Remove `visit_subpats` parameter from `check_pat`

[varkor]

The visit_subpats parameter is a hack (to fix a problem with range patterns) that should be refactored. The idea is that the state should be contained in EllipsisInclusiveRangePatterns instead.

This should be straightforward to fix.

1. Remove the &mut bool parameter from
   

   rust/src/librustc/lint/mod.rs

   Line 374 in 6ed6f14

   fn check_pat(a: &ast::Pat, b: &mut bool); // FIXME: &mut bool looks just broken

   
   (You'll also need to update the signature of check_pat in src/librustc_lint/unused.rs.
2. Add a check_pat_post method with an identical signature below it.
3. Remove the visit_subpats condition from

   rust/src/librustc/lint/context.rs

   Lines 1166 to 1173 in 6ed6f14

   	fn visit_pat(&mut self, p: &'a ast::Pat) {
   	let mut visit_subpats = true;
   	run_early_pass!(self, check_pat, p, &mut visit_subpats);
   	self.check_id(p.id);
   	if visit_subpats {
   	ast_visit::walk_pat(self, p);
   	}
   	}

4. Run an early lint pass for check_pat_post at the end.
5. Add a Option<NodeId> field to EllipsisInclusiveRangePatterns in

   rust/src/librustc_lint/builtin.rs

   Line 1289 in 6ed6f14

   declare_lint_pass!(EllipsisInclusiveRangePatterns => [ELLIPSIS_INCLUSIVE_RANGE_PATTERNS]);

   
   (Look at DeprecatedAttr for an example of a lint with fields.)
   You'll also need to initialise it with None here:
   

   rust/src/librustc_lint/lib.rs

   Line 97 in 6ed6f14

   EllipsisInclusiveRangePatterns: EllipsisInclusiveRangePatterns,

6. This should be replaced with an assignment of the new field in EllipsisInclusiveRangePatterns to pat.id:
   

   rust/src/librustc_lint/builtin.rs

   Line 1315 in 6ed6f14

   *visit_subpats = false;

7. check_pat should be early-exited if the field is Some(..).
8. Add a check_pat_post implementation to EllipsisInclusiveRangePatterns and reset the field to None if the pattern ID matches the field ID.
9. Make sure the UI tests still pass with no changes.

[stepnivlk]

hi @varkor, I'm happy to take care of this one if it's not taken already.

[varkor]

@stepnivlk: go ahead! Let me know if you have any questions or problems with the implementation! :)