Implement "if-available" option for download-ci-llvm

src/bootstrap/bootstrap.py

@@ -447,7 +447,8 @@ class RustBuild(object):
 
     def downloading_llvm(self):
         opt = self.get_toml('download-ci-llvm', 'llvm')
-        return opt == "true"
+        return opt == "true" \
+            or (opt == "if-available" and self.build == "x86_64-unknown-linux-gnu")
 
     def _download_stage0_helper(self, filename, pattern, tarball_suffix, date=None):
         if date is None:
@@ -892,7 +893,7 @@ class RustBuild(object):
         submodules_names = []
         for module in submodules:
             if module.endswith("llvm-project"):
-                if self.get_toml('llvm-config') or self.get_toml('download-ci-llvm') == 'true':
+                if self.get_toml('llvm-config') or self.downloading_llvm():
                     if self.get_toml('lld') != 'true':
                         continue
             check = self.check_submodule(module, slow_submodules)

src/bootstrap/config.rs

@@ -391,7 +391,7 @@ struct Llvm {
     use_libcxx: Option<bool>,
     use_linker: Option<String>,
     allow_old_toolchain: Option<bool>,
-    download_ci_llvm: Option<bool>,
+    download_ci_llvm: Option<StringOrBool>,
 }
 
 #[derive(Deserialize, Default, Clone, Merge)]
@@ -735,7 +735,14 @@ impl Config {
             set(&mut config.llvm_use_libcxx, llvm.use_libcxx);
             config.llvm_use_linker = llvm.use_linker.clone();
             config.llvm_allow_old_toolchain = llvm.allow_old_toolchain;
-            config.llvm_from_ci = llvm.download_ci_llvm.unwrap_or(false);
+            config.llvm_from_ci = match llvm.download_ci_llvm {
+                Some(StringOrBool::String(s)) => {
+                    assert!(s == "if-available", "unknown option `{}` for download-ci-llvm", s);
+                    config.build.triple == "x86_64-unknown-linux-gnu"
+                }
+                Some(StringOrBool::Bool(b)) => b,
+                None => false,
+            };
 
             if config.llvm_from_ci {
                 // None of the LLVM options, except assertions, are supported

src/bootstrap/defaults/config.compiler.toml

@@ -6,3 +6,8 @@
 debug-logging = true
 # This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
 incremental = true
+
+[llvm]
+# Will download LLVM from CI if available on your platform (Linux only for now)
+# https://github.com/rust-lang/rust/issues/77084 tracks support for more platforms
+download-ci-llvm = "if-available"

src/bootstrap/defaults/config.library.toml

@@ -8,3 +8,8 @@ bench-stage = 0
 [rust]
 # This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
 incremental = true
+
+[llvm]
+# Will download LLVM from CI if available on your platform (Linux only for now)
+# https://github.com/rust-lang/rust/issues/77084 tracks support for more platforms
+download-ci-llvm = "if-available"