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

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(),