Move linkage type check to HIR analysis and fix semantics issues.

This ensures that the error is printed even for unused variables,
as well as unifying the handling between the LLVM and GCC backends.

This also fixes unusual behavior around exported Rust-defined variables
with linkage attributes. With the previous behavior, it appears to be
impossible to define such a variable such that it can actually be imported
and used by another crate. This is because on the importing side, the
variable is required to be a pointer, but on the exporting side, the
type checker rejects static variables of pointer type because they do
not implement `Sync`. Even if it were possible to import such a type, it
appears that code generation on the importing side would add an unexpected
additional level of pointer indirection, which would break type safety.

This highlighted that the semantics of linkage on Rust-defined variables
is different to linkage on foreign items. As such, we now model the
difference with two different codegen attributes: linkage for Rust-defined
variables, and import_linkage for foreign items.

This change gives semantics to the test
src/test/ui/linkage-attr/auxiliary/def_illtyped_external.rs which was
previously expected to fail to compile. Therefore, convert it into a
test that is expected to successfully compile.

The update to the GCC backend is speculative and untested.

Author: pcc

src/consts.rs

@@ -8,13 +8,11 @@ use rustc_middle::mir::mono::MonoItem;
 use rustc_middle::ty::{self, Instance, Ty};
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::mir::interpret::{self, ConstAllocation, ErrorHandled, Scalar as InterpScalar, read_target_uint};
-use rustc_span::Span;
 use rustc_span::def_id::DefId;
 use rustc_target::abi::{self, Align, HasDataLayout, Primitive, Size, WrappingRange};
 
 use crate::base;
 use crate::context::CodegenCx;
-use crate::errors::LinkageConstOrMutType;
 use crate::type_of::LayoutGccExt;
 
 impl<'gcc, 'tcx> CodegenCx<'gcc, 'tcx> {
@@ -239,12 +237,12 @@ impl<'gcc, 'tcx> CodegenCx<'gcc, 'tcx> {
                     }
 
                     Node::ForeignItem(&hir::ForeignItem {
-                        span,
+                        span: _,
                         kind: hir::ForeignItemKind::Static(..),
                         ..
                     }) => {
                         let fn_attrs = self.tcx.codegen_fn_attrs(def_id);
-                        check_and_apply_linkage(&self, &fn_attrs, ty, sym, span)
+                        check_and_apply_linkage(&self, &fn_attrs, ty, sym)
                     }
 
                     item => bug!("get_static: expected static, found {:?}", item),
@@ -257,8 +255,7 @@ impl<'gcc, 'tcx> CodegenCx<'gcc, 'tcx> {
                 //debug!("get_static: sym={} item_attr={:?}", sym, self.tcx.item_attrs(def_id));
 
                 let attrs = self.tcx.codegen_fn_attrs(def_id);
-                let span = self.tcx.def_span(def_id);
-                let global = check_and_apply_linkage(&self, &attrs, ty, sym, span);
+                let global = check_and_apply_linkage(&self, &attrs, ty, sym);
 
                 let needs_dll_storage_attr = false; // TODO(antoyo)
 
@@ -355,24 +352,12 @@ pub fn codegen_static_initializer<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, def_id
     Ok((const_alloc_to_gcc(cx, alloc), alloc))
 }
 
-fn check_and_apply_linkage<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, attrs: &CodegenFnAttrs, ty: Ty<'tcx>, sym: &str, span: Span) -> LValue<'gcc> {
+fn check_and_apply_linkage<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, attrs: &CodegenFnAttrs, ty: Ty<'tcx>, sym: &str) -> LValue<'gcc> {
     let is_tls = attrs.flags.contains(CodegenFnAttrFlags::THREAD_LOCAL);
     let llty = cx.layout_of(ty).gcc_type(cx, true);
-    if let Some(linkage) = attrs.linkage {
-        // If this is a static with a linkage specified, then we need to handle
-        // it a little specially. The typesystem prevents things like &T and
-        // extern "C" fn() from being non-null, so we can't just declare a
-        // static and call it a day. Some linkages (like weak) will make it such
-        // that the static actually has a null value.
-        let llty2 =
-            if let ty::RawPtr(ref mt) = ty.kind() {
-                cx.layout_of(mt.ty).gcc_type(cx, true)
-            }
-            else {
-                cx.sess().emit_fatal(LinkageConstOrMutType { span: span })
-            };
+    if let Some(linkage) = attrs.import_linkage {
         // Declare a symbol `foo` with the desired linkage.
-        let global1 = cx.declare_global_with_linkage(&sym, llty2, base::global_linkage_to_gcc(linkage));
+        let global1 = cx.declare_global_with_linkage(&sym, cx.type_i8(), base::global_linkage_to_gcc(linkage));
 
         // Declare an internal global `extern_with_linkage_foo` which
         // is initialized with the address of `foo`.  If `foo` is

src/errors.rs

@@ -211,13 +211,6 @@ pub(crate) struct InvalidMonomorphizationUnsupportedOperation<'a> {
     pub in_elem: Ty<'a>,
 }
 
-#[derive(Diagnostic)]
-#[diag(codegen_gcc_linkage_const_or_mut_type)]
-pub(crate) struct LinkageConstOrMutType {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(codegen_gcc_lto_not_supported)]
 pub(crate) struct LTONotSupported;