Add long error explanation for E0542

compiler/rustc_error_codes/src/error_codes.rs

@@ -285,6 +285,7 @@ E0537: include_str!("./error_codes/E0537.md"),
 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"),
 E0546: include_str!("./error_codes/E0546.md"),
 E0550: include_str!("./error_codes/E0550.md"),
 E0551: include_str!("./error_codes/E0551.md"),
@@ -602,7 +603,6 @@ E0781: include_str!("./error_codes/E0781.md"),
     E0523,
 //  E0526, // shuffle indices are not constant
 //  E0540, // multiple rustc_deprecated attributes
-    E0542, // missing 'since'
     E0543, // missing 'reason'
     E0544, // multiple stability levels
     E0545, // incorrect 'issue'

compiler/rustc_error_codes/src/error_codes/E0542.md

@@ -0,0 +1,47 @@
+A feature since is missing.
+
+Erroneous code example:
+
+```compile_fail,E0542
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[stable(feature = "_stable_fn")] // invalid
+fn _stable_fn() {}
+
+#[rustc_const_stable(feature = "_stable_const_fn")] // invalid
+fn _stable_const_fn() {}
+
+#[stable(feature = "_deprecated_fn", since = "0.1.0")]
+#[rustc_deprecated( // invalid
+    reason = "explanation for deprecation"
+)]
+fn _deprecated_fn() {}
+```
+
+To fix the issue you need to provide the since field of the feature.
+
+```
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[stable(feature = "_stable_fn", since = "1.0.0")]
+fn _stable_fn() {}
+
+#[rustc_const_stable(feature = "_stable_const_fn", since = "1.0.0")]
+fn _stable_const_fn() {}
+
+#[stable(feature = "_deprecated_fn", since = "0.1.0")]
+#[rustc_deprecated(
+    since = "1.0.0",
+    reason = "explanation for deprecation"
+)]
+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