Auto merge of #14587 - weihanglo:cargo-rustc, r=epage

fix(cargo-rustc): give trailing flags higher precedence on nightly

### What does this PR try to resolve?

Previously `cargo rustc -- <flags>` got a lower precedence than
some of the flags set by cargo internal.
This is a bit unintuitive as Cargo generally treats user-provided
CLI flags with the highest priority.

This commit changes `cargo rustc -- <flags>` to a higher precedence:
higher than most of flags set by Cargo, and only lower than
`build.rustflags` family.

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

Unsure if this affects people's workflow, so this behavior is only
enabled on nightly for collectin feedback. A environment variable
`__CARGO_RUSTC_ORIG_ARGS_PRIO=1` is provided for users to opt-out.
If everything goes well, the nightly gate will be removed after a
few of releases.

### Additional information

See discussion on https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/rustflags.20precendence.20of.20.60cargo.20rustc.60

Fixes #14346

src/cargo/core/compiler/mod.rs

@@ -694,6 +694,11 @@ fn prepare_rustc(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> CargoResult
     base.inherit_jobserver(&build_runner.jobserver);
     build_deps_args(&mut base, build_runner, unit)?;
     add_cap_lints(build_runner.bcx, unit, &mut base);
+    if cargo_rustc_higher_args_precedence(build_runner) {
+        if let Some(args) = build_runner.bcx.extra_args_for(unit) {
+            base.args(args);
+        }
+    }
     base.args(&unit.rustflags);
     if build_runner.bcx.gctx.cli_unstable().binary_dep_depinfo {
         base.arg("-Z").arg("binary-dep-depinfo");
@@ -753,8 +758,11 @@ fn prepare_rustdoc(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> CargoResu
     }
 
     rustdoc.args(unit.pkg.manifest().lint_rustflags());
-    if let Some(args) = build_runner.bcx.extra_args_for(unit) {
-        rustdoc.args(args);
+
+    if !cargo_rustc_higher_args_precedence(build_runner) {
+        if let Some(args) = build_runner.bcx.extra_args_for(unit) {
+            rustdoc.args(args);
+        }
     }
 
     let metadata = build_runner.metadata_for_doc_units[unit];
@@ -795,6 +803,11 @@ fn prepare_rustdoc(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> CargoResu
 
     rustdoc::add_output_format(build_runner, unit, &mut rustdoc)?;
 
+    if cargo_rustc_higher_args_precedence(build_runner) {
+        if let Some(args) = build_runner.bcx.extra_args_for(unit) {
+            rustdoc.args(args);
+        }
+    }
     rustdoc.args(&unit.rustdocflags);
 
     if !crate_version_flag_already_present(&rustdoc) {
@@ -1097,8 +1110,10 @@ fn build_base_args(
 
     cmd.args(unit.pkg.manifest().lint_rustflags());
     cmd.args(&profile_rustflags);
-    if let Some(args) = build_runner.bcx.extra_args_for(unit) {
-        cmd.args(args);
+    if !cargo_rustc_higher_args_precedence(build_runner) {
+        if let Some(args) = build_runner.bcx.extra_args_for(unit) {
+            cmd.args(args);
+        }
     }
 
     // `-C overflow-checks` is implied by the setting of `-C debug-assertions`,
@@ -1969,3 +1984,19 @@ fn scrape_output_path(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> CargoR
         .outputs(unit)
         .map(|outputs| outputs[0].path.clone())
 }
+
+/// Provides a way to change the precedence of `cargo rustc -- <flags>`.
+///
+/// This is intended to be a short-live function.
+///
+/// See <https://github.com/rust-lang/cargo/issues/14346>
+fn cargo_rustc_higher_args_precedence(build_runner: &BuildRunner<'_, '_>) -> bool {
+    build_runner.bcx.gctx.nightly_features_allowed
+        && build_runner
+            .bcx
+            .gctx
+            .get_env("__CARGO_RUSTC_ORIG_ARGS_PRIO")
+            .ok()
+            .as_deref()
+            != Some("1")
+}

tests/testsuite/rustc.rs

@@ -795,3 +795,66 @@ windows
         )
         .run();
 }
+
+#[cargo_test]
+fn precedence() {
+    // Ensure that the precedence of cargo-rustc is only lower than RUSTFLAGS,
+    // but higher than most flags set by cargo.
+    //
+    // See rust-lang/cargo#14346
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            edition = "2021"
+
+            [lints.rust]
+            unexpected_cfgs = "allow"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("rustc --release -v -- --cfg cargo_rustc -C strip=symbols")
+        .env("RUSTFLAGS", "--cfg from_rustflags")
+        .masquerade_as_nightly_cargo(&["cargo-rustc-precedence"])
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[RUNNING] `rustc [..]-C strip=debuginfo [..]--cfg cargo_rustc -C strip=symbols --cfg from_rustflags`
+[FINISHED] `release` profile [optimized] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+
+    // Ensure the short-live env var to work
+    p.cargo("clean").run();
+    p.cargo("rustc --release -v -- --cfg cargo_rustc -C strip=symbols")
+        .env("RUSTFLAGS", "--cfg from_rustflags")
+        .env("__CARGO_RUSTC_ORIG_ARGS_PRIO", "1")
+        .masquerade_as_nightly_cargo(&["cargo-rustc-precedence"])
+        .with_stderr_data(
+            str![[r#"
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[RUNNING] `rustc [..]--cfg cargo_rustc -C strip=symbols [..]-C strip=debuginfo [..]--cfg from_rustflags`
+[FINISHED] `release` profile [optimized] target(s) in [ELAPSED]s
+
+"#]]
+        )
+        .run();
+
+    // Ensure non-nightly to work as before
+    p.cargo("clean").run();
+    p.cargo("rustc --release -v -- --cfg cargo_rustc -C strip=symbols")
+        .env("RUSTFLAGS", "--cfg from_rustflags")
+        .with_stderr_data(
+            str![[r#"
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[RUNNING] `rustc [..]--cfg cargo_rustc -C strip=symbols [..]-C strip=debuginfo [..]--cfg from_rustflags`
+[FINISHED] `release` profile [optimized] target(s) in [ELAPSED]s
+
+"#]]
+        )
+        .run();
+}