Add custom = <function> container attribute and allow optional ignore = "<reason>" for fields
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See the module docs. Basically, we allow bypassing the built-in parsing to customize one or more attributes. If you're going to customize all of them, then there's no point in using the derive macro, however I found there are cases where all but one attribute can be derived, and it stinks to resort to needing to manually re-implement when that happens.
Allows someone to leave behind a comment for why something is ignored as well as if someone uses `ignore = "custom"` we'll enforce that the struct must have a `cache_diff(custom = <function>)` defined on the struct.
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Trying to get version resolution with the build packs-ruby crate to work with manually specified git
```
cache_diff = { features = ["bullet_stream"], branch = "schneems/custom_fn", git = "https://github.com/heroku-buildpacks/cache_diff" }
```
Not sure if it will work.
Even though the derive macro knows which attributes it owns:
```
#[proc_macro_derive(CacheDiff, attributes(cache_diff))]
pub fn cache_diff(item: TokenStream) -> TokenStream {
//...
}
```
It passes in the full AST. The non-cache-diff attributes must be filtered out. This is how the `CacheDiffAttributes` (which holds Field attributes) is implemented.
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This was referenced Jan 27, 2025
Malax
approved these changes
Jan 29, 2025
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#[cache_diff(custom = <function>)]to containers (structs) to allow for customizing + deriving diffs.#[cache_diff(ignore = "<reason>")]. Usingignore = "custom"requires the container (struct) to implementcustom = <function>.For a real-life test drive you can see heroku/buildpacks-go#327 which uses this branch. Overall I'm pretty confident and happy with the state of this PR and feel it's ready for merge pending user feedback or doc fixups.
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