codegen_llvm: avoid Deref impls w/ extern type

`rustc_codegen_llvm` relied on `Deref` impls where `Deref::Target` was
or contained an extern type - in my experimental implementation of
rust-lang/rfcs#3729, this isn't possible as the `Target` associated
type's `?Sized` bound cannot be relaxed backwards compatibly (unless we
come up with some way of doing this).

In later pull requests with the rust-lang/rfcs#3729 implementation,
breakage like this could only occur for nightly users relying on the
`extern_types` feature.

Upstreaming this to avoid needing to keep carrying this patch locally,
and I think it'll necessarily need to change eventually.

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -793,7 +793,9 @@ pub(crate) unsafe fn optimize_thin_module(
         {
             let _timer =
                 cgcx.prof.generic_activity_with_arg("LLVM_thin_lto_rename", thin_module.name());
-            unsafe { llvm::LLVMRustPrepareThinLTORename(thin_module.shared.data.0, llmod, target) };
+            unsafe {
+                llvm::LLVMRustPrepareThinLTORename(thin_module.shared.data.0, llmod, target.raw())
+            };
             save_temp_bitcode(cgcx, &module, "thin-lto-after-rename");
         }
 
@@ -823,7 +825,7 @@ pub(crate) unsafe fn optimize_thin_module(
             let _timer =
                 cgcx.prof.generic_activity_with_arg("LLVM_thin_lto_import", thin_module.name());
             if unsafe {
-                !llvm::LLVMRustPrepareThinLTOImport(thin_module.shared.data.0, llmod, target)
+                !llvm::LLVMRustPrepareThinLTOImport(thin_module.shared.data.0, llmod, target.raw())
             } {
                 return Err(write::llvm_err(dcx, LlvmError::PrepareThinLtoModule));
             }

compiler/rustc_codegen_llvm/src/back/owned_target_machine.rs

@@ -1,6 +1,5 @@
 use std::ffi::{CStr, c_char};
 use std::marker::PhantomData;
-use std::ops::Deref;
 use std::ptr::NonNull;
 
 use rustc_data_structures::small_c_str::SmallCStr;
@@ -80,12 +79,12 @@ impl OwnedTargetMachine {
             .map(|tm_unique| Self { tm_unique, phantom: PhantomData })
             .ok_or_else(|| LlvmError::CreateTargetMachine { triple: SmallCStr::from(triple) })
     }
-}
 
-impl Deref for OwnedTargetMachine {
-    type Target = llvm::TargetMachine;
-
-    fn deref(&self) -> &Self::Target {
+    /// Returns inner `llvm::TargetMachine` type.
+    ///
+    /// This could be a `Deref` implementation, but `llvm::TargetMachine` is an extern type and
+    /// `Deref::Target: ?Sized`.
+    pub fn raw(&self) -> &llvm::TargetMachine {
         // SAFETY: constructing ensures we have a valid pointer created by
         // llvm::LLVMRustCreateTargetMachine.
         unsafe { self.tm_unique.as_ref() }

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -637,7 +637,7 @@ pub(crate) unsafe fn llvm_optimize(
     let result = unsafe {
         llvm::LLVMRustOptimize(
             module.module_llvm.llmod(),
-            &*module.module_llvm.tm,
+            &*module.module_llvm.tm.raw(),
             to_pass_builder_opt_level(opt_level),
             opt_stage,
             cgcx.opts.cg.linker_plugin_lto.enabled(),
@@ -875,7 +875,7 @@ pub(crate) unsafe fn codegen(
             };
             write_output_file(
                 dcx,
-                tm,
+                tm.raw(),
                 config.no_builtins,
                 llmod,
                 &path,
@@ -909,7 +909,7 @@ pub(crate) unsafe fn codegen(
 
                 write_output_file(
                     dcx,
-                    tm,
+                    tm.raw(),
                     config.no_builtins,
                     llmod,
                     &obj_out,