bootstrap: add Builder::rustc_cmd that includes the lib path

When building with `rust.rpath = false`, every `rustc` invocation needs
to include the library path as well. I particularly ran into this in
`generate_target_spec_json_schema` when testing 1.91-beta in Fedora,
where we do disable rpath for our system builds. The new helper function
will hopefully encourage the right thing going forward.

(cherry picked from commit 03cdcb5cd5)

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

@@ -519,7 +519,7 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, cargo: &mut Car
         // Query rustc for the deployment target, and the associated env var.
         // The env var is one of the standard `*_DEPLOYMENT_TARGET` vars, i.e.
         // `MACOSX_DEPLOYMENT_TARGET`, `IPHONEOS_DEPLOYMENT_TARGET`, etc.
-        let mut cmd = command(builder.rustc(cargo.compiler()));
+        let mut cmd = builder.rustc_cmd(cargo.compiler());
         cmd.arg("--target").arg(target.rustc_target_arg());
         cmd.arg("--print=deployment-target");
         let output = cmd.run_capture_stdout(builder).stdout();

src/bootstrap/src/core/build_steps/dist.rs

@@ -625,7 +625,7 @@ fn generate_target_spec_json_schema(builder: &Builder<'_>, sysroot: &Path) {
     // We do this by using the stage 1 compiler, which is always compiled for the host,
     // even in a cross build.
     let stage1_host = builder.compiler(1, builder.host_target);
-    let mut rustc = command(builder.rustc(stage1_host)).fail_fast();
+    let mut rustc = builder.rustc_cmd(stage1_host).fail_fast();
     rustc
         .env("RUSTC_BOOTSTRAP", "1")
         .args(["--print=target-spec-json-schema", "-Zunstable-options"]);

src/bootstrap/src/core/build_steps/synthetic_targets.rs

@@ -10,7 +10,6 @@
 use crate::Compiler;
 use crate::core::builder::{Builder, ShouldRun, Step};
 use crate::core::config::TargetSelection;
-use crate::utils::exec::command;
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub(crate) struct MirOptPanicAbortSyntheticTarget {
@@ -55,7 +54,7 @@ fn create_synthetic_target(
         return TargetSelection::create_synthetic(&name, path.to_str().unwrap());
     }
 
-    let mut cmd = command(builder.rustc(compiler));
+    let mut cmd = builder.rustc_cmd(compiler);
     cmd.arg("--target").arg(base.rustc_target_arg());
     cmd.args(["-Zunstable-options", "--print", "target-spec-json"]);
 

src/bootstrap/src/core/builder/cargo.rs

@@ -722,10 +722,8 @@ impl Builder<'_> {
                 // Build proc macros both for the host and the target unless proc-macros are not
                 // supported by the target.
                 if target != compiler.host && cmd_kind != Kind::Check {
-                    let mut rustc_cmd = command(self.rustc(compiler));
-                    self.add_rustc_lib_path(compiler, &mut rustc_cmd);
-
-                    let error = rustc_cmd
+                    let error = self
+                        .rustc_cmd(compiler)
                         .arg("--target")
                         .arg(target.rustc_target_arg())
                         .arg("--print=file-names")

src/bootstrap/src/core/builder/mod.rs

@@ -1591,6 +1591,14 @@ Alternatively, you can set `build.local-rebuild=true` and use a stage0 compiler
         }
     }
 
+    /// Gets a command to run the compiler specified, including the dynamic library
+    /// path in case the executable has not been build with `rpath` enabled.
+    pub fn rustc_cmd(&self, compiler: Compiler) -> BootstrapCommand {
+        let mut cmd = command(self.rustc(compiler));
+        self.add_rustc_lib_path(compiler, &mut cmd);
+        cmd
+    }
+
     /// Gets the paths to all of the compiler's codegen backends.
     fn codegen_backends(&self, compiler: Compiler) -> impl Iterator<Item = PathBuf> {
         fs::read_dir(self.sysroot_codegen_backends(compiler))