Add all tier 1 platforms to supported platforms for "if-available"

... and update the comment in `config.toml.example`

config.toml.example

@@ -35,9 +35,11 @@ changelog-seen = 2
 # Unless you're developing for a target where Rust CI doesn't build a compiler
 # toolchain or changing LLVM locally, you probably want to set this to true.
 #
-# It's currently false by default due to being newly added; please file bugs if
-# enabling this did not work for you on x86_64-unknown-linux-gnu.
-# Other target triples are currently not supported; see #77084.
+# This is false by default so that distributions don't unexpectedly download
+# LLVM from the internet.
+#
+# All tier 1 targets are currently supported; set this to `"if-supported"` if
+# you are not sure whether you're on a tier 1 target.
 #
 # We also currently only support this when building LLVM for the build triple.
 #

src/bootstrap/bootstrap.py

@@ -465,8 +465,21 @@ class RustBuild(object):
 
     def downloading_llvm(self):
         opt = self.get_toml('download-ci-llvm', 'llvm')
+        # This is currently all tier 1 targets (since others may not have CI
+        # artifacts)
+        # https://doc.rust-lang.org/rustc/platform-support.html#tier-1
+        supported_platforms = [
+            "aarch64-unknown-linux-gnu",
+            "i686-pc-windows-gnu",
+            "i686-pc-windows-msvc",
+            "i686-unknown-linux-gnu",
+            "x86_64-unknown-linux-gnu",
+            "x86_64-apple-darwin",
+            "x86_64-pc-windows-gnu",
+            "x86_64-pc-windows-msvc",
+        ]
         return opt == "true" \
-            or (opt == "if-available" and self.build == "x86_64-unknown-linux-gnu")
+            or (opt == "if-available" and self.build in supported_platforms)
 
     def _download_stage0_helper(self, filename, pattern, tarball_suffix, date=None):
         if date is None: