Allow targets to override default codegen backend

1 files changed, 63 insertions, 13 deletions

diff --git a/compiler/rustc_interface/src/interface.rs b/compiler/rustc_interface/src/interface.rs
index 8a4705e0056..f53b320f2e0 100644
--- a/compiler/rustc_interface/src/interface.rs
+++ b/compiler/rustc_interface/src/interface.rs
@@ -16,7 +16,7 @@ use rustc_parse::maybe_new_parser_from_source_str;
 use rustc_query_impl::QueryCtxt;
 use rustc_query_system::query::print_query_stack;
 use rustc_session::config::{self, Cfg, CheckCfg, ExpectedValues, Input, OutFileName};
-use rustc_session::filesearch::sysroot_candidates;
+use rustc_session::filesearch::{self, sysroot_candidates};
 use rustc_session::parse::ParseSess;
 use rustc_session::{lint, CompilerIO, EarlyDiagCtxt, Session};
 use rustc_span::source_map::FileLoader;
@@ -336,14 +336,66 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
 
             let early_dcx = EarlyDiagCtxt::new(config.opts.error_format);
 
-            let codegen_backend = if let Some(make_codegen_backend) = config.make_codegen_backend {
-                make_codegen_backend(&config.opts)
-            } else {
-                util::get_codegen_backend(
-                    &early_dcx,
-                    &config.opts.maybe_sysroot,
-                    config.opts.unstable_opts.codegen_backend.as_deref(),
-                )
+            let sysroot = match &config.opts.maybe_sysroot {
+                Some(sysroot) => sysroot.clone(),
+                None => filesearch::get_or_default_sysroot().expect("Failed finding sysroot"),
+            };
+
+            let (codegen_backend, target_cfg) = match config.make_codegen_backend {
+                None => {
+                    // Build a target without override, so that it can override the backend if needed
+                    let target =
+                        config::build_target_config(&early_dcx, &config.opts, None, &sysroot);
+
+                    let backend = util::get_codegen_backend(
+                        &early_dcx,
+                        &sysroot,
+                        config.opts.unstable_opts.codegen_backend.as_deref(),
+                        &target,
+                    );
+
+                    // target_override is documented to be called before init(), so this is okay
+                    let target_override = backend.target_override(&config.opts);
+
+                    // Assert that we don't use target's override of the backend and
+                    // backend's override of the target at the same time
+                    if config.opts.unstable_opts.codegen_backend.is_none()
+                        && target.default_codegen_backend.is_some()
+                        && target_override.is_some()
+                    {
+                        rustc_middle::bug!(
+                            "Codegen backend requested target override even though the target requested the backend"
+                        );
+                    }
+
+                    // Re-build target with the (potential) override
+                    let target = config::build_target_config(
+                        &early_dcx,
+                        &config.opts,
+                        target_override,
+                        &sysroot,
+                    );
+
+                    (backend, target)
+                }
+                Some(make_codegen_backend) => {
+                    // N.B. `make_codegen_backend` takes precedence over `target.default_codegen_backend`,
+                    //      which is ignored in this case.
+
+                    let backend = make_codegen_backend(&config.opts);
+
+                    // target_override is documented to be called before init(), so this is okay
+                    let target_override = backend.target_override(&config.opts);
+
+                    let target = config::build_target_config(
+                        &early_dcx,
+                        &config.opts,
+                        target_override,
+                        &sysroot,
+                    );
+
+                    (backend, target)
+                }
             };
 
             let temps_dir = config.opts.unstable_opts.temps_dir.as_deref().map(PathBuf::from);
@@ -364,9 +416,6 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
             let mut locale_resources = Vec::from(config.locale_resources);
             locale_resources.push(codegen_backend.locale_resource());
 
-            // target_override is documented to be called before init(), so this is okay
-            let target_override = codegen_backend.target_override(&config.opts);
-
             let mut sess = rustc_session::build_session(
                 early_dcx,
                 config.opts,
@@ -381,7 +430,8 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
                 locale_resources,
                 config.lint_caps,
                 config.file_loader,
-                target_override,
+                target_cfg,
+                sysroot,
                 util::rustc_version_str().unwrap_or("unknown"),
                 config.ice_file,
                 config.using_internal_features,