Add long explanation of E0546

Author: sasurau4

compiler/rustc_error_codes/src/error_codes.rs

@@ -283,6 +283,7 @@ 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"),
+E0546: include_str!("./error_codes/E0546.md"),
 E0550: include_str!("./error_codes/E0550.md"),
 E0551: include_str!("./error_codes/E0551.md"),
 E0552: include_str!("./error_codes/E0552.md"),
@@ -603,7 +604,6 @@ E0779: include_str!("./error_codes/E0779.md"),
     E0543, // missing 'reason'
     E0544, // multiple stability levels
     E0545, // incorrect 'issue'
-    E0546, // missing 'feature'
     E0547, // missing 'issue'
 //  E0548, // replaced with a generic attribute input check
     // rustc_deprecated attribute must be paired with either stable or unstable

compiler/rustc_error_codes/src/error_codes/E0546.md

@@ -0,0 +1,27 @@
+A feature name is missing.
+
+Erroneous code example:
+
+```compile_fail,E0546
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[unstable(issue = "none")] // invalid
+fn unstable_fn() {}
+
+#[stable(since = "1.0.0")] // invalid
+fn stable_fn() {}
+```
+
+To fix the issue you need to provide a feature name.
+
+```
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[unstable(feature = "unstable_fn", issue = "none")] // ok!
+fn unstable_fn() {}
+
+#[stable(feature = "stable_fn", since = "1.0.0")] // ok!
+fn stable_fn() {}
+```