Fix update_rust when there's no fallback

The version is a combination of the rust version and clang version
concat'd together.

Bug: 1409334
Change-Id: Iac89e4f50b42fb18d4988e091f94535795cfc38c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4188796
Reviewed-by: Adrian Taylor <[email protected]>
Commit-Queue: danakj <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1095645}
NOKEYCHECK=True
GitOrigin-RevId: ad0573e0592046e182166a123f297ef65d0862a0

Author: danakj

update_rust.py

@@ -75,14 +75,19 @@
 
 
 # Package version built in build_rust.py, which is always built against the
-# latest Clang and never uses the FALLBACK_CLANG_VERSION.
+# current Clang. Typically Clang and Rust revisions are both updated together
+# and this picks the Clang that has just been built.
 def GetPackageVersionForBuild():
     from update import (CLANG_REVISION, CLANG_SUB_REVISION)
-    return GetPackageVersion(f'{CLANG_REVISION}-{CLANG_SUB_REVISION}')
+    return (f'{RUST_REVISION}-{RUST_SUB_REVISION}'
+            '-{CLANG_REVISION}-{CLANG_SUB_REVISION}')
 
 
-# Package version for download, which may differ from GetUploadPackageVersion()
-# if FALLBACK_CLANG_VERSION is set.
+# Package version for download. Ideally this is the latest Clang+Rust roll,
+# which was built successfully and is returned from GetPackageVersionForBuild().
+# However at this time Clang rolls even if Rust fails to build, so we have Rust
+# pinned to the last known successful build with FALLBACK_REVISION. This should
+# go away once we block Clang rolls on Rust also being built.
 def GetDownloadPackageVersion():
     return FALLBACK_REVISION \
         if FALLBACK_REVISION else GetPackageVersionForBuild()