needless_borrow: do not contradict dangerous_implicit_autorefs

Rust 1.88 introduces the `dangerous_implicit_autorefs` lint which warns
about using implicit autorefs on a place obtained from a raw pointer,
as this may create aliasing issues.

Prevent `clippy::needless_borrow` from triggering in this case,
by disabling the lint when taking a reference on a raw pointer
dereference. There might be a better way for doing this in the long run
with a finer way of distinguish the problematic cases, but this will
prevent Clippy from contradicting the compiler in the meantime.

Author: samueltardieu

clippy_lints/src/dereference.rs

@@ -997,6 +997,15 @@ fn report<'tcx>(
             );
         },
         State::DerefedBorrow(state) => {
+            // Do not suggest removing a non-mandatory `&` in `&*rawptr` in an `unsafe` context,
+            // as this may make rustc trigger its `dangerous_implicit_autorefs` lint.
+            if let ExprKind::AddrOf(BorrowKind::Ref, _, subexpr) = data.first_expr.kind
+                && let ExprKind::Unary(UnOp::Deref, subsubexpr) = subexpr.kind
+                && cx.typeck_results().expr_ty_adjusted(subsubexpr).is_raw_ptr()
+            {
+                return;
+            }
+
             let mut app = Applicability::MachineApplicable;
             let (snip, snip_is_macro) =
                 snippet_with_context(cx, expr.span, data.first_expr.span.ctxt(), "..", &mut app);

tests/ui/needless_borrow.fixed

@@ -107,9 +107,6 @@ fn main() {
     let x = (1, 2);
     let _ = x.0;
     //~^ needless_borrow
-    let x = &x as *const (i32, i32);
-    let _ = unsafe { (*x).0 };
-    //~^ needless_borrow
 
     // Issue #8367
     trait Foo {
@@ -289,3 +286,15 @@ fn issue_12268() {
 
     // compiler
 }
+
+fn issue_14743<T>(slice: &[T]) {
+    let _ = slice.len();
+    //~^ needless_borrow
+
+    let slice = slice as *const [T];
+    let _ = unsafe { (&*slice).len() };
+
+    // Check that rustc would actually warn if Clippy had suggested removing the reference
+    #[expect(dangerous_implicit_autorefs)]
+    let _ = unsafe { (*slice).len() };
+}

tests/ui/needless_borrow.rs

@@ -107,9 +107,6 @@ fn main() {
     let x = (1, 2);
     let _ = (&x).0;
     //~^ needless_borrow
-    let x = &x as *const (i32, i32);
-    let _ = unsafe { (&*x).0 };
-    //~^ needless_borrow
 
     // Issue #8367
     trait Foo {
@@ -289,3 +286,15 @@ fn issue_12268() {
 
     // compiler
 }
+
+fn issue_14743<T>(slice: &[T]) {
+    let _ = (&slice).len();
+    //~^ needless_borrow
+
+    let slice = slice as *const [T];
+    let _ = unsafe { (&*slice).len() };
+
+    // Check that rustc would actually warn if Clippy had suggested removing the reference
+    #[expect(dangerous_implicit_autorefs)]
+    let _ = unsafe { (*slice).len() };
+}

tests/ui/needless_borrow.stderr

@@ -103,71 +103,71 @@ error: this expression borrows a value the compiler would automatically borrow
 LL |     let _ = (&x).0;
    |             ^^^^ help: change this to: `x`
 
-error: this expression borrows a value the compiler would automatically borrow
-  --> tests/ui/needless_borrow.rs:111:22
-   |
-LL |     let _ = unsafe { (&*x).0 };
-   |                      ^^^^^ help: change this to: `(*x)`
-
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> tests/ui/needless_borrow.rs:122:5
+  --> tests/ui/needless_borrow.rs:119:5
    |
 LL |     (&&()).foo();
    |     ^^^^^^ help: change this to: `(&())`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> tests/ui/needless_borrow.rs:132:5
+  --> tests/ui/needless_borrow.rs:129:5
    |
 LL |     (&&5).foo();
    |     ^^^^^ help: change this to: `(&5)`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> tests/ui/needless_borrow.rs:159:23
+  --> tests/ui/needless_borrow.rs:156:23
    |
 LL |     let x: (&str,) = (&"",);
    |                       ^^^ help: change this to: `""`
 
 error: this expression borrows a value the compiler would automatically borrow
-  --> tests/ui/needless_borrow.rs:202:13
+  --> tests/ui/needless_borrow.rs:199:13
    |
 LL |             (&self.f)()
    |             ^^^^^^^^^ help: change this to: `(self.f)`
 
 error: this expression borrows a value the compiler would automatically borrow
-  --> tests/ui/needless_borrow.rs:212:13
+  --> tests/ui/needless_borrow.rs:209:13
    |
 LL |             (&mut self.f)()
    |             ^^^^^^^^^^^^^ help: change this to: `(self.f)`
 
 error: this expression borrows a value the compiler would automatically borrow
-  --> tests/ui/needless_borrow.rs:250:22
+  --> tests/ui/needless_borrow.rs:247:22
    |
 LL |         let _ = &mut (&mut { x.u }).x;
    |                      ^^^^^^^^^^^^^^ help: change this to: `{ x.u }`
 
 error: this expression borrows a value the compiler would automatically borrow
-  --> tests/ui/needless_borrow.rs:258:22
+  --> tests/ui/needless_borrow.rs:255:22
    |
 LL |         let _ = &mut (&mut { x.u }).x;
    |                      ^^^^^^^^^^^^^^ help: change this to: `{ x.u }`
 
 error: this expression borrows a value the compiler would automatically borrow
-  --> tests/ui/needless_borrow.rs:263:22
+  --> tests/ui/needless_borrow.rs:260:22
    |
 LL |         let _ = &mut (&mut x.u).x;
    |                      ^^^^^^^^^^ help: change this to: `x.u`
 
 error: this expression borrows a value the compiler would automatically borrow
-  --> tests/ui/needless_borrow.rs:265:22
+  --> tests/ui/needless_borrow.rs:262:22
    |
 LL |         let _ = &mut (&mut { x.u }).x;
    |                      ^^^^^^^^^^^^^^ help: change this to: `{ x.u }`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> tests/ui/needless_borrow.rs:287:23
+  --> tests/ui/needless_borrow.rs:284:23
    |
 LL |     option.unwrap_or((&x.0,));
    |                       ^^^^ help: change this to: `x.0`
 
+error: this expression creates a reference which is immediately dereferenced by the compiler
+  --> tests/ui/needless_borrow.rs:291:13
+   |
+LL |     let _ = (&slice).len();
+   |             ^^^^^^^^ help: change this to: `slice`
+
 error: aborting due to 28 previous errors