Auto merge of rust-lang#103092 - petrochenkov:weaklto, r=wesleywiser

linker: Fix weak lang item linking with combination windows-gnu + LLD + LTO

In rust-lang#100404 this logic was originally disabled for MSVC due to issues with LTO, but the same issues appear on windows-gnu with LLD because that LLD uses the same underlying logic as MSVC LLD, just with re-syntaxed command line options.

So this PR just disables it for LTO builds in general.

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -2715,7 +2715,7 @@ fn relevant_lib(sess: &Session, lib: &NativeLib) -> bool {
     }
 }
 
-fn are_upstream_rust_objects_already_included(sess: &Session) -> bool {
+pub(crate) fn are_upstream_rust_objects_already_included(sess: &Session) -> bool {
     match sess.lto() {
         config::Lto::Fat => true,
         config::Lto::Thin => {

compiler/rustc_codegen_ssa/src/base.rs

@@ -1,3 +1,4 @@
+use crate::back::link::are_upstream_rust_objects_already_included;
 use crate::back::metadata::create_compressed_metadata_file;
 use crate::back::write::{
     compute_per_cgu_lto_type, start_async_codegen, submit_codegened_module_to_llvm,
@@ -892,10 +893,14 @@ impl CrateInfo {
 
         // Handle circular dependencies in the standard library.
         // See comment before `add_linked_symbol_object` function for the details.
-        // With msvc-like linkers it's both unnecessary (they support circular dependencies),
-        // and causes linking issues (when weak lang item symbols are "privatized" by LTO).
+        // If global LTO is enabled then almost everything (*) is glued into a single object file,
+        // so this logic is not necessary and can cause issues on some targets (due to weak lang
+        // item symbols being "privatized" to that object file), so we disable it.
+        // (*) Native libs, and `#[compiler_builtins]` and `#[no_builtins]` crates are not glued,
+        // and we assume that they cannot define weak lang items. This is not currently enforced
+        // by the compiler, but that's ok because all this stuff is unstable anyway.
         let target = &tcx.sess.target;
-        if !target.is_like_msvc {
+        if !are_upstream_rust_objects_already_included(tcx.sess) {
             let missing_weak_lang_items: FxHashSet<&Symbol> = info
                 .used_crates
                 .iter()