Add long explanation for E0544.

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"),
 E0543: include_str!("./error_codes/E0543.md"),
+E0544: include_str!("./error_codes/E0544.md"),
 E0545: include_str!("./error_codes/E0545.md"),
 E0546: include_str!("./error_codes/E0546.md"),
 E0547: include_str!("./error_codes/E0547.md"),
@@ -610,7 +611,6 @@ E0783: include_str!("./error_codes/E0783.md"),
     E0523,
 //  E0526, // shuffle indices are not constant
 //  E0540, // multiple rustc_deprecated attributes
-    E0544, // multiple stability levels
 //  E0548, // replaced with a generic attribute input check
 //  E0553, // multiple rustc_const_unstable attributes
 //  E0555, // replaced with a generic attribute input check

compiler/rustc_error_codes/src/error_codes/E0544.md

@@ -0,0 +1,29 @@
+Multiple stability attributes were declared on the same item.
+
+Erroneous code example:
+
+```compile_fail,E0544
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "rust1")]
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[stable(feature = "test", since = "2.0.0")] // invalid
+fn foo() {}
+```
+
+To fix this issue, ensure that each item has at most one stability attribute.
+
+```
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "rust1")]
+
+#[stable(feature = "test", since = "2.0.0")] // ok!
+fn foo() {}
+```
+
+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

@@ -122,5 +122,5 @@ 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, E0549, E0550.
+Some errors have detailed explanations: E0539, E0541, E0542, E0543, E0544, E0546, E0547, E0549, E0550.
 For more information about an error, try `rustc --explain E0539`.