Rollup merge of rust-lang#58609 - gabi-250:mutable-refs, r=oli-obk

Allow Self::Module to be mutated.

This only changes `&Self::Module` to `&mut Self::Module` in a couple of places.

`codegen_allocator` and `write_metadata` from `ExtraBackendMethods` mutate the underlying LLVM module. As such, it makes sense for these two functions to receive a mutable reference to the module (as opposed to an immutable one).

I am trying to implement `codegen_allocator` for my backend, and I need to be able to mutate `Self::Module`:
https://github.com/rust-lang/rust/blob/f66e4697ae286985ddefc53c3a047614568458bb/src/librustc_codegen_ssa/traits/backend.rs#L41
Modifying the module in `codegen_allocator`/`write_metadata` is not a problem for the LLVM backend, because [ModuleLlvm](https://github.com/rust-lang/rust/blob/master/src/librustc_codegen_llvm/lib.rs#L357) contains a raw pointer to the underlying LLVM module, so it can easily be mutated through FFI calls.

I am trying to avoid interior mutability and `unsafe` as much as I can. What do you think? Does this change make sense, or is there a reason why this should stay the way it is?

src/librustc_codegen_llvm/allocator.rs

@@ -9,7 +9,7 @@ use rustc_allocator::{ALLOCATOR_METHODS, AllocatorTy};
 use crate::ModuleLlvm;
 use crate::llvm::{self, False, True};
 
-pub(crate) unsafe fn codegen(tcx: TyCtxt, mods: &ModuleLlvm, kind: AllocatorKind) {
+pub(crate) unsafe fn codegen(tcx: TyCtxt, mods: &mut ModuleLlvm, kind: AllocatorKind) {
     let llcx = &*mods.llcx;
     let llmod = mods.llmod();
     let usize = match &tcx.sess.target.target.target_pointer_width[..] {

src/librustc_codegen_llvm/base.rs

@@ -46,7 +46,7 @@ use crate::value::Value;
 
 pub fn write_metadata<'a, 'gcx>(
     tcx: TyCtxt<'a, 'gcx, 'gcx>,
-    llvm_module: &ModuleLlvm
+    llvm_module: &mut ModuleLlvm
 ) -> EncodedMetadata {
     use std::io::Write;
     use flate2::Compression;

src/librustc_codegen_llvm/lib.rs

@@ -120,11 +120,11 @@ impl ExtraBackendMethods for LlvmCodegenBackend {
     fn write_metadata<'b, 'gcx>(
         &self,
         tcx: TyCtxt<'b, 'gcx, 'gcx>,
-        metadata: &ModuleLlvm
+        metadata: &mut ModuleLlvm
     ) -> EncodedMetadata {
         base::write_metadata(tcx, metadata)
     }
-    fn codegen_allocator(&self, tcx: TyCtxt, mods: &ModuleLlvm, kind: AllocatorKind) {
+    fn codegen_allocator(&self, tcx: TyCtxt, mods: &mut ModuleLlvm, kind: AllocatorKind) {
         unsafe { allocator::codegen(tcx, mods, kind) }
     }
     fn compile_codegen_unit<'a, 'tcx: 'a>(

src/librustc_codegen_ssa/base.rs

@@ -551,9 +551,9 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
                                                             &["crate"],
                                                             Some("metadata")).as_str()
                                                                              .to_string();
-    let metadata_llvm_module = backend.new_metadata(tcx, &metadata_cgu_name);
+    let mut metadata_llvm_module = backend.new_metadata(tcx, &metadata_cgu_name);
     let metadata = time(tcx.sess, "write metadata", || {
-        backend.write_metadata(tcx, &metadata_llvm_module)
+        backend.write_metadata(tcx, &mut metadata_llvm_module)
     });
     tcx.sess.profiler(|p| p.end_activity(ProfileCategory::Codegen));
 
@@ -636,9 +636,9 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
                                                        &["crate"],
                                                        Some("allocator")).as_str()
                                                                          .to_string();
-        let modules = backend.new_metadata(tcx, &llmod_id);
+        let mut modules = backend.new_metadata(tcx, &llmod_id);
         time(tcx.sess, "write allocator module", || {
-            backend.codegen_allocator(tcx, &modules, kind)
+            backend.codegen_allocator(tcx, &mut modules, kind)
         });
 
         Some(ModuleCodegen {

src/librustc_codegen_ssa/traits/backend.rs

@@ -36,9 +36,9 @@ pub trait ExtraBackendMethods: CodegenBackend + WriteBackendMethods + Sized + Se
     fn write_metadata<'b, 'gcx>(
         &self,
         tcx: TyCtxt<'b, 'gcx, 'gcx>,
-        metadata: &Self::Module,
+        metadata: &mut Self::Module,
     ) -> EncodedMetadata;
-    fn codegen_allocator(&self, tcx: TyCtxt, mods: &Self::Module, kind: AllocatorKind);
+    fn codegen_allocator(&self, tcx: TyCtxt, mods: &mut Self::Module, kind: AllocatorKind);
     fn compile_codegen_unit<'a, 'tcx: 'a>(
         &self,
         tcx: TyCtxt<'a, 'tcx, 'tcx>,