Add long explanation for E0549

compiler/rustc_error_codes/src/error_codes.rs

@@ -290,6 +290,7 @@ E0543: include_str!("./error_codes/E0543.md"),
 E0545: include_str!("./error_codes/E0545.md"),
 E0546: include_str!("./error_codes/E0546.md"),
 E0547: include_str!("./error_codes/E0547.md"),
+E0549: include_str!("./error_codes/E0549.md"),
 E0550: include_str!("./error_codes/E0550.md"),
 E0551: include_str!("./error_codes/E0551.md"),
 E0552: include_str!("./error_codes/E0552.md"),
@@ -608,9 +609,6 @@ E0781: include_str!("./error_codes/E0781.md"),
 //  E0540, // multiple rustc_deprecated attributes
     E0544, // multiple stability levels
 //  E0548, // replaced with a generic attribute input check
-    // rustc_deprecated attribute must be paired with either stable or unstable
-    // attribute
-    E0549,
     E0553, // multiple rustc_const_unstable attributes
 //  E0555, // replaced with a generic attribute input check
 //  E0558, // replaced with a generic attribute input check

compiler/rustc_error_codes/src/error_codes/E0549.md

@@ -0,0 +1,37 @@
+A `rustc_deprecated` attribute wasn't paired with a `stable`/`unstable`
+attribute. 
+
+Erroneous code example:
+
+```compile_fail,E0549
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[rustc_deprecated(
+    since = "1.0.1",
+    reason = "explanation for deprecation"
+)] // invalid
+fn _deprecated_fn() {}
+```
+
+To fix this issue, you need to add also an attribute `stable` or `unstable`.
+Example:
+
+```
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[stable(since = "1.0.0", feature = "test")]
+#[rustc_deprecated(
+    since = "1.0.1",
+    reason = "explanation for deprecation"
+)] // ok!
+fn _deprecated_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, E0543, E0546, E0547, E0550.
+Some errors have detailed explanations: E0539, E0541, E0542, E0543, E0546, E0547, E0549, E0550.
 For more information about an error, try `rustc --explain E0539`.