Remove `CodegenBackend::target_override`.

Backend and target selection is a mess: the target can override the
backend (via `Target::default_codegen_backend`), *and* the backend can
override the target (via `CodegenBackend::target_override`).

The code that handles this is ugly. It calls `build_target_config`
twice, once before getting the backend and once again afterward. It also
must check that both overrides aren't triggering at the same time.

This commit removes the latter override. It's used in rust-gpu but
@eddyb said via Zulip that removing it would be ok. This simplifies the
code greatly, and will allow some nice follow-up refactorings.

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index 938f9f0beaa..716e31080dd 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -890,7 +890,7 @@ pub fn version_at_macro_invocation(
         let backend_name = debug_flags.iter().find_map(|x| x.strip_prefix("codegen-backend="));
         let opts = config::Options::default();
         let sysroot = filesearch::materialize_sysroot(opts.maybe_sysroot.clone());
-        let target = config::build_target_config(early_dcx, &opts, None, &sysroot);
+        let target = config::build_target_config(early_dcx, &opts, &sysroot);
 
         get_codegen_backend(early_dcx, &sysroot, backend_name, &target).print_version();
     }
@@ -1100,7 +1100,7 @@ pub fn describe_flag_categories(early_dcx: &EarlyDiagCtxt, matches: &Matches) ->
 
         let opts = config::Options::default();
         let sysroot = filesearch::materialize_sysroot(opts.maybe_sysroot.clone());
-        let target = config::build_target_config(early_dcx, &opts, None, &sysroot);
+        let target = config::build_target_config(early_dcx, &opts, &sysroot);
 
         get_codegen_backend(early_dcx, &sysroot, backend_name, &target).print_passes();
         return true;