Rollup merge of #145256 - GuillaumeGomez:bootstrap-test-codegen-backend, r=Kobzol,bjorn3

Add new `--test-codegen-backend` bootstrap option

This new bootstrap command line flag allows to do:

```shell
./x.py test tests/ui/intrinsics/panic-uninitialized-zeroed.rs --stage 1 -j8 --test-codegen-backend gcc
```

This is the last step before running it into the CI.

Supersedes rust-lang/rust#144687.

r? ``````@Kobzol``````

6 files changed, 58 insertions, 7 deletions

diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index e7a57a7f375..56e7582a6ff 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -1830,10 +1830,27 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         cmd.arg("--host").arg(&*compiler.host.triple);
         cmd.arg("--llvm-filecheck").arg(builder.llvm_filecheck(builder.config.host_target));
 
-        if let Some(codegen_backend) = builder.config.default_codegen_backend(compiler.host) {
-            // Tells compiletest which codegen backend is used by default by the compiler.
+        if let Some(codegen_backend) = builder.config.cmd.test_codegen_backend() {
+            if !builder.config.enabled_codegen_backends(compiler.host).contains(codegen_backend) {
+                eprintln!(
+                    "\
+ERROR: No configured backend named `{name}`
+HELP: You can add it into `bootstrap.toml` in `rust.codegen-backends = [{name:?}]`",
+                    name = codegen_backend.name(),
+                );
+                crate::exit!(1);
+            }
+            // Tells compiletest that we want to use this codegen in particular and to override
+            // the default one.
+            cmd.arg("--override-codegen-backend").arg(codegen_backend.name());
+            // Tells compiletest which codegen backend to use.
+            // It is used to e.g. ignore tests that don't support that codegen backend.
+            cmd.arg("--default-codegen-backend").arg(codegen_backend.name());
+        } else {
+            // Tells compiletest which codegen backend to use.
             // It is used to e.g. ignore tests that don't support that codegen backend.
-            cmd.arg("--codegen-backend").arg(codegen_backend.name());
+            cmd.arg("--default-codegen-backend")
+                .arg(builder.config.default_codegen_backend(compiler.host).unwrap().name());
         }
 
         if builder.build.config.llvm_enzyme {
diff --git a/src/bootstrap/src/core/builder/cargo.rs b/src/bootstrap/src/core/builder/cargo.rs
index 69a744a86cb..72192403412 100644
--- a/src/bootstrap/src/core/builder/cargo.rs
+++ b/src/bootstrap/src/core/builder/cargo.rs
@@ -1326,7 +1326,12 @@ impl Builder<'_> {
 
             if let Some(limit) = limit
                 && (build_compiler_stage == 0
-                    || self.config.default_codegen_backend(target).unwrap_or_default().is_llvm())
+                    || self
+                        .config
+                        .default_codegen_backend(target)
+                        .cloned()
+                        .unwrap_or_default()
+                        .is_llvm())
             {
                 rustflags.arg(&format!("-Cllvm-args=-import-instr-limit={limit}"));
             }
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index 5eea5436023..d0647537e56 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -1755,8 +1755,8 @@ impl Config {
 
     /// Returns the codegen backend that should be configured as the *default* codegen backend
     /// for a rustc compiled by bootstrap.
-    pub fn default_codegen_backend(&self, target: TargetSelection) -> Option<CodegenBackendKind> {
-        self.enabled_codegen_backends(target).first().cloned()
+    pub fn default_codegen_backend(&self, target: TargetSelection) -> Option<&CodegenBackendKind> {
+        self.enabled_codegen_backends(target).first()
     }
 
     pub fn jemalloc(&self, target: TargetSelection) -> bool {
diff --git a/src/bootstrap/src/core/config/flags.rs b/src/bootstrap/src/core/config/flags.rs
index 17bfb388280..c01b71b9260 100644
--- a/src/bootstrap/src/core/config/flags.rs
+++ b/src/bootstrap/src/core/config/flags.rs
@@ -15,7 +15,7 @@ use crate::core::build_steps::setup::Profile;
 use crate::core::builder::{Builder, Kind};
 use crate::core::config::Config;
 use crate::core::config::target_selection::{TargetSelectionList, target_selection_list};
-use crate::{Build, DocTests};
+use crate::{Build, CodegenBackendKind, DocTests};
 
 #[derive(Copy, Clone, Default, Debug, ValueEnum)]
 pub enum Color {
@@ -419,6 +419,9 @@ pub enum Subcommand {
         #[arg(long)]
         /// don't capture stdout/stderr of tests
         no_capture: bool,
+        #[arg(long)]
+        /// Use a different codegen backend when running tests.
+        test_codegen_backend: Option<CodegenBackendKind>,
     },
     /// Build and run some test suites *in Miri*
     Miri {
@@ -658,6 +661,13 @@ impl Subcommand {
             _ => vec![],
         }
     }
+
+    pub fn test_codegen_backend(&self) -> Option<&CodegenBackendKind> {
+        match self {
+            Subcommand::Test { test_codegen_backend, .. } => test_codegen_backend.as_ref(),
+            _ => None,
+        }
+    }
 }
 
 /// Returns the shell completion for a given shell, if the result differs from the current
diff --git a/src/bootstrap/src/lib.rs b/src/bootstrap/src/lib.rs
index 8f766ed00a5..ec7edbf7531 100644
--- a/src/bootstrap/src/lib.rs
+++ b/src/bootstrap/src/lib.rs
@@ -165,6 +165,20 @@ impl CodegenBackendKind {
     }
 }
 
+impl std::str::FromStr for CodegenBackendKind {
+    type Err = &'static str;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.to_lowercase().as_str() {
+            "" => Err("Invalid empty backend name"),
+            "gcc" => Ok(Self::Gcc),
+            "llvm" => Ok(Self::Llvm),
+            "cranelift" => Ok(Self::Cranelift),
+            _ => Ok(Self::Custom(s.to_string())),
+        }
+    }
+}
+
 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 pub enum DocTests {
     /// Run normal tests and doc tests (default).
diff --git a/src/bootstrap/src/utils/change_tracker.rs b/src/bootstrap/src/utils/change_tracker.rs
index b2f9960a449..4fb5891ed18 100644
--- a/src/bootstrap/src/utils/change_tracker.rs
+++ b/src/bootstrap/src/utils/change_tracker.rs
@@ -507,6 +507,11 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         summary: "It is no longer possible to `x clippy` with stage 0. All clippy commands have to be on stage 1+.",
     },
     ChangeInfo {
+        change_id: 145256,
+        severity: ChangeSeverity::Info,
+        summary: "Added `--test-codegen-backend` CLI option for tests",
+    },
+    ChangeInfo {
         change_id: 145379,
         severity: ChangeSeverity::Info,
         summary: "Build/check now supports forwarding `--timings` flag to cargo.",