fix: emit warnings as warnings when learning target info from rustc

This is a horrible hack,
but at least it unblocks people who pass `-Awarnings` via RUSTFLAGS.

A long-term solution is a better interface
between build systems and the compiler.
See the discussion on Zulip:
https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Improving.20communication.20interface.20between.20cargo.2Frustc

Fixes rust-lang#8010

src/cargo/core/compiler/build_context/target_info.rs

@@ -206,6 +206,10 @@ impl TargetInfo {
             process.arg("--print=crate-name"); // `___` as a delimiter.
             process.arg("--print=cfg");
 
+            // parse_crate_type() relies on "unsupported/unknown crate type" error message,
+            // so make warnings always emitted as warnings.
+            process.arg("-Wwarnings");
+
             let (output, error) = rustc
                 .cached_output(&process, extra_fingerprint)
                 .with_context(|| {

tests/testsuite/cross_compile.rs

@@ -1283,11 +1283,11 @@ fn always_emit_warnings_as_warnings_when_learning_target_info() {
     p.cargo("build -v --target")
         .env("RUSTFLAGS", "-Awarnings")
         .arg(&target)
-        .with_status(101)
         .with_stderr_data(str![[r#"
-[ERROR] output of --print=file-names missing when learning about target-specific information from rustc
-command was: `rustc - --crate-name ___ --print=file-names -Awarnings --target wasm32-unknown-unknown --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg`
-...
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name foo [..]-Awarnings[..]`
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
 "#]])
         .run();
 }