compiletest: Don't pass --out-dir if the compile flags include -o

src/tools/compiletest/src/runtest.rs

@@ -2254,6 +2254,9 @@ fn make_compile_args(
         }
 
         match output_file {
+            // If the test's compile flags specify an output path with `-o`,
+            // avoid a compiler warning about `--out-dir` being ignored.
+            _ if self.props.compile_flags.iter().any(|flag| flag == "-o") => {}
             TargetLocation::ThisFile(path) => {
                 rustc.arg("-o").arg(path);
             }

tests/ui/io-checks/non-ice-error-on-worker-io-fail.stderr

@@ -1,6 +1,4 @@
-warning: ignoring --out-dir flag due to -o flag
-
 error: io error modifying ./does-not-exist/
 
-error: aborting due to 1 previous error; 1 warning emitted
+error: aborting due to 1 previous error