Make download-ci-llvm = true check if CI llvm is available

and make it the default for the compiler profile, as to prevent
unnecessarily checking out `src/llvm-project` with `"if-unchanged"`.

config.example.toml

@@ -42,6 +42,9 @@
 # Unless you're developing for a target where Rust CI doesn't build a compiler
 # toolchain or changing LLVM locally, you probably want to leave this enabled.
 #
+# Set this to `true` to download if CI llvm available otherwise it builds
+# from `src/llvm-project`.
+#
 # Set this to `"if-unchanged"` to download only if the llvm-project has not
 # been modified. You can also use this if you are unsure whether you're on a
 # tier 1 target. All tier 1 targets are currently supported.

src/bootstrap/defaults/config.compiler.toml

@@ -27,4 +27,5 @@ assertions = false
 # Enable warnings during the LLVM compilation (when LLVM is changed, causing a compilation)
 enable-warnings = true
 # Will download LLVM from CI if available on your platform.
-download-ci-llvm = "if-unchanged"
+# If you intend to modify `src/llvm-project`, use `"if-unchanged"` or `false` instead.
+download-ci-llvm = true

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

@@ -2781,7 +2781,8 @@ impl Config {
                     );
                 }
 
-                b
+                // If download-ci-llvm=true we also want to check that CI llvm is available
+                b && llvm::is_ci_llvm_available(self, asserts)
             }
             Some(StringOrBool::String(s)) if s == "if-unchanged" => if_unchanged(),
             Some(StringOrBool::String(other)) => {

src/bootstrap/src/utils/change_tracker.rs

@@ -250,4 +250,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "New option `llvm.enzyme` to control whether the llvm based autodiff tool (Enzyme) is built.",
     },
+    ChangeInfo {
+        change_id: 129473,
+        severity: ChangeSeverity::Warning,
+        summary: "`download-ci-llvm = true` now checks if CI llvm is available and has become the default for the compiler profile",
+    },
 ];