For "stage 1" ui-fulldeps, use the stage 1 compiler to query target info

Testing ui-fulldeps in "stage 1" actually uses the stage 0 compiler, so that
test programs can link against stage 1 rustc crates.

Unfortunately, using the stage 0 compiler causes problems when compiletest
tries to obtain target information from the compiler, but the output format has
changed since the last bootstrap beta bump.

We can work around this by also providing compiletest with a stage 1 compiler,
and having it use that compiler to query for target information.

Author: Zalathar

src/bootstrap/src/core/build_steps/test.rs

@@ -1663,7 +1663,11 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         // bootstrap compiler.
         // NOTE: Only stage 1 is special cased because we need the rustc_private artifacts to match the
         // running compiler in stage 2 when plugins run.
+        let query_compiler;
         let (stage, stage_id) = if suite == "ui-fulldeps" && compiler.stage == 1 {
+            // Even when using the stage 0 compiler, we also need to provide the stage 1 compiler
+            // so that compiletest can query it for target information.
+            query_compiler = Some(compiler);
             // At stage 0 (stage - 1) we are using the stage0 compiler. Using `self.target` can lead
             // finding an incorrect compiler path on cross-targets, as the stage 0 is always equal to
             // `build.build` in the configuration.
@@ -1672,6 +1676,7 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
             let test_stage = compiler.stage + 1;
             (test_stage, format!("stage{test_stage}-{build}"))
         } else {
+            query_compiler = None;
             let stage = compiler.stage;
             (stage, format!("stage{stage}-{target}"))
         };
@@ -1716,6 +1721,9 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         cmd.arg("--compile-lib-path").arg(builder.rustc_libdir(compiler));
         cmd.arg("--run-lib-path").arg(builder.sysroot_target_libdir(compiler, target));
         cmd.arg("--rustc-path").arg(builder.rustc(compiler));
+        if let Some(query_compiler) = query_compiler {
+            cmd.arg("--query-rustc-path").arg(builder.rustc(query_compiler));
+        }
 
         // Minicore auxiliary lib for `no_core` tests that need `core` stubs in cross-compilation
         // scenarios.

src/tools/compiletest/src/common.rs

@@ -279,6 +279,15 @@ pub struct Config {
     /// [`Self::rustc_path`].
     pub stage0_rustc_path: Option<Utf8PathBuf>,
 
+    /// Path to the stage 1 or higher `rustc` used to obtain target information via
+    /// `--print=all-target-specs-json` and similar queries.
+    ///
+    /// Normally this is unset, because [`Self::rustc_path`] can be used instead.
+    /// But when running "stage 1" ui-fulldeps tests, `rustc_path` is a stage 0
+    /// compiler, whereas target specs must be obtained from a stage 1+ compiler
+    /// (in case the JSON format has changed since the last bootstrap bump).
+    pub query_rustc_path: Option<Utf8PathBuf>,
+
     /// Path to the `rustdoc`-under-test. Like [`Self::rustc_path`], this `rustdoc` is *staged*.
     pub rustdoc_path: Option<Utf8PathBuf>,
 
@@ -712,6 +721,7 @@ impl Config {
             rustc_path: Utf8PathBuf::default(),
             cargo_path: Default::default(),
             stage0_rustc_path: Default::default(),
+            query_rustc_path: Default::default(),
             rustdoc_path: Default::default(),
             coverage_dump_path: Default::default(),
             python: Default::default(),
@@ -917,7 +927,7 @@ pub struct TargetCfgs {
 
 impl TargetCfgs {
     fn new(config: &Config) -> TargetCfgs {
-        let mut targets: HashMap<String, TargetCfg> = serde_json::from_str(&rustc_output(
+        let mut targets: HashMap<String, TargetCfg> = serde_json::from_str(&query_rustc_output(
             config,
             &["--print=all-target-specs-json", "-Zunstable-options"],
             Default::default(),
@@ -950,7 +960,7 @@ impl TargetCfgs {
             if config.target.ends_with(".json") || !envs.is_empty() {
                 targets.insert(
                     config.target.clone(),
-                    serde_json::from_str(&rustc_output(
+                    serde_json::from_str(&query_rustc_output(
                         config,
                         &[
                             "--print=target-spec-json",
@@ -1009,10 +1019,13 @@ impl TargetCfgs {
         // which are respected for `--print=cfg` but not for `--print=all-target-specs-json`. The
         // code below extracts them from `--print=cfg`: make sure to only override fields that can
         // actually be changed with `-C` flags.
-        for config in
-            rustc_output(config, &["--print=cfg", "--target", &config.target], Default::default())
-                .trim()
-                .lines()
+        for config in query_rustc_output(
+            config,
+            &["--print=cfg", "--target", &config.target],
+            Default::default(),
+        )
+        .trim()
+        .lines()
         {
             let (name, value) = config
                 .split_once("=\"")
@@ -1113,7 +1126,7 @@ pub enum Endian {
 }
 
 fn builtin_cfg_names(config: &Config) -> HashSet<String> {
-    rustc_output(
+    query_rustc_output(
         config,
         &["--print=check-cfg", "-Zunstable-options", "--check-cfg=cfg()"],
         Default::default(),
@@ -1128,7 +1141,7 @@ pub const KNOWN_CRATE_TYPES: &[&str] =
     &["bin", "cdylib", "dylib", "lib", "proc-macro", "rlib", "staticlib"];
 
 fn supported_crate_types(config: &Config) -> HashSet<String> {
-    let crate_types: HashSet<_> = rustc_output(
+    let crate_types: HashSet<_> = query_rustc_output(
         config,
         &["--target", &config.target, "--print=supported-crate-types", "-Zunstable-options"],
         Default::default(),
@@ -1149,8 +1162,10 @@ fn supported_crate_types(config: &Config) -> HashSet<String> {
     crate_types
 }
 
-fn rustc_output(config: &Config, args: &[&str], envs: HashMap<String, String>) -> String {
-    let mut command = Command::new(&config.rustc_path);
+fn query_rustc_output(config: &Config, args: &[&str], envs: HashMap<String, String>) -> String {
+    let query_rustc_path = config.query_rustc_path.as_deref().unwrap_or(&config.rustc_path);
+
+    let mut command = Command::new(query_rustc_path);
     add_dylib_path(&mut command, iter::once(&config.compile_lib_path));
     command.args(&config.target_rustcflags).args(args);
     command.env("RUSTC_BOOTSTRAP", "1");

src/tools/compiletest/src/lib.rs

@@ -63,6 +63,12 @@ pub fn parse_config(args: Vec<String>) -> Config {
             "path to rustc to use for compiling run-make recipes",
             "PATH",
         )
+        .optopt(
+            "",
+            "query-rustc-path",
+            "path to rustc to use for querying target information (defaults to `--rustc-path`)",
+            "PATH",
+        )
         .optopt("", "rustdoc-path", "path to rustdoc to use for compiling", "PATH")
         .optopt("", "coverage-dump-path", "path to coverage-dump to use in tests", "PATH")
         .reqopt("", "python", "path to python to use for doc tests", "PATH")
@@ -354,6 +360,7 @@ pub fn parse_config(args: Vec<String>) -> Config {
         rustc_path: opt_path(matches, "rustc-path"),
         cargo_path: matches.opt_str("cargo-path").map(Utf8PathBuf::from),
         stage0_rustc_path: matches.opt_str("stage0-rustc-path").map(Utf8PathBuf::from),
+        query_rustc_path: matches.opt_str("query-rustc-path").map(Utf8PathBuf::from),
         rustdoc_path: matches.opt_str("rustdoc-path").map(Utf8PathBuf::from),
         coverage_dump_path: matches.opt_str("coverage-dump-path").map(Utf8PathBuf::from),
         python: matches.opt_str("python").unwrap(),