Add long explanation for E0545

compiler/rustc_error_codes/src/error_codes.rs

@@ -286,6 +286,7 @@ E0538: include_str!("./error_codes/E0538.md"),
 E0539: include_str!("./error_codes/E0539.md"),
 E0541: include_str!("./error_codes/E0541.md"),
 E0542: include_str!("./error_codes/E0542.md"),
+E0545: include_str!("./error_codes/E0545.md"),
 E0546: include_str!("./error_codes/E0546.md"),
 E0547: include_str!("./error_codes/E0547.md"),
 E0550: include_str!("./error_codes/E0550.md"),
@@ -606,7 +607,6 @@ E0781: include_str!("./error_codes/E0781.md"),
 //  E0540, // multiple rustc_deprecated attributes
     E0543, // missing 'reason'
     E0544, // multiple stability levels
-    E0545, // incorrect 'issue'
 //  E0548, // replaced with a generic attribute input check
     // rustc_deprecated attribute must be paired with either stable or unstable
     // attribute

compiler/rustc_error_codes/src/error_codes/E0545.md

@@ -0,0 +1,35 @@
+The `issue` value is incorrect in a stability attribute.
+
+Erroneous code example:
+
+```compile_fail,E0545
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[unstable(feature = "_unstable_fn", issue = "0")] // invalid
+fn _unstable_fn() {}
+
+#[rustc_const_unstable(feature = "_unstable_const_fn", issue = "0")] // invalid
+fn _unstable_const_fn() {}
+```
+
+To fix this issue, you need to provide a correct value in the `issue` field.
+Example:
+
+```
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[unstable(feature = "_unstable_fn", issue = "none")] // ok!
+fn _unstable_fn() {}
+
+#[rustc_const_unstable(feature = "_unstable_const_fn", issue = "1")] // ok!
+fn _unstable_const_fn() {}
+```
+
+See the [How Rust is Made and “Nightly Rust”][how-rust-made-nightly] appendix
+of the Book and the [Stability attributes][stability-attributes] section of the
+Rustc Dev Guide for more details.
+
+[how-rust-made-nightly]: https://doc.rust-lang.org/book/appendix-07-nightly-rust.html
+[stability-attributes]: https://rustc-dev-guide.rust-lang.org/stability.html

src/test/ui/feature-gates/unstable-attribute-allow-issue-0.stderr

@@ -16,3 +16,4 @@ LL | #[unstable(feature = "unstable_test_feature", issue = "something")]
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0545`.

src/test/ui/stability-attribute/stability-attribute-sanity-2.stderr

@@ -20,5 +20,5 @@ LL | #[unstable(feature = "a", issue = "no")]
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0538, E0541.
+Some errors have detailed explanations: E0538, E0541, E0545.
 For more information about an error, try `rustc --explain E0538`.