Rollup merge of #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 #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 #59706

r? @eddyb

Note that this doesn't actually remove tables from libcore, to avoid conflict with https://github.com/rust-lang/rust/pull/62641.

cc https://github.com/unicode-rs/unicode-xid/pull/11

</details>

1 files changed, 1 insertions, 2 deletions

diff --git a/src/libsyntax/ext/proc_macro_server.rs b/src/libsyntax/ext/proc_macro_server.rs
index 1a26b17dac7..544ec789d80 100644
--- a/src/libsyntax/ext/proc_macro_server.rs
+++ b/src/libsyntax/ext/proc_macro_server.rs
@@ -322,8 +322,7 @@ impl Ident {
     fn is_valid(string: &str) -> bool {
         let mut chars = string.chars();
         if let Some(start) = chars.next() {
-            (start == '_' || start.is_xid_start())
-                && chars.all(|cont| cont == '_' || cont.is_xid_continue())
+            rustc_lexer::is_id_start(start) && chars.all(rustc_lexer::is_id_continue)
         } else {
             false
         }