fix: emit warnings as warnings when learning rust target info (#15036)

### What does this PR try to resolve?

This is a horrible hack,
which lets the rustc invocation for learning target info always emit
warnings as warnings.
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/cargo#8010

### How should we test and review this PR?

Ensure `CFG_DISABLE_CROSS_TESTS` is not set,
and run `cargo t --test testsuite
always_emit_warnings_as_warnings_when_learning_target_info`

This also additionally adds `wasm32-unknown-unknown` target to Cargo's
CI.

### Additional information

.github/workflows/main.yml

@@ -161,6 +161,7 @@ jobs:
     - run: rustup update --no-self-update stable
     - run: rustup update --no-self-update ${{ matrix.rust }} && rustup default ${{ matrix.rust }}
     - run: rustup target add ${{ matrix.other }}
+    - run: rustup target add wasm32-unknown-unknown
     - run: rustup target add aarch64-unknown-none # need this for build-std mock tests
       if: startsWith(matrix.rust, 'nightly')
     - run: rustup component add rustc-dev llvm-tools-preview rust-docs
@@ -225,6 +226,7 @@ jobs:
       - uses: actions/checkout@v4
       - run: rustup update --no-self-update stable && rustup default stable
       - run: rustup target add i686-unknown-linux-gnu
+      - run: rustup target add wasm32-unknown-unknown
       - run: sudo apt update -y && sudo apt install gcc-multilib libsecret-1-0 libsecret-1-dev -y
       - run: rustup component add rustfmt || echo "rustfmt not available"
       - run: cargo test -p cargo

Cargo.lock

@@ -456,7 +456,7 @@ version = "0.4.0"
 
 [[package]]
 name = "cargo-test-support"
-version = "0.7.0"
+version = "0.7.1"
 dependencies = [
  "anstream",
  "anstyle",

Cargo.toml

@@ -33,7 +33,7 @@ cargo-credential-macos-keychain = { version = "0.4.7", path = "credential/cargo-
 cargo-credential-wincred = { version = "0.4.7", path = "credential/cargo-credential-wincred" }
 cargo-platform = { path = "crates/cargo-platform", version = "0.2.0" }
 cargo-test-macro = { version = "0.4.0", path = "crates/cargo-test-macro" }
-cargo-test-support = { version = "0.7.0", path = "crates/cargo-test-support" }
+cargo-test-support = { version = "0.7.1", path = "crates/cargo-test-support" }
 cargo-util = { version = "0.2.14", path = "crates/cargo-util" }
 cargo-util-schemas = { version = "0.7.3", path = "crates/cargo-util-schemas" }
 cargo_metadata = "0.19.0"

crates/cargo-test-support/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cargo-test-support"
-version = "0.7.0"
+version = "0.7.1"
 edition.workspace = true
 rust-version = "1.83"  # MSRV:1
 license.workspace = true

crates/cargo-test-support/src/cross_compile.rs

@@ -267,3 +267,33 @@ pub fn can_run_on_host() -> bool {
         return true;
     }
 }
+
+/// Check if the given target has been installed.
+///
+/// Generally [`disabled`] should be used to check if cross-compilation is allowed.
+/// And [`alternate`] to get the cross target.
+///
+/// You should only use this as a last resort to skip tests,
+/// because it doesn't report skipped tests as ignored.
+pub fn requires_target_installed(target: &str) -> bool {
+    let has_target = std::process::Command::new("rustup")
+        .args(["target", "list", "--installed"])
+        .output()
+        .ok()
+        .map(|output| {
+            String::from_utf8(output.stdout)
+                .map(|stdout| stdout.contains(target))
+                .unwrap_or_default()
+        })
+        .unwrap_or_default();
+    if !has_target {
+        let msg =
+            format!("to run this test, run `rustup target add {target} --toolchain <toolchain>`",);
+        if cargo_util::is_ci() {
+            panic!("{msg}");
+        } else {
+            eprintln!("{msg}");
+        }
+    }
+    has_target
+}

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

@@ -1260,3 +1260,38 @@ fn doctest_xcompile_linker() {
 "#]])
         .run();
 }
+
+#[cargo_test]
+fn always_emit_warnings_as_warnings_when_learning_target_info() {
+    if cross_compile::disabled() {
+        return;
+    }
+
+    let target = "wasm32-unknown-unknown";
+    if !cross_compile::requires_target_installed(target) {
+        return;
+    }
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                edition = "2015"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("build -v --target")
+        .env("RUSTFLAGS", "-Awarnings")
+        .arg(target)
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name foo [..]-Awarnings[..]`
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}

tests/testsuite/standard_lib.rs

@@ -6,6 +6,7 @@
 
 use std::path::{Path, PathBuf};
 
+use cargo_test_support::cross_compile;
 use cargo_test_support::prelude::*;
 use cargo_test_support::registry::{Dependency, Package};
 use cargo_test_support::ProjectBuilder;
@@ -391,24 +392,8 @@ fn check_core() {
 
 #[cargo_test(build_std_mock)]
 fn build_std_with_no_arg_for_core_only_target() {
-    let has_rustup_aarch64_unknown_none = std::process::Command::new("rustup")
+    let target = "aarch64-unknown-none";
-        .args(["target", "list", "--installed"])
+    if !cross_compile::requires_target_installed(target) {
-        .output()
-        .ok()
-        .map(|output| {
-            String::from_utf8(output.stdout)
-                .map(|stdout| stdout.contains("aarch64-unknown-none"))
-                .unwrap_or_default()
-        })
-        .unwrap_or_default();
-    if !has_rustup_aarch64_unknown_none {
-        let msg =
-            "to run this test, run `rustup target add aarch64-unknown-none --toolchain nightly`";
-        if cargo_util::is_ci() {
-            panic!("{msg}");
-        } else {
-            eprintln!("{msg}");
-        }
         return;
     }
 
@@ -427,7 +412,8 @@ fn build_std_with_no_arg_for_core_only_target() {
         .build();
 
     p.cargo("build -v")
-        .arg("--target=aarch64-unknown-none")
+        .arg("--target")
+        .arg(target)
         .build_std(&setup)
         .with_stderr_data(
             str![[r#"
@@ -457,7 +443,8 @@ fn build_std_with_no_arg_for_core_only_target() {
     // Note that we don't  download std dependencies for the second call
     // because `-Zbuild-std` downloads them all also when building for core only.
     p.cargo("build -v")
-        .arg("--target=aarch64-unknown-none")
+        .arg("--target")
+        .arg(target)
         .target_host()
         .build_std(&setup)
         .with_stderr_data(