Auto merge of #998 - RalfJung:comment, r=RalfJung

explain our shell encoding

Follow-up to https://github.com/rust-lang/miri/pull/980

src/bin/cargo-miri.rs

@@ -340,6 +340,11 @@ path = "lib.rs"
     let sysroot = if is_host { dir.join("HOST") } else { PathBuf::from(dir) };
     std::env::set_var("MIRI_SYSROOT", &sysroot); // pass the env var to the processes we spawn, which will turn it into "--sysroot" flags
     if print_env {
+        // Escape an arbitrary string for the shell: by wrapping it in `'`, the only special
+        // character we have to worry about is `'` itself. Everything else is taken literally
+        // in these strings. `'` is encoded as `'"'"'`: the outer `'` end and being a
+        // `'`-quoted string, respectively; the `"'"` in the middle represents a single `'`.
+        // (We could use `'\''` instead of `'"'"'` if we wanted but let's avoid backslashes.)
         println!("MIRI_SYSROOT='{}'", sysroot.display().to_string().replace('\'', r#"'"'"'"#));
     } else if !ask_user {
         println!("A libstd for Miri is now available in `{}`.", sysroot.display());