From e6aa2010ef1cf5bc1fc772b644a4b9422501eac9 Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Mon, 16 Jun 2025 09:20:57 +0000
Subject: [PATCH] Use the .drectve section for exporting symbols from dlls on
 Windows

While it would be reasonable to expect the Windows linker to handle
linker args in the .drectve section identical to cli arguments, as it
turns out exporting weak symbols only works when the /EXPORT is in the
.drectve section, not when it is a linker argument or when a .DEF file
is used.
---
 compiler/rustc_codegen_ssa/src/back/link.rs   | 42 +++++++++++++++++--
 compiler/rustc_codegen_ssa/src/back/linker.rs | 21 ++--------
 2 files changed, 43 insertions(+), 20 deletions(-)

diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index 8c52ed6ed1234..44bfc746a741f 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -1976,9 +1976,11 @@ 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: &[String],
 ) {
-    if symbols.is_empty() {
+    if linked_symbols.is_empty() && exported_symbols.is_empty() {
         return;
     }
 
@@ -2015,7 +2017,7 @@ 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,
@@ -2073,6 +2075,38 @@ fn add_linked_symbol_object(
         }
     }
 
+    if sess.target.is_like_msvc {
+        // Symbol visibility takes care of this for executables typically
+        let should_filter_symbols = if crate_type == CrateType::Executable {
+            sess.opts.unstable_opts.export_executable_symbols
+        } else {
+            true
+        };
+        if should_filter_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.
+            let drectve = exported_symbols
+                .into_iter()
+                .map(|sym| format!(" /EXPORT:\"{sym}\""))
+                .collect::<Vec<_>>()
+                .join("");
+
+            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 {
@@ -2247,7 +2281,9 @@ fn linker_with_args(
         cmd,
         sess,
         tmpdir,
+        crate_type,
         &codegen_results.crate_info.linked_symbols[&crate_type],
+        &codegen_results.crate_info.exported_symbols[&crate_type],
     );
 
     // Sanitizer libraries.
diff --git a/compiler/rustc_codegen_ssa/src/back/linker.rs b/compiler/rustc_codegen_ssa/src/back/linker.rs
index 8fc83908efbcc..f0ebc2a6cc87d 100644
--- a/compiler/rustc_codegen_ssa/src/back/linker.rs
+++ b/compiler/rustc_codegen_ssa/src/back/linker.rs
@@ -1086,19 +1086,10 @@ impl<'a> Linker for MsvcLinker<'a> {
         }
     }
 
-    // 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 function
-    // generates a DEF file which lists all the symbols.
-    //
-    // The linker will read this `*.def` file 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 export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType, symbols: &[String]) {
+    fn export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType, _symbols: &[String]) {
+        // We already add /EXPORT arguments to the .drectve section of symbols.o. We generate
+        // a .DEF file here anyway as it might prevent auto-export of some symbols.
+
         // Symbol visibility takes care of this typically
         if crate_type == CrateType::Executable {
             let should_export_executable_symbols =
@@ -1116,10 +1107,6 @@ impl<'a> Linker for MsvcLinker<'a> {
             // straight to exports.
             writeln!(f, "LIBRARY")?;
             writeln!(f, "EXPORTS")?;
-            for symbol in symbols {
-                debug!("  _{symbol}");
-                writeln!(f, "  {symbol}")?;
-            }
         };
         if let Err(error) = res {
             self.sess.dcx().emit_fatal(errors::LibDefWriteFailure { error });