rustfmt leaves behind a too-long line inside a closure

Here's some example code:

impl<'tcx> LateLintPass<'tcx> for UnqualifiedLocalImports {
    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
        let hir::ItemKind::Use(path, _kind) = item.kind else { return };
        let is_local_import = |res: &Res| {
            matches!(res, hir::def::Res::Def(def_kind, def_id) if def_id.is_local() && !matches!(def_kind, DefKind::Macro(_)))
        };
    }
}

It's not the best code, that line is too long. So let's format it. Now we have:

impl<'tcx> LateLintPass<'tcx> for UnqualifiedLocalImports {
    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
        let hir::ItemKind::Use(path, _kind) = item.kind else {
            return;
        };
        let is_local_import = |res: &Res| matches!(res, hir::def::Res::Def(def_kind, def_id) if def_id.is_local() && !matches!(def_kind, DefKind::Macro(_)));
    }
}

Now the line is even longer! It's way past the 100 character limit.

I guess rustfmt is reluctant to introduce linebreaks inside a macro, but then it should just avoid touching this code -- but instead it makes things worse by putting the entire closure on a single line.

Start by formatting the Rust snippet from the issue and compare the resulting closure and line length with the 100-character limit. Trace rustfmt's handling of closures containing matches! macros, then add a regression test for this input. Done means formatting no longer produces the longer single-line closure or otherwise violates the stated limit.