Rollup merge of #71808 - unexge:long-err-expl-for-e0539, r=GuillaumeGomez

Add long error explanation for E0539

since this error is similar to [E0551](https://github.com/rust-lang/rust/blob/master/src/librustc_error_codes/error_codes/E0551.md) most of the content was copied from it. part of #61137.

1 files changed, 48 insertions, 0 deletions

diff --git a/src/librustc_error_codes/error_codes/E0539.md b/src/librustc_error_codes/error_codes/E0539.md
new file mode 100644
index 00000000000..df2d7d910bb
--- /dev/null
+++ b/src/librustc_error_codes/error_codes/E0539.md
@@ -0,0 +1,48 @@
+An invalid meta-item was used inside an attribute.
+
+Erroneous code example:
+
+```compile_fail,E0539
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[rustc_deprecated(reason)] // error!
+#[unstable(feature = "deprecated_fn", issue = "123")]
+fn deprecated() {}
+
+#[unstable(feature = "unstable_struct", issue)] // error!
+struct Unstable;
+
+#[rustc_const_unstable(feature)] // error!
+const fn unstable_fn() {}
+
+#[stable(feature = "stable_struct", since)] // error!
+struct Stable;
+
+#[rustc_const_stable(feature)] // error!
+const fn stable_fn() {}
+```
+
+Meta items are the key-value pairs inside of an attribute.
+To fix these issues you need to give required key-value pairs.
+
+```
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[rustc_deprecated(since = "1.39.0", reason = "reason")] // ok!
+#[unstable(feature = "deprecated_fn", issue = "123")]
+fn deprecated() {}
+
+#[unstable(feature = "unstable_struct", issue = "123")] // ok!
+struct Unstable;
+
+#[rustc_const_unstable(feature = "unstable_fn", issue = "124")] // ok!
+const fn unstable_fn() {}
+
+#[stable(feature = "stable_struct", since = "1.39.0")] // ok!
+struct Stable;
+
+#[rustc_const_stable(feature = "stable_fn", since = "1.39.0")] // ok!
+const fn stable_fn() {}
+```