Rollup merge of #49345 - davidtwco:issue-44109, r=nikomatsakis

RFC 2008: Finishing Touches

Part of #44109.

r? @nikomatsakis
(not sure who was best for this PR).

src/librustc/diagnostics.rs

@@ -2058,6 +2058,33 @@ fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y> + 'x
 ```
 "##,
 
+E0910: r##"
+This error indicates that a `#[non_exhaustive]` attribute was incorrectly placed
+on something other than a struct or enum.
+
+Examples of erroneous code:
+
+```compile_fail,E0910
+# #![feature(non_exhaustive)]
+
+#[non_exhaustive]
+trait Foo { }
+```
+"##,
+
+E0911: r##"
+This error indicates that a `#[non_exhaustive]` attribute had a value. The
+`#[non_exhaustive]` should be empty.
+
+Examples of erroneous code:
+
+```compile_fail,E0911
+# #![feature(non_exhaustive)]
+
+#[non_exhaustive(anything)]
+struct Foo;
+```
+"##,
 
 }
 

src/librustc/hir/check_attr.rs

@@ -66,6 +66,8 @@ fn check_attributes(&self, item: &hir::Item, target: Target) {
         for attr in &item.attrs {
             if attr.check_name("inline") {
                 self.check_inline(attr, &item.span, target)
+            } else if attr.check_name("non_exhaustive") {
+                self.check_non_exhaustive(attr, item, target)
             } else if attr.check_name("wasm_import_module") {
                 has_wasm_import_module = true;
                 if attr.value_str().is_none() {
@@ -113,6 +115,31 @@ fn check_inline(&self, attr: &hir::Attribute, span: &Span, target: Target) {
         }
     }
 
+    /// Check if the `#[non_exhaustive]` attribute on an `item` is valid.
+    fn check_non_exhaustive(&self, attr: &hir::Attribute, item: &hir::Item, target: Target) {
+        match target {
+            Target::Struct | Target::Enum => { /* Valid */ },
+            _ => {
+                struct_span_err!(self.tcx.sess,
+                                 attr.span,
+                                 E0910,
+                                 "attribute can only be applied to a struct or enum")
+                    .span_label(item.span, "not a struct or enum")
+                    .emit();
+                return;
+            }
+        }
+
+        if attr.meta_item_list().is_some() || attr.value_str().is_some() {
+            struct_span_err!(self.tcx.sess,
+                             attr.span,
+                             E0911,
+                             "attribute should be empty")
+                .span_label(item.span, "not empty")
+                .emit();
+        }
+    }
+
     /// Check if the `#[repr]` attributes on `item` are valid.
     fn check_repr(&self, item: &hir::Item, target: Target) {
         // Extract the names of all repr hints, e.g., [foo, bar, align] for:

src/test/compile-fail/rfc-2008-non-exhaustive/invalid-attribute.rs

@@ -0,0 +1,28 @@
+// Copyright 2012 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.
+
+#![feature(non_exhaustive)]
+
+#[non_exhaustive(anything)]
+//~^ ERROR attribute should be empty [E0911]
+struct Foo;
+
+#[non_exhaustive]
+//~^ ERROR attribute can only be applied to a struct or enum [E0910]
+trait Bar { }
+
+#[non_exhaustive]
+//~^ ERROR attribute can only be applied to a struct or enum [E0910]
+union Baz {
+    f1: u16,
+    f2: u16
+}
+
+fn main() { }