x.py build fails with `download-ci-llvm = true`

[jyn514]

I just started running into this recently - maybe llvm-dwp stopped being packaged recently? I think the fix is just to make it optional in bootstrap.

$ x.py build
Copying stage0 rustc from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Assembling stage1 compiler (x86_64-unknown-linux-gnu)
thread 'main' panicked at 'src.symlink_metadata() failed with No such file or directory (os error 2)', src/bootstrap/lib.rs:1216:24

Backtrace

stack backtrace:
   0: rust_begin_unwind
             at /rustc/21dea46d8347c8b4117c5567949703f0fbb51649/library/std/src/panicking.rs:495:5
   1: std::panicking::begin_panic_fmt
             at /rustc/21dea46d8347c8b4117c5567949703f0fbb51649/library/std/src/panicking.rs:437:5
   2: bootstrap::Build::copy
             at ./src/bootstrap/lib.rs:1216:24
   3: <bootstrap::compile::Assemble as bootstrap::builder::Step>::run
             at ./src/bootstrap/compile.rs:990:13
   4: bootstrap::builder::Builder::ensure
             at ./src/bootstrap/builder.rs:1474:23
   5: bootstrap::builder::Builder::compiler
             at ./src/bootstrap/builder.rs:578:9
   6: <bootstrap::compile::Std as bootstrap::builder::Step>::make_run
             at ./src/bootstrap/compile.rs:48:23
   7: bootstrap::builder::StepDescription::maybe_run
             at ./src/bootstrap/builder.rs:179:13
   8: bootstrap::builder::StepDescription::run
             at ./src/bootstrap/builder.rs:200:25
   9: bootstrap::builder::Builder::run_step_descriptions
             at ./src/bootstrap/builder.rs:570:9
  10: bootstrap::builder::Builder::execute_cli
             at ./src/bootstrap/builder.rs:561:9
  11: bootstrap::Build::build
             at ./src/bootstrap/lib.rs:509:13
  12: bootstrap::main
             at ./src/bootstrap/bin/main.rs:30:5
  13: core::ops::function::FnOnce::call_once
             at /rustc/21dea46d8347c8b4117c5567949703f0fbb51649/library/core/src/ops/function.rs:227:5

A workaround is touch build/x86_64-unknown-linux-gnu/ci-llvm/bin/llvm-dwp.

[jyn514]

git merge-base HEAD master reports 2ba7ca2.

[jyn514]

This is likely a regression from #77117, cc @davidtwco

[tgnottingham]

Hitting this despite doing rm -r build and updating to latest. The workaround works though.

[jyn514]

Yup, this still fails for me after b32e6e6.

[Mark-Simulacrum]

Oh, we forgot to bump the stamp file so we're not downloading a fresh copy.

[Mark-Simulacrum]

https://github.com/rust-lang/rust/blob/master/src/bootstrap/download-ci-llvm-stamp should be bumped in a PR to fix this.