Always build miri for the host in x run miri

src/bootstrap/src/core/build_steps/run.rs

@@ -5,13 +5,14 @@
 
 use std::path::PathBuf;
 
+use build_helper::exit;
 use clap_complete::{Generator, shells};
 
 use crate::core::build_steps::dist::distdir;
 use crate::core::build_steps::test;
 use crate::core::build_steps::tool::{self, RustcPrivateCompilers, SourceType, Tool};
 use crate::core::build_steps::vendor::{Vendor, default_paths_to_vendor};
-use crate::core::builder::{Builder, Kind, RunConfig, ShouldRun, Step};
+use crate::core::builder::{Builder, Kind, RunConfig, ShouldRun, Step, StepMetadata};
 use crate::core::config::TargetSelection;
 use crate::core::config::flags::get_completion;
 use crate::utils::exec::command;
@@ -100,8 +101,17 @@ impl Step for ReplaceVersionPlaceholder {
     }
 }
 
+/// Invoke the Miri tool on a specified file.
+///
+/// Note that Miri always executed on the host, as it is an interpreter.
+/// That means that `x run miri --target FOO` will build miri for the host,
+/// prepare a miri sysroot for the target `FOO` and then execute miri with
+/// the target `FOO`.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct Miri {
+    /// The build compiler that will build miri and the target compiler to which miri links.
+    compilers: RustcPrivateCompilers,
+    /// The target which will miri interpret.
     target: TargetSelection,
 }
 
@@ -113,14 +123,9 @@ impl Step for Miri {
     }
 
     fn make_run(run: RunConfig<'_>) {
-        run.builder.ensure(Miri { target: run.target });
-    }
+        let builder = run.builder;
 
-    fn run(self, builder: &Builder<'_>) {
-        let host = builder.build.host_target;
-        let target = self.target;
-
-        // `x run` uses stage 0 by default but miri does not work well with stage 0.
+        // `x run` uses stage 0 by default, but miri does not work well with stage 0.
         // Change the stage to 1 if it's not set explicitly.
         let stage = if builder.config.is_explicit_stage() || builder.top_stage >= 1 {
             builder.top_stage
@@ -129,14 +134,22 @@ impl Step for Miri {
         };
 
         if stage == 0 {
-            eprintln!("miri cannot be run at stage 0");
-            std::process::exit(1);
+            eprintln!("ERROR: miri cannot be run at stage 0");
+            exit!(1);
         }
 
-        // This compiler runs on the host, we'll just use it for the target.
-        let compilers = RustcPrivateCompilers::new(builder, stage, target);
-        let miri_build = builder.ensure(tool::Miri::from_compilers(compilers));
-        let host_compiler = miri_build.build_compiler;
+        // Miri always runs on the host, because it can interpret code for any target
+        let compilers = RustcPrivateCompilers::new(builder, stage, builder.host_target);
+
+        run.builder.ensure(Miri { compilers, target: run.target });
+    }
+
+    fn run(self, builder: &Builder<'_>) {
+        let host = builder.build.host_target;
+        let compilers = self.compilers;
+        let target = self.target;
+
+        builder.ensure(tool::Miri::from_compilers(compilers));
 
         // Get a target sysroot for Miri.
         let miri_sysroot =
@@ -147,7 +160,7 @@ impl Step for Miri {
         // add_rustc_lib_path does not add the path that contains librustc_driver-<...>.so.
         let mut miri = tool::prepare_tool_cargo(
             builder,
-            host_compiler,
+            compilers.build_compiler(),
             Mode::ToolRustc,
             host,
             Kind::Run,
@@ -167,6 +180,10 @@ impl Step for Miri {
 
         miri.into_cmd().run(builder);
     }
+
+    fn metadata(&self) -> Option<StepMetadata> {
+        Some(StepMetadata::run("miri", self.target).built_by(self.compilers.build_compiler()))
+    }
 }
 
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]

src/bootstrap/src/core/builder/mod.rs

@@ -182,6 +182,10 @@ impl StepMetadata {
         Self::new(name, target, Kind::Test)
     }
 
+    pub fn run(name: &str, target: TargetSelection) -> Self {
+        Self::new(name, target, Kind::Run)
+    }
+
     fn new(name: &str, target: TargetSelection, kind: Kind) -> Self {
         Self { name: name.to_string(), kind, target, built_by: None, stage: None, metadata: None }
     }

src/bootstrap/src/core/builder/tests.rs

@@ -2372,6 +2372,24 @@ mod snapshot {
         [dist] src <>
         ");
     }
+
+    // Check that `x run miri --target FOO` actually builds miri for the host.
+    #[test]
+    fn run_miri() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("run")
+                .path("miri")
+                .stage(1)
+                .targets(&[TEST_TRIPLE_1])
+                .render_steps(), @r"
+        [build] llvm <host>
+        [build] rustc 0 <host> -> rustc 1 <host>
+        [build] rustc 0 <host> -> miri 1 <host>
+        [build] rustc 0 <host> -> cargo-miri 1 <host>
+        [run] rustc 0 <host> -> miri 1 <target1>
+        ");
+    }
 }
 
 struct ExecutedSteps {

src/bootstrap/src/core/config/config.rs

@@ -961,8 +961,8 @@ impl Config {
             Subcommand::Dist => flags_stage.or(build_dist_stage).unwrap_or(2),
             Subcommand::Install => flags_stage.or(build_install_stage).unwrap_or(2),
             Subcommand::Perf { .. } => flags_stage.unwrap_or(1),
-            // These are all bootstrap tools, which don't depend on the compiler.
-            // The stage we pass shouldn't matter, but use 0 just in case.
+            // Most of the run commands execute bootstrap tools, which don't depend on the compiler.
+            // Other commands listed here should always use bootstrap tools.
             Subcommand::Clean { .. }
             | Subcommand::Run { .. }
             | Subcommand::Setup { .. }