Auto merge of #72049 - mati865:mingw-lld, r=petrochenkov

MinGW: enable dllexport/dllimport

Fixes (only when using LLD) https://github.com/rust-lang/rust/issues/50176
Fixes https://github.com/rust-lang/rust/issues/72319

This makes `windows-gnu` on pair with `windows-msvc` when it comes to symbol exporting.
For MinGW it means both good things like correctly working dllimport/dllexport, ability to link with LLD and bad things like https://github.com/rust-lang/rust/issues/27438.

Not sure but maybe this should land behind unstable compiler option (`-Z`) or environment variable?

3 files changed, 19 insertions, 5 deletions

diff --git a/src/librustc_codegen_llvm/callee.rs b/src/librustc_codegen_llvm/callee.rs
index 75b4f2e3ca5..afdc8dc6187 100644
--- a/src/librustc_codegen_llvm/callee.rs
+++ b/src/librustc_codegen_llvm/callee.rs
@@ -172,7 +172,12 @@ pub fn get_fn(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'tcx>) -> &'ll Value
             }
         }
 
-        if cx.use_dll_storage_attrs && tcx.is_dllimport_foreign_item(instance_def_id) {
+        // MinGW: For backward compatibility we rely on the linker to decide whether it
+        // should use dllimport for functions.
+        if cx.use_dll_storage_attrs
+            && tcx.is_dllimport_foreign_item(instance_def_id)
+            && tcx.sess.target.target.target_env != "gnu"
+        {
             unsafe {
                 llvm::LLVMSetDLLStorageClass(llfn, llvm::DLLStorageClass::DllImport);
             }
diff --git a/src/librustc_codegen_llvm/consts.rs b/src/librustc_codegen_llvm/consts.rs
index 3e9e5d9c8c1..1288870f55f 100644
--- a/src/librustc_codegen_llvm/consts.rs
+++ b/src/librustc_codegen_llvm/consts.rs
@@ -287,7 +287,7 @@ impl CodegenCx<'ll, 'tcx> {
             // argument validation.
             debug_assert!(
                 !(self.tcx.sess.opts.cg.linker_plugin_lto.enabled()
-                    && self.tcx.sess.target.target.options.is_like_msvc
+                    && self.tcx.sess.target.target.options.is_like_windows
                     && self.tcx.sess.opts.cg.prefer_dynamic)
             );
 
diff --git a/src/librustc_codegen_llvm/context.rs b/src/librustc_codegen_llvm/context.rs
index 33a3cdbfa9b..26707fdf839 100644
--- a/src/librustc_codegen_llvm/context.rs
+++ b/src/librustc_codegen_llvm/context.rs
@@ -217,7 +217,16 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         // attributes in LLVM IR as well as native dependencies (in C these
         // correspond to `__declspec(dllimport)`).
         //
-        // Whenever a dynamic library is built by MSVC it must have its public
+        // LD (BFD) in MinGW mode can often correctly guess `dllexport` but
+        // relying on that can result in issues like #50176.
+        // LLD won't support that and expects symbols with proper attributes.
+        // Because of that we make MinGW target emit dllexport just like MSVC.
+        // When it comes to dllimport we use it for constants but for functions
+        // rely on the linker to do the right thing. Opposed to dllexport this
+        // task is easy for them (both LD and LLD) and allows us to easily use
+        // symbols from static libraries in shared libraries.
+        //
+        // Whenever a dynamic library is built on Windows it must have its public
         // interface specified by functions tagged with `dllexport` or otherwise
         // they're not available to be linked against. This poses a few problems
         // for the compiler, some of which are somewhat fundamental, but we use
@@ -254,8 +263,8 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         // this effect) by marking very little as `dllimport` and praying the
         // linker will take care of everything. Fixing this problem will likely
         // require adding a few attributes to Rust itself (feature gated at the
-        // start) and then strongly recommending static linkage on MSVC!
-        let use_dll_storage_attrs = tcx.sess.target.target.options.is_like_msvc;
+        // start) and then strongly recommending static linkage on Windows!
+        let use_dll_storage_attrs = tcx.sess.target.target.options.is_like_windows;
 
         let check_overflow = tcx.sess.overflow_checks();