don't see issue #0

The unstable-feature attribute requires an issue (neglecting it is
E0547), which gets used in the error messages. Unfortunately, there are
some cases where "0" is apparently used a placeholder where no issue
exists, directing the user to see the (nonexistent) issue #0. (It would
have been better to either let `issue` be optional—compare to how issue
is an `Option<u32>` in the feature-gate declarations in
libsyntax/feature-gate.rs—or actually require that an issue be created.)
Rather than endeavoring to change how `#[unstable]` works at this time
(given competing contributor and reviewer priorities), this simple patch
proposes the less-ambitious solution of just not adding the "(see
issue)" note when the number is zero.

Resolves #49983.

src/libsyntax/feature_gate.rs

@@ -1228,10 +1228,9 @@ fn leveled_feature_err<'a>(sess: &'a ParseSess, feature: &str, span: Span, issue
         GateIssue::Library(lib) => lib,
     };
 
-    let explanation = if let Some(n) = issue {
-        format!("{} (see issue #{})", explain, n)
-    } else {
-        explain.to_owned()
+    let explanation = match issue {
+        None | Some(0) => explain.to_owned(),
+        Some(n) => format!("{} (see issue #{})", explain, n)
     };
 
     let mut err = match level {

src/test/ui/feature-gate/issue-49983-see-issue-0.rs

+// Copyright 2018 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.
+
+extern crate core;
+
+// error should not say "(see issue #0)"
+#[allow(unused_imports)] use core::ptr::Unique; //~ ERROR use of unstable library feature
+
+fn main() {}

src/test/ui/feature-gate/issue-49983-see-issue-0.stderr

+error[E0658]: use of unstable library feature 'ptr_internals': use NonNull instead and consider PhantomData<T> (if you also use #[may_dangle]), Send, and/or Sync
+  --> $DIR/issue-49983-see-issue-0.rs:14:30
+   |
+LL | #[allow(unused_imports)] use core::ptr::Unique; //~ ERROR use of unstable library feature
+   |                              ^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(ptr_internals)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.