`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.
This is a bit unfortunate, but code needs to be able to fatally error
early on (in particular, syntax_pos after we move SourceMap there). It's
also a tiny bit of code, which means it's ultimately not that bad.
