Module wrapper 

Fat-build runner + captured-args loader.

The other half of the rustc/linker hijack started in I4g-4a. whisker-rustc-shim writes a JSON file per rustc invocation into a cache dir; this module:

1. Spawns the fat build — a normal cargo build with RUSTC_WORKSPACE_WRAPPER=whisker-rustc-shim set, so the cache fills up.
2. Loads those JSON files back into a HashMap<String, CapturedRustcInvocation> keyed by crate name, picking the most recent timestamp when a crate was rebuilt mid-session.
3. (Future, I4g-5) hands the captured args to a thin-rebuild driver that only recompiles the changed crate and re-links.

CapturedRustcInvocation is currently defined here, not in whisker-cli, so that the shim binary doesn’t need to pull in the whole dev-server dep tree (tokio / axum / notify / object). The shim has its own copy of the struct shape; serde keeps the wire format compatible. A future cleanup will extract a tiny whisker-hotpatch-types crate and dedupe both sides — see TODO.

Structs

CapturedLinkerInvocation

Mirrors whisker_cli::linker_shim::CapturedLinkerInvocation exactly. Same duplication rationale as CapturedRustcInvocation.

CapturedRustcInvocation

Mirrors whisker_cli::rustc_shim::CapturedRustcInvocation exactly. Kept duplicated (rather than imported) so the shim binary stays dep-light. JSON wire format is what binds them — both sides go through serde, so a field rename in one without the other will trip the deserialize step at run time and emit a clear error.

LinkerCaptureConfig

Optional linker-shim wiring for run_fat_build. Provide all three when you want the linker invocation captured (Tier 1 needs it); leave this None for plain Tier 2 / Tier 0 fat builds.

Functions

default_cache_dir

Convenience: best-effort default cache dir under the workspace’s target/.whisker/rustc-args/. Created on demand.

default_linker_cache_dir

Counterpart of default_cache_dir for the linker side: target/.whisker/linker-args/. Created on demand.

keep_newest

Pure helper for the load loop’s “keep most-recent per crate” decision. Pulled out so unit tests don’t have to write JSON to disk to exercise the merge.

keep_newest_linker

Pure helper for load_captured_linker_args — same “keep most-recent per key” pattern as keep_newest, but the key is the output filename rather than a crate name.

load_captured_args

Walk cache_dir, deserialise every *.json produced by whisker-rustc-shim, and collapse duplicates per crate by keeping the most-recent timestamp. Empty / unparseable files are skipped with a warning rather than aborting the whole load — a partial fat build shouldn’t take the dev loop down.

load_captured_linker_args

Walk a whisker-linker-shim cache dir and collapse duplicates per output filename, keeping the most-recent timestamp. Same shape as load_captured_args but for the linker side. Empty / unparseable files get a warning, not an abort.

resolve_host_linker

Resolve the system linker driver we want to forward to from the shim. Same logic the integration test uses, lifted into one place so production and tests agree:

run_fat_build

Spawn a cargo build for the given target with RUSTC_WORKSPACE_WRAPPER pointed at shim_path and WHISKER_RUSTC_CACHE_DIR pointed at cache_dir. Inherits stdout/stderr so cargo’s progress is visible. After the build completes successfully, load_captured_args can read the cache.