Auto merge of rust-lang#10896 - y21:eager-or-lazy-autoderef, r=blyxyas,xFrednet

consider autoderef through user-defined `Deref` in `eager_or_lazy`

Fixes rust-lang#10462

This PR handles autoderef in the `eager_or_lazy` util module and stops suggesting to change lazy to eager if autoderef in an expression goes through user defined `Deref` impls, e.g.
```rs
struct S;
impl Deref for S {
  type Target = ();
  fn deref(&self) -> &Self::Target { &() }
}

let _ = Some(()).as_ref().unwrap_or_else(|| &S); // autoderef `&S` -> `&()`
```

changelog: [`unnecessary_lazy_evaluations`]: don't suggest changing lazy evaluation to eager if autoderef goes through user-defined `Deref`

r? `@xFrednet`  (because of the earlier review in rust-lang#10864, might help for context here)

Author: bors

clippy_utils/src/eager_or_lazy.rs

@@ -15,6 +15,7 @@ use rustc_hir::def::{DefKind, Res};
 use rustc_hir::intravisit::{walk_expr, Visitor};
 use rustc_hir::{def_id::DefId, Block, Expr, ExprKind, QPath, UnOp};
 use rustc_lint::LateContext;
+use rustc_middle::ty::adjustment::Adjust;
 use rustc_middle::ty::{self, PredicateKind};
 use rustc_span::{sym, Symbol};
 use std::cmp;
@@ -114,6 +115,20 @@ fn expr_eagerness<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> EagernessS
             if self.eagerness == ForceNoChange {
                 return;
             }
+
+            // Autoderef through a user-defined `Deref` impl can have side-effects,
+            // so don't suggest changing it.
+            if self
+                .cx
+                .typeck_results()
+                .expr_adjustments(e)
+                .iter()
+                .any(|adj| matches!(adj.kind, Adjust::Deref(Some(_))))
+            {
+                self.eagerness |= NoChange;
+                return;
+            }
+
             match e.kind {
                 ExprKind::Call(
                     &Expr {

tests/ui/unnecessary_lazy_eval.fixed

@@ -4,6 +4,7 @@
 #![allow(clippy::redundant_closure)]
 #![allow(clippy::bind_instead_of_map)]
 #![allow(clippy::map_identity)]
+#![allow(clippy::needless_borrow)]
 
 use std::ops::Deref;
 
@@ -81,6 +82,9 @@ fn main() {
     let _ = Some(1).unwrap_or(*r);
     let b = Box::new(1);
     let _ = Some(1).unwrap_or(*b);
+    // Should lint - Builtin deref through autoderef
+    let _ = Some(1).as_ref().unwrap_or(&r);
+    let _ = Some(1).as_ref().unwrap_or(&b);
 
     // Cases when unwrap is not called on a simple variable
     let _ = Some(10).unwrap_or(2);
@@ -113,6 +117,9 @@ fn main() {
     let _ = Some(1).unwrap_or_else(|| *Issue10437); // Issue10437 has a deref impl
     let _ = Some(1).unwrap_or(*Issue10437);
 
+    let _ = Some(1).as_ref().unwrap_or_else(|| &Issue10437);
+    let _ = Some(1).as_ref().unwrap_or(&Issue10437);
+
     // Should not lint - bool
     let _ = (0 == 1).then(|| Issue9427(0)); // Issue9427 has a significant drop
     let _ = false.then(|| Issue9427FollowUp); // Issue9427FollowUp has a significant drop

tests/ui/unnecessary_lazy_eval.rs

@@ -4,6 +4,7 @@
 #![allow(clippy::redundant_closure)]
 #![allow(clippy::bind_instead_of_map)]
 #![allow(clippy::map_identity)]
+#![allow(clippy::needless_borrow)]
 
 use std::ops::Deref;
 
@@ -81,6 +82,9 @@ fn main() {
     let _ = Some(1).unwrap_or_else(|| *r);
     let b = Box::new(1);
     let _ = Some(1).unwrap_or_else(|| *b);
+    // Should lint - Builtin deref through autoderef
+    let _ = Some(1).as_ref().unwrap_or_else(|| &r);
+    let _ = Some(1).as_ref().unwrap_or_else(|| &b);
 
     // Cases when unwrap is not called on a simple variable
     let _ = Some(10).unwrap_or_else(|| 2);
@@ -113,6 +117,9 @@ fn main() {
     let _ = Some(1).unwrap_or_else(|| *Issue10437); // Issue10437 has a deref impl
     let _ = Some(1).unwrap_or(*Issue10437);
 
+    let _ = Some(1).as_ref().unwrap_or_else(|| &Issue10437);
+    let _ = Some(1).as_ref().unwrap_or(&Issue10437);
+
     // Should not lint - bool
     let _ = (0 == 1).then(|| Issue9427(0)); // Issue9427 has a significant drop
     let _ = false.then(|| Issue9427FollowUp); // Issue9427FollowUp has a significant drop