Fix checking of LLVM prebuilt status

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

@@ -1393,7 +1393,7 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
     if builder.config.llvm_enabled(target) {
         let building_llvm_is_expensive = prebuilt_llvm_output(builder, target).is_none();
 
-        let skip_llvm = (builder.kind == Kind::Check) && building_llvm_is_expensive;
+        let skip_llvm = (cargo.kind() == Kind::Check) && building_llvm_is_expensive;
         if !skip_llvm {
             rustc_llvm_env(builder, cargo, target)
         }

src/bootstrap/src/core/build_steps/llvm.rs

@@ -168,7 +168,13 @@ pub fn prebuilt_llvm_output(builder: &Builder<'_>, target: TargetSelection) -> O
             kind: LlvmKind::External,
         });
     }
-    None
+
+    // If LLVM is not available from CI, not externally, it is still possible that it was already
+    // built locally before. In that case we still treat it as prebuilt config.
+    match get_locally_built_llvm_build_status(builder, target) {
+        LlvmBuildStatus::AlreadyBuilt(output) => Some(output),
+        LlvmBuildStatus::ShouldBuild(_) => None,
+    }
 }
 
 /// This returns whether we've already previously built LLVM.
@@ -186,6 +192,16 @@ pub fn get_llvm_build_status(builder: &Builder<'_>, target: TargetSelection) ->
     // If submodules are disabled, this does nothing.
     builder.config.update_submodule("src/llvm-project");
 
+    get_locally_built_llvm_build_status(builder, target)
+}
+
+/// Return build status of LLVM, considering only the (possibly) locally built LLVM.
+///
+/// Calling this function should never attempt to checkout the LLVM submodule.
+fn get_locally_built_llvm_build_status(
+    builder: &Builder<'_>,
+    target: TargetSelection,
+) -> LlvmBuildStatus {
     let out_dir = builder.llvm_out(target);
 
     let build_llvm_config = if let Some(build_llvm_config) = builder

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

@@ -152,6 +152,7 @@ pub(crate) struct Cargo {
     build_compiler_stage: u32,
     extra_rustflags: Vec<String>,
     profile: Option<&'static str>,
+    kind: Kind,
 }
 
 impl Cargo {
@@ -203,6 +204,10 @@ impl Cargo {
         self.into()
     }
 
+    pub(crate) fn kind(&self) -> Kind {
+        self.kind
+    }
+
     /// Same as [`Cargo::new`] except this one doesn't configure the linker with
     /// [`Cargo::configure_linker`].
     #[track_caller]
@@ -1530,6 +1535,7 @@ impl Builder<'_> {
             build_compiler_stage,
             extra_rustflags,
             profile,
+            kind: cmd_kind,
         }
     }
 }

src/bootstrap/src/core/builder/tests.rs

@@ -2704,8 +2704,7 @@ mod snapshot {
             ctx.config("clippy")
                 .path("miri")
                 .stage(1)
-                .render_steps(), @r"
-        [build] llvm <host>
+                .render_steps(), @"
         [check] rustc 0 <host> -> rustc 1 <host>
         [clippy] rustc 0 <host> -> miri 1 <host>
         ");