fix lifetime error and comment

tests/run-make/reset-codegen-1/rmake.rs

@@ -1,7 +1,8 @@
-// When rustc receives 4 codegen-units, an output path and an emit flag all simultaneously,
+// When rustc received 4 codegen-units, an output path and an emit flag all simultaneously,
 // this could cause an annoying recompilation issue, uselessly lengthening the build process.
-// This test tries to reproduce this issue with varied emit flags - if it passes, the codegen-units
-// are being appropriately reset to 1 after each pass.
+// A fix was delivered, which resets codegen-units to 1 when necessary,
+// but as it directly affected the way codegen-units are manipulated,
+// this test was created to check that this fix did not cause compilation failures.
 // See https://github.com/rust-lang/rust/issues/30063
 
 //@ ignore-cross-compile
@@ -10,7 +11,8 @@ use run_make_support::{rustc, tmp_dir};
 use std::fs;
 
 fn compile(emit: Option<&str>, output_dir: &str) {
-    let mut rustc = rustc().codegen_units(4).output(tmp_dir().join(output_dir)).input("foo.rs");
+    let rustc = rustc();
+    let rustc = rustc.codegen_units(4).output(tmp_dir().join(output_dir)).input("foo.rs");
     if let Some(emit) = emit {
         rustc.emit(emit);
     }