adjust miri sysroot determination

Author: RalfJung

src/bootstrap/test.rs

@@ -427,7 +427,7 @@ impl Step for Miri {
             // (We do this separately from the above so that when the setup actually
             // happens we get some output.)
             // We re-use the `cargo` from above.
-            cargo.arg("--env");
+            cargo.arg("--print-sysroot");
 
             // FIXME: Is there a way in which we can re-use the usual `run` helpers?
             let miri_sysroot = if builder.config.dry_run {
@@ -437,13 +437,11 @@ impl Step for Miri {
                 let out = cargo.output()
                     .expect("We already ran `cargo miri setup` before and that worked");
                 assert!(out.status.success(), "`cargo miri setup` returned with non-0 exit code");
-                // Output is "MIRI_SYSROOT=<str>\n".
+                // Output is "<sysroot>\n".
                 let stdout = String::from_utf8(out.stdout)
                     .expect("`cargo miri setup` stdout is not valid UTF-8");
-                let stdout = stdout.trim();
-                builder.verbose(&format!("`cargo miri setup --env` returned: {:?}", stdout));
-                let sysroot = stdout.splitn(2, '=')
-                    .nth(1).expect("`cargo miri setup` stdout did not contain '='");
+                let sysroot = stdout.trim_end();
+                builder.verbose(&format!("`cargo miri setup --print-sysroot` said: {:?}", sysroot));
                 sysroot.to_owned()
             };