Auto merge of #66919 - dtolnay:description, r=KodrAus

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 PR:

- 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.

---

![description](https://user-images.githubusercontent.com/1940490/69910369-3bbaca80-13bf-11ea-94f7-2fe27a7ea333.png)

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libstd/path.rs b/src/libstd/path.rs
index 42bca0a9575..580ff1610ac 100644
--- a/src/libstd/path.rs
+++ b/src/libstd/path.rs
@@ -2786,6 +2786,7 @@ impl_cmp_os_str!(Cow<'a, Path>, OsString);
 
 #[stable(since = "1.7.0", feature = "strip_prefix")]
 impl fmt::Display for StripPrefixError {
+    #[allow(deprecated, deprecated_in_future)]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.description().fmt(f)
     }
@@ -2793,6 +2794,7 @@ impl fmt::Display for StripPrefixError {
 
 #[stable(since = "1.7.0", feature = "strip_prefix")]
 impl Error for StripPrefixError {
+    #[allow(deprecated)]
     fn description(&self) -> &str {
         "prefix not found"
     }