Support non-stage0 check

Author: Mark-Simulacrum

src/bootstrap/check.rs

@@ -73,7 +73,7 @@ impl Step for Std {
 
     fn run(self, builder: &Builder<'_>) {
         let target = self.target;
-        let compiler = builder.compiler(0, builder.config.build);
+        let compiler = builder.compiler(builder.top_stage, builder.config.build);
 
         let mut cargo = builder.cargo(
             compiler,
@@ -94,9 +94,13 @@ impl Step for Std {
             true,
         );
 
-        let libdir = builder.sysroot_libdir(compiler, target);
-        let hostdir = builder.sysroot_libdir(compiler, compiler.host);
-        add_to_sysroot(&builder, &libdir, &hostdir, &libstd_stamp(builder, compiler, target));
+        // We skip populating the sysroot in non-zero stage because that'll lead
+        // to rlib/rmeta conflicts if std gets built during this session.
+        if compiler.stage == 0 {
+            let libdir = builder.sysroot_libdir(compiler, target);
+            let hostdir = builder.sysroot_libdir(compiler, compiler.host);
+            add_to_sysroot(&builder, &libdir, &hostdir, &libstd_stamp(builder, compiler, target));
+        }
 
         // Then run cargo again, once we've put the rmeta files for the library
         // crates into the sysroot. This is needed because e.g., core's tests
@@ -163,10 +167,20 @@ impl Step for Rustc {
     /// the `compiler` targeting the `target` architecture. The artifacts
     /// created will also be linked into the sysroot directory.
     fn run(self, builder: &Builder<'_>) {
-        let compiler = builder.compiler(0, builder.config.build);
+        let compiler = builder.compiler(builder.top_stage, builder.config.build);
         let target = self.target;
 
-        builder.ensure(Std { target });
+        if compiler.stage != 0 {
+            // If we're not in stage 0, then we won't have a std from the beta
+            // compiler around. That means we need to make sure there's one in
+            // the sysroot for the compiler to find. Otherwise, we're going to
+            // fail when building crates that need to generate code (e.g., build
+            // scripts and their dependencies).
+            builder.ensure(crate::compile::Std { target: compiler.host, compiler });
+            builder.ensure(crate::compile::Std { target, compiler });
+        } else {
+            builder.ensure(Std { target });
+        }
 
         let mut cargo = builder.cargo(
             compiler,
@@ -225,7 +239,7 @@ impl Step for CodegenBackend {
     }
 
     fn run(self, builder: &Builder<'_>) {
-        let compiler = builder.compiler(0, builder.config.build);
+        let compiler = builder.compiler(builder.top_stage, builder.config.build);
         let target = self.target;
         let backend = self.backend;
 
@@ -280,7 +294,7 @@ macro_rules! tool_check_step {
             }
 
             fn run(self, builder: &Builder<'_>) {
-                let compiler = builder.compiler(0, builder.config.build);
+                let compiler = builder.compiler(builder.top_stage, builder.config.build);
                 let target = self.target;
 
                 builder.ensure(Rustc { target });

src/bootstrap/flags.rs

@@ -614,14 +614,10 @@ Arguments:
         };
 
         if let Subcommand::Check { .. } = &cmd {
-            if matches.opt_str("stage").is_some() {
-                println!("--stage not supported for x.py check, always treated as stage 0");
-                process::exit(1);
-            }
             if matches.opt_str("keep-stage").is_some()
                 || matches.opt_str("keep-stage-std").is_some()
             {
-                println!("--keep-stage not supported for x.py check, only one stage available");
+                println!("--keep-stage not yet supported for x.py check");
                 process::exit(1);
             }
         }