Rollup merge of #58057 - michaelwoerister:stabilize-xlto, r=alexcrichton

Stabilize linker-plugin based LTO (aka cross-language LTO)

This PR stabilizes [linker plugin based LTO](https://github.com/rust-lang/rust/issues/49879), also known as "cross-language LTO" because it allows for doing inlining and other optimizations across language boundaries in mixed Rust/C/C++ projects.

As described in the tracking issue, it works by making `rustc` emit LLVM bitcode instead of machine code, the same as `clang` does. A linker with the proper plugin (like LLD) can then run (Thin)LTO across all modules.

The feature has been implemented over a number of pull requests and there are various [codegen](https://github.com/rust-lang/rust/blob/master/src/test/codegen/no-dllimport-w-cross-lang-lto.rs) and [run](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-clang)-[make](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-upstream-rlibs) [tests](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto) that make sure that it keeps working.

It also works for building big projects like [Firefox](https://treeherder.mozilla.org/#/jobs?repo=try&revision=2ce2d5ddcea6fbff790503eac406954e469b2f5d).

The PR makes the feature available under the `-C linker-plugin-lto` flag. As discussed in the tracking issue it is not cross-language specific and also not LLD specific. `-C linker-plugin-lto` is descriptive of what it does. If someone has a better name, let me know `:)`

3 files changed, 4 insertions, 4 deletions

diff --git a/src/librustc_codegen_llvm/back/link.rs b/src/librustc_codegen_llvm/back/link.rs
index 725009e1377..548c94abc07 100644
--- a/src/librustc_codegen_llvm/back/link.rs
+++ b/src/librustc_codegen_llvm/back/link.rs
@@ -857,7 +857,7 @@ fn link_args(cmd: &mut dyn Linker,
              codegen_results: &CodegenResults) {
 
     // Linker plugins should be specified early in the list of arguments
-    cmd.cross_lang_lto();
+    cmd.linker_plugin_lto();
 
     // The default library location, we need this to find the runtime.
     // The location of crates will be determined as needed.
@@ -1491,7 +1491,7 @@ fn are_upstream_rust_objects_already_included(sess: &Session) -> bool {
         Lto::Thin => {
             // If we defer LTO to the linker, we haven't run LTO ourselves, so
             // any upstream object files have not been copied yet.
-            !sess.opts.debugging_opts.cross_lang_lto.enabled()
+            !sess.opts.cg.linker_plugin_lto.enabled()
         }
         Lto::No |
         Lto::ThinLocal => false,
diff --git a/src/librustc_codegen_llvm/back/lto.rs b/src/librustc_codegen_llvm/back/lto.rs
index be7733bf554..ac55244d8d9 100644
--- a/src/librustc_codegen_llvm/back/lto.rs
+++ b/src/librustc_codegen_llvm/back/lto.rs
@@ -159,7 +159,7 @@ pub(crate) fn run_thin(cgcx: &CodegenContext<LlvmCodegenBackend>,
     let symbol_white_list = symbol_white_list.iter()
                                              .map(|c| c.as_ptr())
                                              .collect::<Vec<_>>();
-    if cgcx.opts.debugging_opts.cross_lang_lto.enabled() {
+    if cgcx.opts.cg.linker_plugin_lto.enabled() {
         unreachable!("We should never reach this case if the LTO step \
                       is deferred to the linker");
     }
diff --git a/src/librustc_codegen_llvm/back/write.rs b/src/librustc_codegen_llvm/back/write.rs
index 47637f3c5d7..e7871531972 100644
--- a/src/librustc_codegen_llvm/back/write.rs
+++ b/src/librustc_codegen_llvm/back/write.rs
@@ -366,7 +366,7 @@ pub(crate) unsafe fn optimize(cgcx: &CodegenContext<LlvmCodegenBackend>,
                 let opt_level = config.opt_level.map(|x| to_llvm_opt_settings(x).0)
                     .unwrap_or(llvm::CodeGenOptLevel::None);
                 let prepare_for_thin_lto = cgcx.lto == Lto::Thin || cgcx.lto == Lto::ThinLocal ||
-                    (cgcx.lto != Lto::Fat && cgcx.opts.debugging_opts.cross_lang_lto.enabled());
+                    (cgcx.lto != Lto::Fat && cgcx.opts.cg.linker_plugin_lto.enabled());
                 with_llvm_pmb(llmod, &config, opt_level, prepare_for_thin_lto, &mut |b| {
                     llvm::LLVMPassManagerBuilderPopulateFunctionPassManager(b, fpm);
                     llvm::LLVMPassManagerBuilderPopulateModulePassManager(b, mpm);