…lementation

get_mut allows us to abstract over the implementation detail of inner being
optional.

update polonius-engine

This updates polonius-engine to [version 0.7.0](https://github.com/rust-lang/polonius/blob/master/RELEASES.md#v070), which adds a hybrid algorithm that starts off with performing a cheaper, location-insensitive analysis before proceeding with the full analysis.

r? @nikomatsakis

HirIdify hir::Def

cc @ljedrz
r? @oli-obk

Remove duplicated redundant spans

Fix rust-lang#59895.

…in-pin-docs, r=RalfJung

Remove [mut] syntax in pin docs

Resolves rust-lang#59832

…nnytm

Make BufWriter use get_mut instead of manipulating inner in Write implementation

`get_mut` allows us to abstract over the implementation detail of inner being optional.

Fix cross-crate visibility of fictive variant constructors

After merging rust-lang#59376 I realized that the code in the decoder wasn't entirely correct - we "decoded" fictive variant constructors with their variant's visibility, which could be public, rather than demoted to `pub(crate)`.

Fictive constructors are not directly usable in expression/patterns, but the effect still can be observed with imports.

r? @davidtwco

…ievink

Add missing backtick to Symbol documentation.