Chapter 20: fixups, the answers reindeer cannot work out

Cargo does a few things quietly that buck2 will not do unless told. The biggest is the build script: a build.rs file some crates ship, which cargo compiles and runs before the crate itself, to probe the compiler, find a system library or generate code. reindeer (chapter 19) can see that a crate has one, but not whether its output matters here, so it asks. Each folder here is the answer for one crate, in a fixups.toml.

Most answers are one line. Some are not:

# fixups/alsa-sys/fixups.toml
# alsa-sys links libasound at BUILD time (every other windowing/audio crate
# here dlopens at run time). Its build script finds the library with
# pkg-config and says so in cargo:rustc-link-lib / rustc-link-search, which
# buck2's buildscript runner drops unless asked to keep them - leaving
# snd_pcm_* undefined at the final link.
[buildscript.run]
rustc_link_lib = true
rustc_link_search = true

Aside: the one that is not about a build script. fixups/subsecond/ gives the hot-patching crate (chapter 27) its own -Cdebug-assertions=yes. Under cargo, one profile setting covers the whole dependency graph; under buck2 every third-party crate is its own compiler invocation, and subsecond's patch machinery is a silent no-op unless its own compilation has debug assertions on. Without this file a patch reports "applied" and nothing changes.

Try it. ls third-party/rust/fixups lists every crate that needed telling something (49 of them), and cat third-party/rust/fixups/subsecond/fixups.toml is the one with the longest story.

For the people who maintain it

In this folder

Path	What
`<crate>/fixups.toml`	One folder per crate, named exactly as the crate (reindeer looks them up by name): `buildscript.run = true`/`false`, link directives to keep, extra `rustc_flags`.
README.md	This chapter. reindeer looks fixups up by crate name only, so this file and CLAUDE.md are never read as one.
CLAUDE.md	How to answer a new crate, for agents.

The per-crate folders get no chapters of their own: they are one small config file each, sharded by crate name for reindeer, with no idea to teach beyond this page.

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