Get the sysroot (like compiletest) in Miri itself.

src/bin/miri.rs

@@ -113,12 +113,6 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
     err.emit();
 }
 
-fn main() {
-    init_logger();
-    let args: Vec<String> = std::env::args().collect();
-    rustc_driver::run_compiler(&args, &mut MiriCompilerCalls);
-}
-
 fn init_logger() {
     const NSPACES: usize = 40;
     let format = |record: &log::LogRecord| {
@@ -142,3 +136,28 @@ fn init_logger() {
 
     builder.init().unwrap();
 }
+
+fn find_sysroot() -> String {
+    // Taken from https://github.com/Manishearth/rust-clippy/pull/911.
+    let home = option_env!("RUSTUP_HOME").or(option_env!("MULTIRUST_HOME"));
+    let toolchain = option_env!("RUSTUP_TOOLCHAIN").or(option_env!("MULTIRUST_TOOLCHAIN"));
+    match (home, toolchain) {
+        (Some(home), Some(toolchain)) => format!("{}/toolchains/{}", home, toolchain),
+        _ => option_env!("RUST_SYSROOT")
+            .expect("need to specify RUST_SYSROOT env var or use rustup or multirust")
+            .to_owned(),
+    }
+}
+
+fn main() {
+    init_logger();
+    let mut args: Vec<String> = std::env::args().collect();
+
+    let sysroot_flag = String::from("--sysroot");
+    if !args.contains(&sysroot_flag) {
+        args.push(sysroot_flag);
+        args.push(find_sysroot());
+    }
+
+    rustc_driver::run_compiler(&args, &mut MiriCompilerCalls);
+}