Add long explanation for E0547

Author: jesusprubio

compiler/rustc_error_codes/src/error_codes.rs

@@ -287,6 +287,7 @@ E0539: include_str!("./error_codes/E0539.md"),
 E0541: include_str!("./error_codes/E0541.md"),
 E0542: include_str!("./error_codes/E0542.md"),
 E0546: include_str!("./error_codes/E0546.md"),
+E0547: include_str!("./error_codes/E0547.md"),
 E0550: include_str!("./error_codes/E0550.md"),
 E0551: include_str!("./error_codes/E0551.md"),
 E0552: include_str!("./error_codes/E0552.md"),
@@ -606,7 +607,6 @@ E0781: include_str!("./error_codes/E0781.md"),
     E0543, // missing 'reason'
     E0544, // multiple stability levels
     E0545, // incorrect 'issue'
-    E0547, // missing '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/E0547.md

@@ -0,0 +1,27 @@
+#![stable(since = "1.0.0", feature = "test")]
+#[unstable(feature = "_unstable_fn")] // invalid
+fn _unstable_fn() {}
+#[rustc_const_unstable(feature = "_unstable_const_fn")] // invalid
+fn _unstable_const_fn() {}
+```
+
+To fix the issue you need to provide the `issue` field.
+
+```
+#![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 = "none"
+)] // 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/stability-attribute/stability-attribute-sanity.stderr

@@ -116,5 +116,5 @@ LL | #[rustc_deprecated(since = "a", reason = "text")]
 
 error: aborting due to 19 previous errors
 
-Some errors have detailed explanations: E0539, E0541, E0542, E0546, E0550.
+Some errors have detailed explanations: E0539, E0541, E0542, E0546, E0547, E0550.
 For more information about an error, try `rustc --explain E0539`.