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

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.

Unsure if this affects people's workflow, so this behavior is only
enabled on nightly for collectin feedback. If everything goes well,
the nightly gate will be removed after a few of releases.

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

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 build_runner.bcx.gctx.nightly_features_allowed {
+        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 !build_runner.bcx.gctx.nightly_features_allowed {
+        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 build_runner.bcx.gctx.nightly_features_allowed {
+        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 !build_runner.bcx.gctx.nightly_features_allowed {
+        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`,

tests/testsuite/rustc.rs

@@ -819,13 +819,12 @@ fn precedence() {
 
     p.cargo("rustc --release -v -- --cfg cargo_rustc -C strip=symbols")
         .env("RUSTFLAGS", "--cfg from_rustflags")
-        .with_stderr_data(
-            str![[r#"
+        .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`
+[RUNNING] `rustc [..]-C strip=debuginfo [..]--cfg cargo_rustc -C strip=symbols --cfg from_rustflags`
 [FINISHED] `release` profile [optimized] target(s) in [ELAPSED]s
 
-"#]]
-        )
+"#]])
         .run();
 }