Auto merge of #11387 - y21:issue11371, r=blyxyas

[`unnecessary_unwrap`]: lint on `.as_ref().unwrap()`

Closes #11371

This turned out to be a little more code than I originally thought, because the lint also makes sure to not lint if the user tries to mutate the option:
```rs
if option.is_some() {
  option = None;
  option.unwrap(); // don't lint here
}
```
... which means that even if we taught this lint to recognize `.as_mut()`, it would *still* not lint because that would count as a mutation. So we need to allow `.as_mut()` calls but reject other kinds of mutations.
Unfortunately it doesn't look like this is possible with `is_potentially_mutated` (seeing what kind of mutation happened).
This replaces it with a custom little visitor that does basically what it did before, but also allows `.as_mut()`.

changelog: [`unnecessary_unwrap`]: lint on `.as_ref().unwrap()`

clippy_utils/src/usage.rs

@@ -4,7 +4,7 @@ use core::ops::ControlFlow;
 use hir::def::Res;
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{self as hir, Expr, ExprKind, HirId, HirIdSet};
-use rustc_hir_typeck::expr_use_visitor::{Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};
+use rustc_hir_typeck::expr_use_visitor::{Delegate, ExprUseVisitor, Place, PlaceBase, PlaceWithHirId};
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::LateContext;
 use rustc_middle::hir::nested_filter;
@@ -37,6 +37,17 @@ pub fn is_potentially_mutated<'tcx>(variable: HirId, expr: &'tcx Expr<'_>, cx: &
     mutated_variables(expr, cx).map_or(true, |mutated| mutated.contains(&variable))
 }
 
+pub fn is_potentially_local_place(local_id: HirId, place: &Place<'_>) -> bool {
+    match place.base {
+        PlaceBase::Local(id) => id == local_id,
+        PlaceBase::Upvar(_) => {
+            // Conservatively assume yes.
+            true
+        },
+        _ => false,
+    }
+}
+
 struct MutVarsDelegate {
     used_mutably: HirIdSet,
     skip: bool,