {
+    mod a {
+        extern "C" {
+            fn no_clash(x: u8);
+        }
     }
-    unsafe {
-        no_clash(123);
+    mod b {
+        extern "C" {
+            fn no_clash(x: u8);
+        }
     }
 }
 
-extern "C" {
-    fn extern_fn(x: u64);
-}
-
-fn extern_clash() {
+extern crate external_extern_fn;
+mod extern_no_clash {
+    // Should not clash with external_extern_fn::extern_fn.
     extern "C" {
-        fn extern_fn(x: u32); //~ WARN `extern_fn` redeclared with a different signature
-    }
-    unsafe {
-        extern_fn(123);
+        fn extern_fn(x: u8);
     }
 }
 
-fn extern_no_clash() {
-    unsafe {
-        external_extern_fn::extern_fn(123);
-        crate::extern_fn(123);
-    }
-}
 extern "C" {
     fn some_other_new_name(x: i16);
 
@@ -134,9 +122,9 @@ mod banana {
             weight: u32,
             length: u16,
         } // note: distinct type
-        extern "C" {
           // This should not trigger the lint because two::Banana is structurally equivalent to
           // one::Banana.
+        extern "C" {
             fn weigh_banana(count: *const Banana) -> u64;
         }
     }
@@ -180,7 +168,120 @@ mod sameish_members {
         // always be the case, for every architecture and situation. This is also a really odd
         // thing to do anyway.
         extern "C" {
-            fn draw_point(p: Point); //~ WARN `draw_point` redeclared with a different
+            fn draw_point(p: Point);
+            //~^ WARN `draw_point` redeclared with a different signature
+        }
+    }
+}
+
+mod same_sized_members_clash {
+    mod a {
+        #[repr(C)]
+        struct Point3 {
+            x: f32,
+            y: f32,
+            z: f32,
+        }
+        extern "C" { fn origin() -> Point3; }
+    }
+    mod b {
+        #[repr(C)]
+        struct Point3 {
+            x: i32,
+            y: i32,
+            z: i32, // NOTE: Incorrectly redeclared as i32
+        }
+        extern "C" { fn origin() -> Point3; }
+        //~^ WARN `origin` redeclared with a different signature
+    }
+}
+
+mod transparent {
+    #[repr(transparent)]
+    struct T(usize);
+    mod a {
+        use super::T;
+        extern "C" {
+            fn transparent() -> T;
+            fn transparent_incorrect() -> T;
+        }
+    }
+
+    mod b {
+        extern "C" {
+            // Shouldn't warn here, because repr(transparent) guarantees that T's layout is the
+            // same as just the usize.
+            fn transparent() -> usize;
+
+            // Should warn, because there's a signedness conversion here:
+            fn transparent_incorrect() -> isize;
+            //~^ WARN `transparent_incorrect` redeclared with a different signature
+        }
+    }
+}
+
+mod missing_return_type {
+    mod a {
+        extern "C" {
+            fn missing_return_type() -> usize;
+        }
+    }
+
+    mod b {
+        extern "C" {
+            // This should output a warning because we can't assume that the first declaration is
+            // the correct one -- if this one is the correct one, then calling the usize-returning
+            // version would allow reads into uninitialised memory.
+            fn missing_return_type();
+            //~^ WARN `missing_return_type` redeclared with a different signature
+        }
+    }
+}
+
+mod non_zero_and_non_null {
+    mod a {
+        extern "C" {
+            fn non_zero_usize() -> core::num::NonZeroUsize;
+            fn non_null_ptr() -> core::ptr::NonNull<usize>;
+        }
+    }
+    mod b {
+        extern "C" {
+            // If there's a clash in either of these cases you're either gaining an incorrect
+            // invariant that the value is non-zero, or you're missing out on that invariant. Both
+            // cases are warning for, from both a caller-convenience and optimisation perspective.
+            fn non_zero_usize() -> usize;
+            //~^ WARN `non_zero_usize` redeclared with a different signature
+            fn non_null_ptr() -> *const usize;
+            //~^ WARN `non_null_ptr` redeclared with a different signature
+        }
+    }
+}
+
+mod null_optimised_enums {
+    mod a {
+        extern "C" {
+            fn option_non_zero_usize() -> usize;
+            fn option_non_zero_isize() -> isize;
+            fn option_non_null_ptr() -> *const usize;
+
+            fn option_non_zero_usize_incorrect() -> usize;
+            fn option_non_null_ptr_incorrect() -> *const usize;
+        }
+    }
+    mod b {
+        extern "C" {
+            // This should be allowed, because these conversions are guaranteed to be FFI-safe (see
+            // #60300)
+            fn option_non_zero_usize() -> Option<core::num::NonZeroUsize>;
+            fn option_non_zero_isize() -> Option<core::num::NonZeroIsize>;
+            fn option_non_null_ptr() -> Option<core::ptr::NonNull<usize>>;
+
+            // However, these should be incorrect (note isize instead of usize)
+            fn option_non_zero_usize_incorrect() -> isize;
+            //~^ WARN `option_non_zero_usize_incorrect` redeclared with a different signature
+            fn option_non_null_ptr_incorrect() -> *const isize;
+            //~^ WARN `option_non_null_ptr_incorrect` redeclared with a different signature
         }
     }
 }
diff --git a/src/test/ui/lint/clashing-extern-fn.stderr b/src/test/ui/lint/clashing-extern-fn.stderr
index 96e51ab5a3e..cca0c4c59eb 100644
--- a/src/test/ui/lint/clashing-extern-fn.stderr
+++ b/src/test/ui/lint/clashing-extern-fn.stderr
@@ -1,11 +1,11 @@
 warning: `clash` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:15:9
+  --> $DIR/clashing-extern-fn.rs:14:13
    |
-LL |     fn clash(x: u8);
-   |     ---------------- `clash` previously declared here
+LL |             fn clash(x: u8);
+   |             ---------------- `clash` previously declared here
 ...
-LL |         fn clash(x: u64);
-   |         ^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+LL |             fn clash(x: u64);
+   |             ^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
    |
 note: the lint level is defined here
   --> $DIR/clashing-extern-fn.rs:4:9
@@ -15,20 +15,8 @@ LL | #![warn(clashing_extern_declarations)]
    = note: expected `unsafe extern "C" fn(u8)`
               found `unsafe extern "C" fn(u64)`
 
-warning: `extern_fn` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:39:9
-   |
-LL |     fn extern_fn(x: u64);
-   |     --------------------- `extern_fn` previously declared here
-...
-LL |         fn extern_fn(x: u32);
-   |         ^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
-   |
-   = note: expected `unsafe extern "C" fn(u64)`
-              found `unsafe extern "C" fn(u32)`
-
 warning: `extern_link_name` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:64:9
+  --> $DIR/clashing-extern-fn.rs:52:9
    |
 LL | /     #[link_name = "extern_link_name"]
 LL | |     fn some_new_name(x: i16);
@@ -41,7 +29,7 @@ LL |           fn extern_link_name(x: u32);
               found `unsafe extern "C" fn(u32)`
 
 warning: `some_other_extern_link_name` redeclares `some_other_new_name` with a different signature
-  --> $DIR/clashing-extern-fn.rs:67:9
+  --> $DIR/clashing-extern-fn.rs:55:9
    |
 LL |       fn some_other_new_name(x: i16);
    |       ------------------------------- `some_other_new_name` previously declared here
@@ -55,7 +43,7 @@ LL | |         fn some_other_extern_link_name(x: u32);
               found `unsafe extern "C" fn(u32)`
 
 warning: `other_both_names_different` redeclares `link_name_same` with a different signature
-  --> $DIR/clashing-extern-fn.rs:71:9
+  --> $DIR/clashing-extern-fn.rs:59:9
    |
 LL | /     #[link_name = "link_name_same"]
 LL | |     fn both_names_different(x: i16);
@@ -70,7 +58,7 @@ LL | |         fn other_both_names_different(x: u32);
               found `unsafe extern "C" fn(u32)`
 
 warning: `different_mod` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:84:9
+  --> $DIR/clashing-extern-fn.rs:72:9
    |
 LL |         fn different_mod(x: u8);
    |         ------------------------ `different_mod` previously declared here
@@ -82,7 +70,7 @@ LL |         fn different_mod(x: u64);
               found `unsafe extern "C" fn(u64)`
 
 warning: `variadic_decl` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:94:9
+  --> $DIR/clashing-extern-fn.rs:82:9
    |
 LL |     fn variadic_decl(x: u8, ...);
    |     ----------------------------- `variadic_decl` previously declared here
@@ -94,7 +82,7 @@ LL |         fn variadic_decl(x: u8);
               found `unsafe extern "C" fn(u8)`
 
 warning: `weigh_banana` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:154:13
+  --> $DIR/clashing-extern-fn.rs:142:13
    |
 LL |             fn weigh_banana(count: *const Banana) -> u64;
    |             --------------------------------------------- `weigh_banana` previously declared here
@@ -106,7 +94,7 @@ LL |             fn weigh_banana(count: *const Banana) -> u64;
               found `unsafe extern "C" fn(*const banana::three::Banana) -> u64`
 
 warning: `draw_point` redeclared with a different signature
-  --> $DIR/clashing-extern-fn.rs:183:13
+  --> $DIR/clashing-extern-fn.rs:171:13
    |
 LL |             fn draw_point(p: Point);
    |             ------------------------ `draw_point` previously declared here
@@ -117,5 +105,89 @@ LL |             fn draw_point(p: Point);
    = note: expected `unsafe extern "C" fn(sameish_members::a::Point)`
               found `unsafe extern "C" fn(sameish_members::b::Point)`
 
-warning: 9 warnings emitted
+warning: `origin` redeclared with a different signature
+  --> $DIR/clashing-extern-fn.rs:194:22
+   |
+LL |         extern "C" { fn origin() -> Point3; }
+   |                      ---------------------- `origin` previously declared here
+...
+LL |         extern "C" { fn origin() -> Point3; }
+   |                      ^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+   = note: expected `unsafe extern "C" fn() -> same_sized_members_clash::a::Point3`
+              found `unsafe extern "C" fn() -> same_sized_members_clash::b::Point3`
+
+warning: `transparent_incorrect` redeclared with a different signature
+  --> $DIR/clashing-extern-fn.rs:217:13
+   |
+LL |             fn transparent_incorrect() -> T;
+   |             -------------------------------- `transparent_incorrect` previously declared here
+...
+LL |             fn transparent_incorrect() -> isize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+   = note: expected `unsafe extern "C" fn() -> transparent::T`
+              found `unsafe extern "C" fn() -> isize`
+
+warning: `missing_return_type` redeclared with a different signature
+  --> $DIR/clashing-extern-fn.rs:235:13
+   |
+LL |             fn missing_return_type() -> usize;
+   |             ---------------------------------- `missing_return_type` previously declared here
+...
+LL |             fn missing_return_type();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+   = note: expected `unsafe extern "C" fn() -> usize`
+              found `unsafe extern "C" fn()`
+
+warning: `non_zero_usize` redeclared with a different signature
+  --> $DIR/clashing-extern-fn.rs:253:13
+   |
+LL |             fn non_zero_usize() -> core::num::NonZeroUsize;
+   |             ----------------------------------------------- `non_zero_usize` previously declared here
+...
+LL |             fn non_zero_usize() -> usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+   = note: expected `unsafe extern "C" fn() -> std::num::NonZeroUsize`
+              found `unsafe extern "C" fn() -> usize`
+
+warning: `non_null_ptr` redeclared with a different signature
+  --> $DIR/clashing-extern-fn.rs:255:13
+   |
+LL |             fn non_null_ptr() -> core::ptr::NonNull<usize>;
+   |             ----------------------------------------------- `non_null_ptr` previously declared here
+...
+LL |             fn non_null_ptr() -> *const usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+   = note: expected `unsafe extern "C" fn() -> std::ptr::NonNull<usize>`
+              found `unsafe extern "C" fn() -> *const usize`
+
+warning: `option_non_zero_usize_incorrect` redeclared with a different signature
+  --> $DIR/clashing-extern-fn.rs:281:13
+   |
+LL |             fn option_non_zero_usize_incorrect() -> usize;
+   |             ---------------------------------------------- `option_non_zero_usize_incorrect` previously declared here
+...
+LL |             fn option_non_zero_usize_incorrect() -> isize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+   = note: expected `unsafe extern "C" fn() -> usize`
+              found `unsafe extern "C" fn() -> isize`
+
+warning: `option_non_null_ptr_incorrect` redeclared with a different signature
+  --> $DIR/clashing-extern-fn.rs:283:13
+   |
+LL |             fn option_non_null_ptr_incorrect() -> *const usize;
+   |             --------------------------------------------------- `option_non_null_ptr_incorrect` previously declared here
+...
+LL |             fn option_non_null_ptr_incorrect() -> *const isize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+   = note: expected `unsafe extern "C" fn() -> *const usize`
+              found `unsafe extern "C" fn() -> *const isize`
+
+warning: 15 warnings emitted