show a better error when running Miri with the wrong sysroot

cargo-miri/bin.rs

@@ -374,12 +374,15 @@ fn setup(subcommand: MiriCommand) {
         }
         None => {
             // Check for `rust-src` rustup component.
-            let sysroot = miri()
-                .args(&["--print", "sysroot"])
-                .output()
-                .expect("failed to determine sysroot")
-                .stdout;
-            let sysroot = std::str::from_utf8(&sysroot).unwrap();
+            let output =
+                miri().args(&["--print", "sysroot"]).output().expect("failed to determine sysroot");
+            if !output.status.success() {
+                show_error(format!(
+                    "Failed to determine sysroot; Miri said:\n{}",
+                    String::from_utf8_lossy(&output.stderr).trim_end()
+                ));
+            }
+            let sysroot = std::str::from_utf8(&output.stdout).unwrap();
             let sysroot = Path::new(sysroot.trim_end_matches('\n'));
             // Check for `$SYSROOT/lib/rustlib/src/rust/library`; test if that contains `std/Cargo.toml`.
             let rustup_src =

src/bin/miri.rs

@@ -1,5 +1,5 @@
 #![feature(rustc_private, stmt_expr_attributes)]
-#![allow(clippy::manual_range_contains)]
+#![allow(clippy::manual_range_contains, clippy::useless_format)]
 
 extern crate rustc_data_structures;
 extern crate rustc_driver;
@@ -143,6 +143,11 @@ impl rustc_driver::Callbacks for MiriBeRustCompilerCalls {
     }
 }
 
+fn show_error(msg: String) -> ! {
+    eprintln!("fatal error: {}", msg);
+    std::process::exit(1)
+}
+
 fn init_early_loggers() {
     // Note that our `extern crate log` is *not* the same as rustc's; as a result, we have to
     // initialize them both, and we always initialize `miri`'s first.
@@ -214,13 +219,26 @@ fn compile_time_sysroot() -> Option<String> {
     let home = option_env!("RUSTUP_HOME").or(option_env!("MULTIRUST_HOME"));
     let toolchain = option_env!("RUSTUP_TOOLCHAIN").or(option_env!("MULTIRUST_TOOLCHAIN"));
     Some(match (home, toolchain) {
-        (Some(home), Some(toolchain)) => format!("{}/toolchains/{}", home, toolchain),
-        _ =>
-            option_env!("RUST_SYSROOT")
-                .expect(
+        (Some(home), Some(toolchain)) => {
+            // Check that at runtime, we are still in this toolchain.
+            let toolchain_runtime =
+                env::var_os("RUSTUP_TOOLCHAIN").or_else(|| env::var_os("MULTIRUST_TOOLCHAIN"));
+            if !matches!(toolchain_runtime, Some(r) if r == toolchain) {
+                show_error(format!(
+                    "This Miri got built with local toolchain `{toolchain}`, but now is being run under a different toolchain. \n\
+                    Make sure to run Miri in the toolchain it got built with, e.g. via `cargo +{toolchain} miri`."
+                ));
+            }
+
+            format!("{}/toolchains/{}", home, toolchain)
+        }
+        _ => option_env!("RUST_SYSROOT")
+            .unwrap_or_else(|| {
+                show_error(format!(
                     "To build Miri without rustup, set the `RUST_SYSROOT` env var at build time",
-                )
-                .to_owned(),
+                ))
+            })
+            .to_owned(),
     })
 }