Avoid literal CURRENT_RUSTC_VERSION string

[Mark-Simulacrum]

See

rust-clippy/clippy_dev/src/parse.rs

Line 505 in f91e93e

if s.bytes().all(|c| matches!(c, b'0'..=b'9' | b'.')) || matches!(s, "pre 1.29.0" | "CURRENT_RUSTC_VERSION") {

This gets replaced by https://github.com/rust-lang/rust/blob/5c543b0b8c73c7b72bc8284ced4fb22ead15734d/src/tools/replace-version-placeholder/src/main.rs#L25 -- if we can avoid the literal there by e.g. checking a prefix + suffix or similar, that would be good. Otherwise we'll accidentally replace this and probably miss it in PR review eventually.

[Jarcho]

Replacing it is "fine" (nothing will break) since this is parsing clippy's source and we don't actually use CURRENT_RUSTC_VERSION yet. The PR to actually use this also fixes the problem.

[Mark-Simulacrum]

Sounds good. I'll skip "fixing" the replaced string on beta then. Thanks!