Fix 12969 and fix 9841

Author: tesuji

clippy_lints/src/dereference.rs

@@ -3,7 +3,8 @@ use clippy_utils::source::{snippet_with_applicability, snippet_with_context};
 use clippy_utils::sugg::has_enclosing_paren;
 use clippy_utils::ty::{implements_trait, is_manually_drop, peel_mid_ty_refs};
 use clippy_utils::{
-    expr_use_ctxt, get_parent_expr, is_block_like, is_lint_allowed, path_to_local, DefinedTy, ExprUseNode,
+    expr_use_ctxt, get_parent_expr, is_block_like, is_lint_allowed, path_to_local, peel_middle_ty_refs, DefinedTy,
+    ExprUseNode,
 };
 use core::mem;
 use rustc_ast::util::parser::{PREC_POSTFIX, PREC_PREFIX};
@@ -1044,16 +1045,28 @@ fn report<'tcx>(
                 return;
             }
 
-            let (prefix, precedence) = if let Some(mutability) = mutability
-                && !typeck.expr_ty(expr).is_ref()
+            let ty = typeck.expr_ty(expr);
+
+            // `&&[T; N]`, or `&&..&[T; N]` (src) cannot coerce to `&[T]` (dst).
+            if let ty::Ref(_, dst, _) = data.adjusted_ty.kind()
+                && dst.is_slice()
             {
-                let prefix = match mutability {
-                    Mutability::Not => "&",
-                    Mutability::Mut => "&mut ",
-                };
-                (prefix, PREC_PREFIX)
-            } else {
-                ("", 0)
+                let (src, n_src_refs) = peel_middle_ty_refs(ty);
+                if n_src_refs >= 2 && src.is_array() {
+                    return;
+                }
+            }
+
+            let (prefix, precedence) = match mutability {
+                Some(mutability) if !ty.is_ref() => {
+                    let prefix = match mutability {
+                        Mutability::Not => "&",
+                        Mutability::Mut => "&mut ",
+                    };
+                    (prefix, PREC_PREFIX)
+                },
+                None if !ty.is_ref() && data.adjusted_ty.is_ref() => ("&", 0),
+                _ => ("", 0),
             };
             span_lint_hir_and_then(
                 cx,

clippy_utils/src/lib.rs

@@ -2471,6 +2471,17 @@ pub fn peel_hir_ty_refs<'a>(mut ty: &'a hir::Ty<'a>) -> (&'a hir::Ty<'a>, usize)
     }
 }
 
+/// Peels off all references on the type. Returns the underlying type and the number of references
+/// removed.
+pub fn peel_middle_ty_refs(mut ty: Ty<'_>) -> (Ty<'_>, usize) {
+    let mut count = 0;
+    while let rustc_ty::Ref(_, dest_ty, _) = ty.kind() {
+        ty = *dest_ty;
+        count += 1;
+    }
+    (ty, count)
+}
+
 /// Removes `AddrOf` operators (`&`) or deref operators (`*`), but only if a reference type is
 /// dereferenced. An overloaded deref such as `Vec` to slice would not be removed.
 pub fn peel_ref_operators<'hir>(cx: &LateContext<'_>, mut expr: &'hir Expr<'hir>) -> &'hir Expr<'hir> {

tests/ui/explicit_auto_deref.fixed

@@ -364,13 +364,17 @@ mod issue_12969 {
     fn bar() {
         let wrapped_bar = Wrapper("");
 
-        foo(wrapped_bar);
+        foo(&wrapped_bar);
     }
 }
 
 mod issue_9841 {
     fn takes_array_ref<T, const N: usize>(array: &&[T; N]) {
-        takes_slice(array)
+        takes_slice(*array)
+    }
+
+    fn takes_array_ref_ref<T, const N: usize>(array: &&&[T; N]) {
+        takes_slice(**array)
     }
 
     fn takes_slice<T>(slice: &[T]) {

tests/ui/explicit_auto_deref.rs

@@ -373,6 +373,10 @@ mod issue_9841 {
         takes_slice(*array)
     }
 
+    fn takes_array_ref_ref<T, const N: usize>(array: &&&[T; N]) {
+        takes_slice(**array)
+    }
+
     fn takes_slice<T>(slice: &[T]) {
         todo!()
     }

tests/ui/explicit_auto_deref.stderr

@@ -275,13 +275,7 @@ error: deref which would be done by auto-deref
   --> tests/ui/explicit_auto_deref.rs:367:13
    |
 LL |         foo(&*wrapped_bar);
-   |             ^^^^^^^^^^^^^ help: try: `wrapped_bar`
+   |             ^^^^^^^^^^^^^ help: try: `&wrapped_bar`
 
-error: deref which would be done by auto-deref
-  --> tests/ui/explicit_auto_deref.rs:373:21
-   |
-LL |         takes_slice(*array)
-   |                     ^^^^^^ help: try: `array`
-
-error: aborting due to 47 previous errors
+error: aborting due to 46 previous errors