rustbuild: LLVM is not downloaded from CI after LLVM updates

[petrochenkov]

I tried this code:

./x build

download-ci-llvm in config.toml is not specified explicitly, the default value is used.

I expected to see this happen:

• LLVM is downloaded from CI, when LLVM version is bumped the new LLVM is downloaded from CI as well.

Instead, this happened:

• LLVM starts building from source code after an LLVM update, instead of being downloaded, unless I remove the build directory and do a fresh build, then it starts downloading from CI again.

Meta

rustc --version --verbose:

$ ./build/x86_64-pc-windows-gnu/stage0/bin/rustc --version --verbose
rustc 1.78.0-beta.1 (efd9d2df1 2024-03-18)
binary: rustc
commit-hash: efd9d2df12b5e17fac0b4d0fb89f612ecd79f259
commit-date: 2024-03-18
host: x86_64-pc-windows-gnu
release: 1.78.0-beta.1
LLVM version: 18.1.2

[petrochenkov]

Or maybe it's a stage0 toolchain update that triggers this issue, not LLVM update (I observed this today).
But I've seen this issue several times in the past as well.
cc @onur-ozkan

[petrochenkov]

The [llvm] section in config.toml is completely empty, nothing is overridden.

[petrochenkov]

Update: LLVM is built from source only on the first build attempt, after that it is correctly downloaded from CI.

[petrochenkov]

@onur-ozkan
This is still an issue that I observe regularly.

[onur-ozkan]

There were some issues with the change detection logic for LLVM and I think 2b5a982 should fix it. Let's see if it still happens with the next LLVM update.