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Rollup merge of rust-lang#62848 - matklad:xid-unicode, r=petrochenkov
Use unicode-xid crate instead of libcore This PR proposes to remove `char::is_xid_start` and `char::is_xid_continue` functions from `libcore` and use `unicode_xid` crate from crates.io (note that this crate is already present in rust-lang/rust's Cargo.lock). Reasons to do this: * removing rustc-binary-specific stuff from libcore * making sure that, across the ecosystem, there's a single definition of what rust identifier is (`unicode-xid` has almost 10 million downs, as a `proc_macro2` dependency) * making it easier to share `rustc_lexer` crate with rust-analyzer: no need to `#[cfg]` if we are building as a part of the compiler Reasons not to do this: * increased maintenance burden: we'll need to upgrade unicode version both in libcore and in unicode-xid. However, this shouldn't be a too heavy burden: just running `./unicode.py` after new unicode version. I (@matklad) am ready to be a t-compiler side maintainer of unicode-xid. Moreover, given that xid-unicode is an important dependency of syn, *someone* needs to maintain it anyway. * xid-unicode implementation is significantly slower. It uses a more compact table with binary search, instead of a trie. However, this shouldn't matter in practice, because we have fast-path for ascii anyway, and code size savings is a plus. Moreover, in rust-lang#59706 not using libcore turned out to be *faster*, presumably beacause checking for whitespace with match is even faster. <details> <summary>old description</summary> Followup to rust-lang#59706 r? @eddyb Note that this doesn't actually remove tables from libcore, to avoid conflict with rust-lang#62641. cc unicode-rs/unicode-xid#11 </details>
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