Investigate `hints.mostly-unused`

[Arnavion]

https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/

Seems very appropriate for k8s-openapi.

[hints]
mostly-unused = true

Apparently this is also backward-compatible with old cargo that doesn't support the hint:

Note that setting a hint does not increase the Minimum Supported Rust Version (MSRV) of your crate. Hints are always ignored if not understood. So, you can safely set this hint immediately, without waiting for this feature to be stabilized, and users of nightly will immediately benefit (if they pass -Zprofile-hint-mostly-unused to cargo to enable the feature).

Need to test that, ie ensure that this does not trip up stable.

If this works it will also fix #77

[Arnavion]

Results of testing reported in https://github.com/rust-lang/cargo/issues/15644#issuecomment-3082202997 and copied below.

Essentially the crate build starts taking ~200ms, whereas without the hint debug builds take ~10s and release builds take ~20s.

Click here

I put [hints] mostly-unused = true in k8s-openapi's Cargo.toml and then did a clean build of its two internal test suite crates with cargo test -Zprofile-hint-mostly-unused --no-run --timings .... k8s-openapi is a very large crate (auto-generated bindings from Kubernetes' OpenAPI spec) but the tests use a very small fraction of its types. For the sake of reproducability, these were the steps:

1. Use this commit (nothing special about it, just latest master at the moment).

2. Add [hints] mostly-unused = true to the root Cargo.toml

3. Run:

   git clean -xffd &&
   time bash -c '
       # test suite 1
       (cd k8s-openapi-tests && cargo test -Zprofile-hint-mostly-unused --no-run --timings --features test_v1_33) &&
   
       # test suite 2
       (cd k8s-openapi-tests-macro-deps && cargo test -Zprofile-hint-mostly-unused --no-run --timings --features test_v1_33)
   '

Results, including the two --timings outputs:

• Without the hint, without -j 1: ~23s

  k8s-openapi v0.25.0 ; Total: 7.1s ; Codegen: 1.1s (16%) ; Features: default, schemars, std, v1_33
  k8s-openapi v0.25.0 ; Total: 7.1s ; Codegen: 1.1s (16%) ; Features: schemars, v1_33

• With the hint, without -j 1: ~20s

  k8s-openapi v0.25.0 ; Total: 6.0s ; Codegen: 0.2s (3%) ; Features: default, schemars, std, v1_33
  k8s-openapi v0.25.0 ; Total: 5.8s ; Codegen: 0.2s (3%) ; Features: schemars, v1_33

• Without the hint, with -j 1: ~58s

  k8s-openapi v0.25.0 ; Total: 10.5s ; Codegen: 4.7s (45%) ; Features: default, schemars, std, v1_33
  k8s-openapi v0.25.0 ; Total: 10.6s ; Codegen: 4.7s (45%) ; Features: schemars, v1_33

• With the hint, with -j 1: ~49s

  k8s-openapi v0.25.0 ; Total: 5.9s ; Codegen: 0.2s (3%) ; Features: default, schemars, std, v1_33
  k8s-openapi v0.25.0 ; Total: 5.9s ; Codegen: 0.2s (3%) ; Features: schemars, v1_33

• Without the hint, with -j 1, with --release: ~144s

  k8s-openapi v0.25.0 ; Total: 27.8s ; Codegen: 22.3s (80%) ; Features: default, schemars, std, v1_33
  k8s-openapi v0.25.0 ; Total: 27.7s ; Codegen: 22.1s (80%) ; Features: schemars, v1_33

• With the hint, with -j 1, with --release: ~101s

  k8s-openapi v0.25.0 ; Total: 5.5s ; Codegen: 0.1s (2%) ; Features: default, schemars, std, v1_33
  k8s-openapi v0.25.0 ; Total: 5.6s ; Codegen: 0.1s (2%) ; Features: schemars, v1_33

---

$ cargo --version

cargo 1.90.0-nightly (6833aa715 2025-07-13)

$ rustc -vV

rustc 1.90.0-nightly (5795086bd 2025-07-16)
binary: rustc
commit-hash: 5795086bdfe7ed988aa53a110bd0692c33d8755b
commit-date: 2025-07-16
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.8