fix: set CARGO env var during rustc -vV probe

The -vV probe goes through RUSTC_WRAPPER and RUSTC_WORKSPACE_WRAPPER
since rust-lang#13659, but the CARGO env var was not set. Tools like sccache
depend on CARGO to detect the workspace wrapper pattern in their
compiler detection logic.

Closes rust-lang#16805

Author: zhibisora

src/cargo/util/rustc.rs

@@ -63,6 +63,7 @@ impl Rustc {
             .wrapped(workspace_wrapper.as_ref())
             .wrapped(wrapper.as_deref());
         apply_env_config(gctx, &mut cmd)?;
+        cmd.env(crate::CARGO_ENV, gctx.cargo_exe()?);
         cmd.arg("-vV");
         let verbose_version = cache.cached_output(&cmd, 0)?.0;
 

tests/testsuite/build.rs

@@ -4581,8 +4581,8 @@ fn rustc_wrapper_vv_probe_has_cargo_env() {
 
     let result = std::fs::read_to_string(marker_dir.join("cargo_env_in_vv")).unwrap();
     assert_eq!(
-        result, "false",
-        "CARGO env var should not be set during -vV probe without the fix"
+        result, "true",
+        "CARGO env var should be set during -vV probe for wrapper compatibility"
     );
 }