Extract/validate packaging tool versions at compile time #241
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We store the versions of the packaging tools used by the buildpack (such as pip, setuptools and wheel) in requirements files in the repo, so that Dependabot can update them. Whilst we can easily include the contents of those files at compile time using `include_str!`, the buildpack actually needs the version substring rather than the whole requirement specifier (such as `foo==1.2.3`). Until now this extraction/validation of this version substring has been performed at runtime, which required using `.expect()` to ignore the "technically fallible but never going to fail for users in practice" result. Now, we use a `const fn` so this extraction/validation can be performed at compile time, allowing us to define these pinned versions as actual `const`s. In addition to compile time checks being preferable to those at runtime, this change will allow us to simplify/remove the `PackagingToolVersions` struct in future PRs. The features we can use in `const fn`s are limited - we can't use: - most methods on `str` - iterators (including for loops) - most methods on `Option` or `Result`, including `expect` or `unwrap*` As such, we convert to bytes and use a while/match pattern to iterate through the string - similar to some of the Rust stdlib `const fn` implementations: https://github.com/rust-lang/rust/blob/6a2cd0d50c9b7e1243d948641758c76d1f22e25e/library/core/src/slice/ascii.rs#L127-L139 The minimum Rust version has also been bumped, since `str::trim_ascii` only became a `const fn` in Rust 1.80.
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We store the versions of the packaging tools used by the buildpack (such as pip, setuptools and wheel) in requirements files in the repo, so that Dependabot can update them.
Whilst we can easily include the contents of those files at compile time using
include_str!, the buildpack actually needs the version substring rather than the whole requirement specifier (the1.2.3fromfoo==1.2.3).Until now the extraction/validation of this version substring has been performed at runtime, which required using
.expect()to ignore the "technically fallible but never going to fail for users in practice" result.Now, we use a
const fnso this extraction/validation can be performed at compile time, allowing us to define these pinned versions as actualconsts. In addition to compile time checks being preferable to those at runtime, this change will allow us to simplify/remove thePackagingToolVersionsstruct in future PRs in favour of referencing Rust constants.The features we can use in
const fns are limited - we can't use:str, includingsplitOptionorResult, includingexpectorunwrap*As such, we convert to bytes and use a while/match pattern to iterate through the string - similar to some of the Rust stdlib
const fnimplementations:https://github.com/rust-lang/rust/blob/6a2cd0d50c9b7e1243d948641758c76d1f22e25e/library/core/src/slice/ascii.rs#L127-L139
The minimum Rust version has also been bumped, since
str::trim_asciionly became aconst fnin Rust 1.80.GUS-W-16436670.