Use in-tree Cargo when testing stage 2+ cargo miri

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

@@ -805,6 +805,20 @@ fn run(self, builder: &Builder<'_>) {
             &[],
         );
 
+        // If we are testing stage 2+ cargo miri, make sure that it works with the in-tree cargo.
+        // We want to do this *somewhere* to ensure that Miri + nightly cargo actually works.
+        if stage >= 2 {
+            let built_cargo = builder
+                .ensure(tool::Cargo::from_build_compiler(
+                    // Build stage 1 cargo here, we don't need it to be built in any special way,
+                    // just that it is built from in-tree sources.
+                    builder.compiler(0, builder.host_target),
+                    builder.host_target,
+                ))
+                .tool_path;
+            cargo.env("CARGO", built_cargo);
+        }
+
         // We're not using `prepare_cargo_test` so we have to do this ourselves.
         // (We're not using that as the test-cargo-miri crate is not known to bootstrap.)
         match builder.doc_tests {

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

@@ -3079,6 +3079,45 @@ fn install_extended() {
         ");
     }
 
+    #[test]
+    fn cargo_miri_stage_1() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("test")
+                .args(&["cargo-miri"])
+                .stage(1)
+                .get_steps()
+                .render(), @"
+        [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>
+        [build] rustdoc 1 <host>
+        [build] rustc 1 <host> -> std 1 <host>
+        ");
+    }
+
+    #[test]
+    fn cargo_miri_stage_2() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("test")
+                .args(&["cargo-miri"])
+                .stage(2)
+                .get_steps()
+                .render(), @"
+        [build] llvm <host>
+        [build] rustc 0 <host> -> rustc 1 <host>
+        [build] rustc 1 <host> -> std 1 <host>
+        [build] rustc 1 <host> -> rustc 2 <host>
+        [build] rustc 1 <host> -> miri 2 <host>
+        [build] rustc 1 <host> -> cargo-miri 2 <host>
+        [build] rustdoc 2 <host>
+        [build] rustc 2 <host> -> std 2 <host>
+        [build] rustc 0 <host> -> cargo 1 <host>
+        ");
+    }
+
     // Check that `x run miri --target FOO` actually builds miri for the host.
     #[test]
     fn run_miri() {