Include debug info for the allocator shim

Issue Details:
In some cases it is necessary to generate an "allocator shim" to forward various Rust allocation functions (e.g., `__rust_alloc`) to an underlying function (e.g., `malloc`). However, since this allocator shim is a manually created LLVM module it is not processed via the normal module processing code and so no debug info is generated for it (if debugging info is enabled).

Fix Details:
* Modify the `debuginfo` code to allow creating debug info for a module without a `CodegenCx` (since it is difficult, and expensive, to create one just to emit some debug info).
* After creating the allocator shim add in basic debug info.

compiler/rustc_codegen_llvm/src/allocator.rs

@@ -3,19 +3,22 @@ use libc::c_uint;
 use rustc_ast::expand::allocator::{AllocatorKind, AllocatorTy, ALLOCATOR_METHODS};
 use rustc_middle::bug;
 use rustc_middle::ty::TyCtxt;
+use rustc_session::config::DebugInfo;
 use rustc_span::symbol::sym;
 
+use crate::debuginfo;
 use crate::llvm::{self, False, True};
 use crate::ModuleLlvm;
 
 pub(crate) unsafe fn codegen(
     tcx: TyCtxt<'_>,
-    mods: &mut ModuleLlvm,
+    module_llvm: &mut ModuleLlvm,
+    module_name: &str,
     kind: AllocatorKind,
     has_alloc_error_handler: bool,
 ) {
-    let llcx = &*mods.llcx;
+    let llcx = &*module_llvm.llcx;
-    let llmod = mods.llmod();
+    let llmod = module_llvm.llmod();
     let usize = match tcx.sess.target.pointer_width {
         16 => llvm::LLVMInt16TypeInContext(llcx),
         32 => llvm::LLVMInt32TypeInContext(llcx),
@@ -132,4 +135,10 @@ pub(crate) unsafe fn codegen(
     llvm::LLVMSetTailCall(ret, True);
     llvm::LLVMBuildRetVoid(llbuilder);
     llvm::LLVMDisposeBuilder(llbuilder);
+
+    if tcx.sess.opts.debuginfo != DebugInfo::None {
+        let dbg_cx = debuginfo::CrateDebugContext::new(llmod);
+        debuginfo::metadata::compile_unit_metadata(tcx, module_name, &dbg_cx);
+        dbg_cx.finalize(tcx.sess);
+    }
 }

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -29,6 +29,7 @@ use rustc_middle::ty::layout::HasTyCtxt;
 use rustc_middle::ty::subst::{GenericArgKind, SubstsRef};
 use rustc_middle::ty::{self, Instance, ParamEnv, Ty, TypeFoldable};
 use rustc_session::config::{self, DebugInfo};
+use rustc_session::Session;
 use rustc_span::symbol::Symbol;
 use rustc_span::{self, BytePos, Pos, SourceFile, SourceFileAndLine, Span};
 use rustc_target::abi::{LayoutOf, Primitive, Size};
@@ -95,45 +96,52 @@ impl<'a, 'tcx> CrateDebugContext<'a, 'tcx> {
             composite_types_completed: Default::default(),
         }
     }
+
+    pub fn finalize(&self, sess: &Session) {
+        unsafe {
+            llvm::LLVMRustDIBuilderFinalize(self.builder);
+
+            // Debuginfo generation in LLVM by default uses a higher
+            // version of dwarf than macOS currently understands. We can
+            // instruct LLVM to emit an older version of dwarf, however,
+            // for macOS to understand. For more info see #11352
+            // This can be overridden using --llvm-opts -dwarf-version,N.
+            // Android has the same issue (#22398)
+            if let Some(version) = sess.target.dwarf_version {
+                llvm::LLVMRustAddModuleFlag(self.llmod, "Dwarf Version\0".as_ptr().cast(), version)
+            }
+
+            // Indicate that we want CodeView debug information on MSVC
+            if sess.target.is_like_msvc {
+                llvm::LLVMRustAddModuleFlag(self.llmod, "CodeView\0".as_ptr().cast(), 1)
+            }
+
+            // Prevent bitcode readers from deleting the debug info.
+            let ptr = "Debug Info Version\0".as_ptr();
+            llvm::LLVMRustAddModuleFlag(
+                self.llmod,
+                ptr.cast(),
+                llvm::LLVMRustDebugMetadataVersion(),
+            );
+        }
+    }
 }
 
 /// Creates any deferred debug metadata nodes
 pub fn finalize(cx: &CodegenCx<'_, '_>) {
-    if cx.dbg_cx.is_none() {
+    if let Some(dbg_cx) = &cx.dbg_cx {
-        return;
+        debug!("finalize");
-    }
 
-    debug!("finalize");
+        if gdb::needs_gdb_debug_scripts_section(cx) {
-
+            // Add a .debug_gdb_scripts section to this compile-unit. This will
-    if gdb::needs_gdb_debug_scripts_section(cx) {
+            // cause GDB to try and load the gdb_load_rust_pretty_printers.py file,
-        // Add a .debug_gdb_scripts section to this compile-unit. This will
+            // which activates the Rust pretty printers for binary this section is
-        // cause GDB to try and load the gdb_load_rust_pretty_printers.py file,
+            // contained in.
-        // which activates the Rust pretty printers for binary this section is
+            gdb::get_or_insert_gdb_debug_scripts_section_global(cx);
-        // contained in.
-        gdb::get_or_insert_gdb_debug_scripts_section_global(cx);
-    }
-
-    unsafe {
-        llvm::LLVMRustDIBuilderFinalize(DIB(cx));
-        // Debuginfo generation in LLVM by default uses a higher
-        // version of dwarf than macOS currently understands. We can
-        // instruct LLVM to emit an older version of dwarf, however,
-        // for macOS to understand. For more info see #11352
-        // This can be overridden using --llvm-opts -dwarf-version,N.
-        // Android has the same issue (#22398)
-        if let Some(version) = cx.sess().target.dwarf_version {
-            llvm::LLVMRustAddModuleFlag(cx.llmod, "Dwarf Version\0".as_ptr().cast(), version)
         }
 
-        // Indicate that we want CodeView debug information on MSVC
+        dbg_cx.finalize(cx.sess());
-        if cx.sess().target.is_like_msvc {
+    }
-            llvm::LLVMRustAddModuleFlag(cx.llmod, "CodeView\0".as_ptr().cast(), 1)
-        }
-
-        // Prevent bitcode readers from deleting the debug info.
-        let ptr = "Debug Info Version\0".as_ptr();
-        llvm::LLVMRustAddModuleFlag(cx.llmod, ptr.cast(), llvm::LLVMRustDebugMetadataVersion());
-    };
 }
 
 impl DebugInfoBuilderMethods for Builder<'a, 'll, 'tcx> {

compiler/rustc_codegen_llvm/src/lib.rs

@@ -92,11 +92,12 @@ impl ExtraBackendMethods for LlvmCodegenBackend {
     fn codegen_allocator<'tcx>(
         &self,
         tcx: TyCtxt<'tcx>,
-        mods: &mut ModuleLlvm,
+        module_llvm: &mut ModuleLlvm,
+        module_name: &str,
         kind: AllocatorKind,
         has_alloc_error_handler: bool,
     ) {
-        unsafe { allocator::codegen(tcx, mods, kind, has_alloc_error_handler) }
+        unsafe { allocator::codegen(tcx, module_llvm, module_name, kind, has_alloc_error_handler) }
     }
     fn compile_codegen_unit(
         &self,

compiler/rustc_codegen_ssa/src/base.rs

@@ -538,12 +538,18 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
     } else if let Some(kind) = tcx.allocator_kind(()) {
         let llmod_id =
             cgu_name_builder.build_cgu_name(LOCAL_CRATE, &["crate"], Some("allocator")).to_string();
-        let mut modules = backend.new_metadata(tcx, &llmod_id);
+        let mut module_llvm = backend.new_metadata(tcx, &llmod_id);
         tcx.sess.time("write_allocator_module", || {
-            backend.codegen_allocator(tcx, &mut modules, kind, tcx.lang_items().oom().is_some())
+            backend.codegen_allocator(
+                tcx,
+                &mut module_llvm,
+                &llmod_id,
+                kind,
+                tcx.lang_items().oom().is_some(),
+            )
         });
 
-        Some(ModuleCodegen { name: llmod_id, module_llvm: modules, kind: ModuleKind::Allocator })
+        Some(ModuleCodegen { name: llmod_id, module_llvm, kind: ModuleKind::Allocator })
     } else {
         None
     };

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -120,7 +120,8 @@ pub trait ExtraBackendMethods: CodegenBackend + WriteBackendMethods + Sized + Se
     fn codegen_allocator<'tcx>(
         &self,
         tcx: TyCtxt<'tcx>,
-        mods: &mut Self::Module,
+        module_llvm: &mut Self::Module,
+        module_name: &str,
         kind: AllocatorKind,
         has_alloc_error_handler: bool,
     );