Use .drectve for MSVC DLL exports

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -62,7 +62,10 @@ use super::metadata::{MetadataPosition, create_wrapper_file};
 use super::rpath::{self, RPathConfig};
 use super::{apple, rmeta_link, versioned_llvm_target};
 use crate::base::needs_allocator_shim_for_linking;
-use crate::{CodegenLintLevelSpecs, CompiledModule, CompiledModules, CrateInfo, NativeLib, errors};
+use crate::{
+    CodegenLintLevelSpecs, CompiledModule, CompiledModules, CrateInfo, NativeLib, SymbolExport,
+    errors,
+};
 
 pub fn ensure_removed(dcx: DiagCtxtHandle<'_>, path: &Path) {
     if let Err(e) = fs::remove_file(path) {
@@ -588,7 +591,7 @@ fn link_staticlib(
             crate_info
                 .exported_symbols
                 .get(&CrateType::StaticLib)
-                .map(|symbols| symbols.iter().map(|(s, _)| s.clone()).collect())
+                .map(|symbols| symbols.iter().map(|symbol| symbol.name.clone()).collect())
         }
     } else {
         None
@@ -2189,9 +2192,15 @@ fn add_linked_symbol_object(
     cmd: &mut dyn Linker,
     sess: &Session,
     tmpdir: &Path,
-    symbols: &[(String, SymbolExportKind)],
+    crate_type: CrateType,
+    linked_symbols: &[(String, SymbolExportKind)],
+    exported_symbols: &[SymbolExport],
 ) {
-    if symbols.is_empty() {
+    let should_export_symbols = sess.target.is_like_msvc
+        && !exported_symbols.is_empty()
+        && (crate_type != CrateType::Executable
+            || sess.opts.unstable_opts.export_executable_symbols);
+    if linked_symbols.is_empty() && !should_export_symbols {
         return;
     }
 
@@ -2228,7 +2237,7 @@ fn add_linked_symbol_object(
         None
     };
 
-    for (sym, kind) in symbols.iter() {
+    for (sym, kind) in linked_symbols.iter() {
         let symbol = file.add_symbol(object::write::Symbol {
             name: sym.clone().into(),
             value: 0,
@@ -2286,6 +2295,37 @@ fn add_linked_symbol_object(
         }
     }
 
+    if should_export_symbols {
+        // Currently the compiler doesn't use `dllexport` (an LLVM attribute) to
+        // export symbols from a dynamic library. When building a dynamic library,
+        // however, we're going to want some symbols exported, so this adds a
+        // `.drectve` section which lists all the symbols using /EXPORT arguments.
+        //
+        // The linker will read these arguments from the `.drectve` section and
+        // export all the symbols from the dynamic library. Note that this is not
+        // as simple as just exporting all the symbols in the current crate (as
+        // specified by `codegen.reachable`) but rather we also need to possibly
+        // export the symbols of upstream crates. Upstream rlibs may be linked
+        // statically to this dynamic library, in which case they may continue to
+        // transitively be used and hence need their symbols exported.
+        fn msvc_drectve_export(symbol: &SymbolExport) -> String {
+            let data = if symbol.kind == SymbolExportKind::Data { ",DATA" } else { "" };
+
+            if let Some(link_name) = symbol.link_name.as_deref() {
+                // The first name is the decorated symbol used by the import library, while
+                // EXPORTAS gives the public name written to the DLL export table.
+                format!(" /EXPORT:\"{link_name}\"{data},EXPORTAS,\"{}\"", symbol.name)
+            } else {
+                format!(" /EXPORT:\"{}\"{data}", symbol.name)
+            }
+        }
+
+        let drectve = exported_symbols.iter().map(msvc_drectve_export).collect::<String>();
+
+        let section = file.add_section(vec![], b".drectve".to_vec(), object::SectionKind::Linker);
+        file.append_section_data(section, drectve.as_bytes(), 1);
+    }
+
     let path = tmpdir.join("symbols.o");
     let result = std::fs::write(&path, file.write().unwrap());
     if let Err(error) = result {
@@ -2416,7 +2456,7 @@ fn add_rpath_args(
 fn add_c_staticlib_symbols(
     sess: &Session,
     lib: &NativeLib,
-    out: &mut Vec<(String, SymbolExportKind)>,
+    out: &mut Vec<SymbolExport>,
 ) -> io::Result<()> {
     let file_path = find_native_static_library(lib.name.as_str(), lib.verbatim, sess);
 
@@ -2469,9 +2509,9 @@ fn add_c_staticlib_symbols(
                 _ => continue,
             };
 
-            // FIXME:The symbol mangle rules are slightly different in Windows(32-bit) and Apple.
-            // Need to be resolved.
-            out.push((name.to_string(), export_kind));
+            // Names read from the object file are already linker-visible.
+            // Do not apply symbol decoration again here.
+            out.push(SymbolExport::new(name.to_string(), export_kind));
         }
     }
 
@@ -2550,7 +2590,14 @@ fn linker_with_args(
     // Pre-link CRT objects.
     add_pre_link_objects(cmd, sess, flavor, link_output_kind, self_contained_crt_objects);
 
-    add_linked_symbol_object(cmd, sess, tmpdir, &crate_info.linked_symbols[&crate_type]);
+    add_linked_symbol_object(
+        cmd,
+        sess,
+        tmpdir,
+        crate_type,
+        &crate_info.linked_symbols[&crate_type],
+        &export_symbols,
+    );
 
     // Sanitizer libraries.
     add_sanitizer_libraries(sess, flavor, crate_type, cmd);