Don't unconditionally build alloc for no-std targets

It's possible for targets to only support `core` and not `alloc`.
Instead of building alloc unconditionally, pass a list of crates to
build into `std_cargo`, and only pass `-p alloc` if the list of crates
wasn't already filtered to a subset.

The original use case was to reuse `std_cargo` for a rustc_driver that
doesn't emit metadata. But this seems like a reasonable change
regardless.

Author: jyn514

src/bootstrap/src/core/build_steps/check.rs

@@ -61,6 +61,9 @@ impl Step for Std {
             return;
         }
 
+        // Explicitly pass -p for all dependencies crates -- this will force cargo
+        // to also check the tests/benches/examples for these crates, rather
+        // than just the leaf crate.
         let crates = std_crates_for_run_make(&run);
         run.builder.ensure(Std {
             build_compiler: prepare_compiler_for_check(run.builder, run.target, Mode::Std)
@@ -83,16 +86,12 @@ impl Step for Std {
             Kind::Check,
         );
 
-        std_cargo(builder, target, &mut cargo);
+        std_cargo(builder, target, &mut cargo, &self.crates);
         if matches!(builder.config.cmd, Subcommand::Fix) {
             // By default, cargo tries to fix all targets. Tell it not to fix tests until we've added `test` to the sysroot.
             cargo.arg("--lib");
         }
 
-        for krate in &*self.crates {
-            cargo.arg("-p").arg(krate);
-        }
-
         let _guard = builder.msg(
             Kind::Check,
             format_args!("library artifacts{}", crate_description(&self.crates)),
@@ -135,14 +134,7 @@ impl Step for Std {
             Kind::Check,
         );
 
-        std_cargo(builder, target, &mut cargo);
-
-        // Explicitly pass -p for all dependencies krates -- this will force cargo
-        // to also check the tests/benches/examples for these crates, rather
-        // than just the leaf crate.
-        for krate in &*self.crates {
-            cargo.arg("-p").arg(krate);
-        }
+        std_cargo(builder, target, &mut cargo, &self.crates);
 
         let stamp =
             build_stamp::libstd_stamp(builder, build_compiler, target).with_prefix("check-test");

src/bootstrap/src/core/build_steps/clippy.rs

@@ -195,11 +195,7 @@ impl Step for Std {
             Kind::Clippy,
         );
 
-        std_cargo(builder, target, &mut cargo);
-
-        for krate in &*self.crates {
-            cargo.arg("-p").arg(krate);
-        }
+        std_cargo(builder, target, &mut cargo, &self.crates);
 
         let _guard = builder.msg(
             Kind::Clippy,

src/bootstrap/src/core/build_steps/compile.rs

@@ -266,10 +266,7 @@ impl Step for Std {
                 target,
                 Kind::Build,
             );
-            std_cargo(builder, target, &mut cargo);
-            for krate in &*self.crates {
-                cargo.arg("-p").arg(krate);
-            }
+            std_cargo(builder, target, &mut cargo, &self.crates);
             cargo
         };
 
@@ -497,7 +494,12 @@ fn compiler_rt_for_profiler(builder: &Builder<'_>) -> PathBuf {
 
 /// Configure cargo to compile the standard library, adding appropriate env vars
 /// and such.
-pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, cargo: &mut Cargo) {
+pub fn std_cargo(
+    builder: &Builder<'_>,
+    target: TargetSelection,
+    cargo: &mut Cargo,
+    crates: &[String],
+) {
     // rustc already ensures that it builds with the minimum deployment
     // target, so ideally we shouldn't need to do anything here.
     //
@@ -605,6 +607,10 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, cargo: &mut Car
         cargo.env("CFG_DISABLE_UNSTABLE_FEATURES", "1");
     }
 
+    for krate in crates {
+        cargo.args(["-p", krate]);
+    }
+
     let mut features = String::new();
 
     if builder.no_std(target) == Some(true) {
@@ -614,8 +620,10 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, cargo: &mut Car
         }
 
         // for no-std targets we only compile a few no_std crates
+        if crates.is_empty() {
+            cargo.args(["-p", "alloc"]);
+        }
         cargo
-            .args(["-p", "alloc"])
             .arg("--manifest-path")
             .arg(builder.src.join("library/alloc/Cargo.toml"))
             .arg("--features")

src/bootstrap/src/core/build_steps/doc.rs

@@ -783,7 +783,7 @@ fn doc_std(
         Kind::Doc,
     );
 
-    compile::std_cargo(builder, target, &mut cargo);
+    compile::std_cargo(builder, target, &mut cargo, requested_crates);
     cargo
         .arg("--no-deps")
         .arg("--target-dir")
@@ -803,10 +803,6 @@ fn doc_std(
         cargo.rustdocflag("--document-private-items").rustdocflag("--document-hidden-items");
     }
 
-    for krate in requested_crates {
-        cargo.arg("-p").arg(krate);
-    }
-
     let description =
         format!("library{} in {} format", crate_description(requested_crates), format.as_str());
     let _guard = builder.msg(Kind::Doc, description, Mode::Std, build_compiler, target);

src/bootstrap/src/core/build_steps/test.rs

@@ -2963,7 +2963,7 @@ impl Step for Crate {
                         .arg("--manifest-path")
                         .arg(builder.src.join("library/sysroot/Cargo.toml"));
                 } else {
-                    compile::std_cargo(builder, target, &mut cargo);
+                    compile::std_cargo(builder, target, &mut cargo, &[]);
                 }
             }
             Mode::Rustc => {