--cap-lints allow switches off can_emit_warnings

src/librustc/session/mod.rs

@@ -660,12 +660,16 @@ pub fn build_session_with_codemap(sopts: config::Options,
     // FIXME: This is not general enough to make the warning lint completely override
     // normal diagnostic warnings, since the warning lint can also be denied and changed
     // later via the source code.
-    let can_print_warnings = sopts.lint_opts
+    let warnings_allow = sopts.lint_opts
         .iter()
         .filter(|&&(ref key, _)| *key == "warnings")
-        .map(|&(_, ref level)| *level != lint::Allow)
+        .map(|&(_, ref level)| *level == lint::Allow)
         .last()
-        .unwrap_or(true);
+        .unwrap_or(false);
+    let cap_lints_allow = sopts.lint_cap.map_or(false, |cap| cap == lint::Allow);
+
+    let can_print_warnings = !(warnings_allow || cap_lints_allow);
+
     let treat_err_as_bug = sopts.debugging_opts.treat_err_as_bug;
 
     let emitter: Box<Emitter> = match (sopts.error_format, emitter_dest) {

src/test/compile-fail/feature-gate-fn_must_use-cap-lints-allow.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.
+
+// compile-flags: --cap-lints allow
+
+// This tests that the fn_must_use feature-gate warning respects the lint
+// cap. (See discussion in Issue #44213.)
+
+#![feature(rustc_attrs)]
+
+#[must_use] // (no feature-gate warning because of the lint cap!)
+fn need_to_use_it() -> bool { true }
+
+#[rustc_error]
+fn main() {} //~ ERROR compilation successful