Respect --all-targets flag when checking the compiler

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

@@ -385,6 +385,16 @@ impl CommandLineStep for Rustc {
         for krate in &*self.crates {
             cargo.arg("-p").arg(krate);
         }
+        // When we run `x check compiler --all-targets`, then the `Rustc` step is executed in
+        // two "modes" - one with all in-tree rustc crates, and a second time with empty crates
+        // in `PrepareRustcRmetaSysroot`, to prepare .rmeta files for RustcPrivate tools.
+        // If we use `--all-targets` for both, then we will end up with a duplicated .rmeta file
+        // in the sysroot, which breaks everything.
+        // So we only use `--all-targets` for the default case where crates are empty.
+        // This will also be used when someone does `x check compiler/<rustc-crate>`.
+        if !self.crates.is_empty() && builder.sess.config.cmd.check_all_targets() {
+            cargo.arg("--all-targets");
+        }
 
         let _guard = builder.msg(
             self.check_kind.to_kind(),

src/bootstrap/src/core/config/flags.rs

@@ -739,6 +739,11 @@ impl Subcommand {
             _ => false,
         }
     }
+
+    /// Are we executing a `check --all-targets` command?
+    pub(crate) fn check_all_targets(&self) -> bool {
+        matches!(self, Subcommand::Check { all_targets: true, .. })
+    }
 }
 
 /// Returns the shell completion for a given shell, if the result differs from the current