_match: correct max_slice_length logic #37603
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The logic used to be wildly wrong, but before the HAIR patch its wrongness was hidden by another bug. Fixes rust-lang#37598.
| // | ||
| // This means that all patterns with length at least | ||
| // `max(max_fixed+1,max_prefix+max_suffix)` are equivalent, so we | ||
| // only need to check patterns from that length and below. |
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This is a good comment!
A minor note: I think it would be marginally improved by a code example, showing what (e.g.) max_fixed etc mean in Rust code.
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Actually, I am a bit confused -- why is it max_fixed_len + 1?
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It must be strictly larger than max_fixed, and >= max_fixed + max_suffix.
| // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies | ||
| // to exactly the subset `Pₜ` of `P` can be transformed to a slice | ||
| // `sₘ` for each sufficiently-large length `m` that applies to exactly | ||
| // the same subset of `P`. |
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This comment is now even more awesome =) But I admit I am kind of getting lost a bit. It would have been helpful (for me) to note that these are byte slices, and that -- effectively -- b"123" is equivalent to &[b'1', b'2', b'3'].
And then maybe some examples like:
match foo { // foo : &[u8]
b"123" => ..., // matches byte strings of length 3
b"1234" => ..., // matches byte strings of length 4
&[b'1', ..c, b'9'] => , // matches byte strings of length 2..infinity
} // here `max_slice_check` yields 5, as `max_fixed_len` is 4, `max_prefix_len` is 1, `max_suffix_len` is 1match foo { // foo : &[u8]
b"123" => ..., // matches byte strings of length 3
b"1234" => ..., // matches byte strings of length 4
&[b'1', b'2', ..c, b'7', b'8, b'9'] => , // matches byte strings of length 5..infinity
} // here `max_slice_check` again yields 5, as `max_fixed_len` is 4, `max_prefix_len` is 2, `max_suffix_len` is 3There was a problem hiding this comment.
These are not byte slices, in general (see examples with boolean slices and Option<()> slices).
If there is a byte literal, then as usual it is treated like the expanded byte slice.
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@bors r+ |
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📌 Commit 1dad4b6 has been approved by |
_match: correct max_slice_length logic The logic used to be wildly wrong, but before the HAIR patch its wrongness was in most cases hidden by another bug. Fixes #37598. r? @nikomatsakis
The logic used to be wildly wrong, but before the HAIR patch its wrongness was in most cases hidden by another bug.
Fixes #37598.
r? @nikomatsakis