Description of problem:
librsvg2 hits several "error: proc-macro derive panicked" when building with Rust 1.66.

Version-Release number of selected component (if applicable):
librsvg2-2.42.7-4.el8

How reproducible:
100%

Steps to Reproduce:
1. Upgrade to Rust 1.66 (bug 2123899)
2. Rebuild the librsvg2 rpm.

Actual results:
Several errors such as this:

error: proc-macro derive panicked
--> /builddir/build/BUILD/librsvg-2.42.7/vendor/cssparser/src/macros.rs:39:17

39 | cssparser_internal_assert_ascii_lowercase_max_len!( $( $match_body )* )

::: /builddir/build/BUILD/librsvg-2.42.7/vendor/cssparser/src/color.rs:213:31

213 | let degrees = match_ignore_ascii_case!

{ &*unit, | _______________________________- 214 | | "deg" => v, 215 | | "grad" => v * 360. / 400., 216 | | "rad" => v * 360. / (2. * PI), 217 | | "turn" => v * 360., 218 | | _ => return Err(location.new_unexpected_token_error(Token::Ident(unit.clone()))), 219 | | }

;

= help: message: expected suffix ").0," not found in "#[allow(unused)] enum ProceduralMasqueradeDummyType\n

{\n Input =\n (0, stringify!\n (\"deg\" => v, \"grad\" => v * 360. / 400., \"rad\" => v * 360. / (2. * PI),\n \"turn\" => v * 360., _ => return\n Err(location.new_unexpected_token_error(Token ::\n Ident(unit.clone()))),)).0\n}

"
= note: this error originates in the macro `match_ignore_ascii_case` (in Nightly builds, run with -Z macro-backtrace for more info)

Expected results:
A successful rebuild.

Additional info:
The errors are in the vendored cssparser crate, but they actually stem from its dependency on procedural-masquerade 0.1.5, which made improper assumptions about macro tokenization, fixed in 0.1.7. The compiler has been maintaining a crude hack to avoid breaking this, until folks could upgrade their dependencies, and I have attached the future-incompatibilities report from Rust 1.62 for reference. Upstream has reduced the scope of that compiler hack as of Rust 1.66.