Rollup merge of #42874 - zackmdavis:overzealous_by_outer_forbid, r=nikomatsakis

only set "overruled by outer forbid" once for lint groups, by group name

Previously, conflicting forbid/allow attributes for a lint group would
result in a separate "allow(L) overruled by outer forbid(L)" error for
every lint L in the group. This was needlessly and annoyingly verbose;
we prefer to just have one error pointing out the conflicting
attributes.

(Also, while we're touching context.rs, clean up some unused arguments.)

Resolves #42873.

src/test/ui/lint/outer-forbid.rs

@@ -0,0 +1,22 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Forbidding a group (here, `unused`) overrules subsequent allowance of both
+// the group, and an individual lint in the group (here, `unused_variables`);
+// and, forbidding an individual lint (here, `non_snake_case`) overrules
+// subsequent allowance of a lint group containing it (here, `bad_style`). See
+// Issue #42873.
+
+#![forbid(unused, non_snake_case)]
+
+#[allow(unused, unused_variables, bad_style)]
+fn main() {
+    println!("hello forbidden world")
+}

src/test/ui/lint/outer-forbid.stderr

@@ -0,0 +1,29 @@
+error[E0453]: allow(unused) overruled by outer forbid(unused)
+  --> $DIR/outer-forbid.rs:19:9
+   |
+17 | #![forbid(unused, non_snake_case)]
+   |           ------ `forbid` level set here
+18 | 
+19 | #[allow(unused, unused_variables, bad_style)]
+   |         ^^^^^^ overruled by previous forbid
+
+error[E0453]: allow(unused_variables) overruled by outer forbid(unused)
+  --> $DIR/outer-forbid.rs:19:17
+   |
+17 | #![forbid(unused, non_snake_case)]
+   |           ------ `forbid` level set here
+18 | 
+19 | #[allow(unused, unused_variables, bad_style)]
+   |                 ^^^^^^^^^^^^^^^^ overruled by previous forbid
+
+error[E0453]: allow(bad_style) overruled by outer forbid(non_snake_case)
+  --> $DIR/outer-forbid.rs:19:35
+   |
+17 | #![forbid(unused, non_snake_case)]
+   |                   -------------- `forbid` level set here
+18 | 
+19 | #[allow(unused, unused_variables, bad_style)]
+   |                                   ^^^^^^^^^ overruled by previous forbid
+
+error: aborting due to previous error(s)
+