Deprecate Error::description for real

`description` has been documented as soft-deprecated since 1.27.0 (17
months ago). There is no longer any reason to call it or implement it.

This commit:

- adds #[rustc_deprecated(since = "1.41.0")] to Error::description;

- moves description (and cause, which is also deprecated) below the
  source and backtrace methods in the Error trait;

- reduces documentation of description and cause to take up much less
  vertical real estate in rustdocs, while preserving the example that
  shows how to render errors without needing to call description;

- removes the description function of all *currently unstable* Error
  impls in the standard library;

- marks #[allow(deprecated)] the description function of all *stable*
  Error impls in the standard library;

- replaces miscellaneous uses of description in example code and the
  compiler.

1 files changed, 15 insertions, 15 deletions

diff --git a/src/librustc_error_codes/error_codes/E0638.md b/src/librustc_error_codes/error_codes/E0638.md
index 29b7586b07b..14cd31502b6 100644
--- a/src/librustc_error_codes/error_codes/E0638.md
+++ b/src/librustc_error_codes/error_codes/E0638.md
@@ -10,23 +10,23 @@ For example, in the below example, since the enum is marked as
 on it.
 
 ```rust,ignore (pseudo-Rust)
-use std::error::Error as StdError;
-
-#[non_exhaustive] pub enum Error {
-   Message(String),
-   Other,
+#[non_exhaustive]
+pub enum Error {
+    Message(String),
+    Other,
 }
 
-impl StdError for Error {
-   fn description(&self) -> &str {
+impl Display for Error {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
         // This will not error, despite being marked as non_exhaustive, as this
         // enum is defined within the current crate, it can be matched
         // exhaustively.
-        match *self {
-           Message(ref s) => s,
-           Other => "other or unknown error",
-        }
-   }
+        let display = match self {
+            Message(s) => s,
+            Other => "other or unknown error",
+        };
+        formatter.write_str(display)
+    }
 }
 ```
 
@@ -38,9 +38,9 @@ use mycrate::Error;
 // This will not error as the non_exhaustive Error enum has been matched with a
 // wildcard.
 match error {
-   Message(ref s) => ...,
-   Other => ...,
-   _ => ...,
+    Message(s) => ...,
+    Other => ...,
+    _ => ...,
 }
 ```