Stabilize #[unix_sigpipe = "sig_dfl"] on fn main()

The attribute is used like this:

    #[unix_sigpipe = "sig_dfl"]
    fn main() {
        // ...
    }

When used, `SIGPIPE` will be set to `SIG_DFL` before `fn main()` is
invoked, rather than being set to `SIG_IGN` which is the default.

This commit does NOT stabilize `#[unix_sigpipe = "sig_ign"]` or
`#[unix_sigpipe = "inherit"]`.

See the PR for more info.

Author: Enselic

compiler/rustc/src/main.rs

@@ -1,4 +1,4 @@
-#![feature(unix_sigpipe)]
+#![cfg_attr(bootstrap, feature(unix_sigpipe))]
 
 // A note about jemalloc: rustc uses jemalloc when built for CI and
 // distribution. The obvious way to do this is with the `#[global_allocator]`

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -200,6 +200,21 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 );
             }
         }
+        // Check unstable flavors of the `#[unix_sigpipe]` attribute.
+        if attr.has_name(sym::unix_sigpipe)
+            && let Some(value) = attr.value_str()
+            && (value == sym::sig_ign || value == sym::inherit)
+        {
+            gate!(
+                self,
+                unix_sigpipe,
+                attr.span,
+                format!(
+                    "the `#[unix_sigpipe = \"{}\"]` attribute is an experimental feature",
+                    value.as_str()
+                )
+            );
+        }
         if !attr.is_doc_comment()
             && let [seg, _] = attr.get_normal_item().path.segments.as_slice()
             && seg.ident.name == sym::diagnostic