Mark .rmeta files as /SAFESEH on x86 Windows.

Chrome links .rlibs with /WHOLEARCHIVE or -Wl,--whole-archive to prevent
the linker from discarding static initializers. This works well, except
on Windows x86, where lld complains:

  error: /safeseh: lib.rmeta is not compatible with SEH

The fix is simply to mark the .rmeta as SAFESEH aware. This is trivially
true, since the metadata file does not contain any executable code.

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -430,6 +430,33 @@ pub fn create_wrapper_file(
         BinaryFormat::Coff => {
             file.section_mut(section).flags =
                 SectionFlags::Coff { characteristics: pe::IMAGE_SCN_LNK_REMOVE };
+
+            // Disable mangling to avoid mangling the fixed symbol name for "@feat.00".
+            // A metadata file has no other symbols, so this is safe.
+            file.set_mangling(object::write::Mangling::None);
+
+            // Add feature flags to the object file. On MSVC this is optional but LLD will complain if
+            // not present.
+            let mut feature = 0;
+
+            if file.architecture() == Architecture::I386 {
+                // Indicate that all SEH handlers are registered in .sxdata section.
+                // We don't have generate any code, so we don't need .sxdata section but LLD still
+                // expects us to set this bit (see #96498).
+                // Reference: https://docs.microsoft.com/en-us/windows/win32/debug/pe-format
+                feature |= 1;
+            }
+
+            file.add_symbol(object::write::Symbol {
+                name: "@feat.00".into(),
+                value: feature,
+                size: 0,
+                kind: object::SymbolKind::Data,
+                scope: object::SymbolScope::Compilation,
+                weak: false,
+                section: object::write::SymbolSection::Absolute,
+                flags: object::SymbolFlags::None,
+            });
         }
         BinaryFormat::Elf => {
             file.section_mut(section).flags =

tests/run-make/windows-safeseh/Makefile

@@ -0,0 +1,19 @@
+# only-windows
+# needs-rust-lld
+
+include ../tools.mk
+
+all: foo bar
+
+# Ensure that LLD can link when an .rlib contains a synthetic object
+# file referencing exported or used symbols.
+foo:
+	$(RUSTC) -C linker=rust-lld foo.rs
+
+# Ensure that LLD can link when /WHOLEARCHIVE: is used with an .rlib.
+# Previously, lib.rmeta was not marked as (trivially) SAFESEH-aware.
+bar: baz
+	$(RUSTC) -C linker=rust-lld -C link-arg=/WHOLEARCHIVE:libbaz.rlib bar.rs
+
+baz:
+	$(RUSTC) baz.rs

tests/run-make/windows-safeseh/bar.rs

@@ -0,0 +1 @@
+fn main() {}

tests/run-make/windows-safeseh/baz.rs

@@ -0,0 +1,4 @@
+#![crate_type = "rlib"]
+
+#[no_mangle]
+extern "C" fn baz() {}