New message for needless_borrow when a borrow could be replaced by auto-borrow

Author: Jarcho

clippy_lints/src/dereference.rs

@@ -145,6 +145,7 @@ enum State {
     DerefedBorrow {
         count: usize,
         required_precedence: i8,
+        msg: &'static str,
     },
 }
 
@@ -259,20 +260,25 @@ impl<'tcx> LateLintPass<'tcx> for Dereferencing {
                         //                None => y
                         //            };
                         //        }
-                        let (required_refs, required_precedence) = if is_auto_borrow_position(parent, expr.hir_id) {
-                            (1, PREC_POSTFIX)
+                        let deref_msg =
+                            "this expression creates a reference which is immediately dereferenced by the compiler";
+                        let borrow_msg = "this expression borrows a value the compiler would automatically borrow";
+
+                        let (required_refs, required_precedence, msg) = if is_auto_borrow_position(parent, expr.hir_id)
+                        {
+                            (1, PREC_POSTFIX, if deref_count == 1 { borrow_msg } else { deref_msg })
                         } else if let Some(&Adjust::Borrow(AutoBorrow::Ref(_, mutability))) =
                             next_adjust.map(|a| &a.kind)
                         {
                             if matches!(mutability, AutoBorrowMutability::Mut { .. })
                                 && !is_auto_reborrow_position(parent)
                             {
-                                (3, 0)
+                                (3, 0, deref_msg)
                             } else {
-                                (2, 0)
+                                (2, 0, deref_msg)
                             }
                         } else {
-                            (2, 0)
+                            (2, 0, deref_msg)
                         };
 
                         if deref_count >= required_refs {
@@ -282,6 +288,7 @@ impl<'tcx> LateLintPass<'tcx> for Dereferencing {
                                     // can't be removed without breaking the code. See earlier comment.
                                     count: deref_count - required_refs,
                                     required_precedence,
+                                    msg,
                                 },
                                 StateData { span: expr.span },
                             ));
@@ -319,6 +326,7 @@ impl<'tcx> LateLintPass<'tcx> for Dereferencing {
                     State::DerefedBorrow {
                         count,
                         required_precedence,
+                        msg,
                     },
                     data,
                 )),
@@ -328,6 +336,7 @@ impl<'tcx> LateLintPass<'tcx> for Dereferencing {
                     State::DerefedBorrow {
                         count: count - 1,
                         required_precedence,
+                        msg,
                     },
                     data,
                 ));
@@ -624,15 +633,17 @@ fn report(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, state: State, data: Stat
             );
         },
         State::DerefedBorrow {
-            required_precedence, ..
+            required_precedence,
+            msg,
+            ..
         } => {
             let mut app = Applicability::MachineApplicable;
             let snip = snippet_with_context(cx, expr.span, data.span.ctxt(), "..", &mut app).0;
             span_lint_and_sugg(
                 cx,
                 NEEDLESS_BORROW,
                 data.span,
-                "this expression creates a reference which is immediately dereferenced by the compiler",
+                msg,
                 "change this to",
                 if required_precedence > expr.precedence().order() && !has_enclosing_paren(&snip) {
                     format!("({})", snip)

tests/ui/needless_borrow.stderr

@@ -84,13 +84,13 @@ error: this expression creates a reference which is immediately dereferenced by
 LL |     let y: &mut i32 = &mut &mut x;
    |                       ^^^^^^^^^^^ help: change this to: `x`
 
-error: this expression creates a reference which is immediately dereferenced by the compiler
+error: this expression borrows a value the compiler would automatically borrow
   --> $DIR/needless_borrow.rs:67:13
    |
 LL |     let _ = (&s).len();
    |             ^^^^ help: change this to: `s`
 
-error: this expression creates a reference which is immediately dereferenced by the compiler
+error: this expression borrows a value the compiler would automatically borrow
   --> $DIR/needless_borrow.rs:68:13
    |
 LL |     let _ = (&s).capacity();
@@ -102,13 +102,13 @@ error: this expression creates a reference which is immediately dereferenced by
 LL |     let _ = (&&s).capacity();
    |             ^^^^^ help: change this to: `s`
 
-error: this expression creates a reference which is immediately dereferenced by the compiler
+error: this expression borrows a value the compiler would automatically borrow
   --> $DIR/needless_borrow.rs:72:13
    |
 LL |     let _ = (&x).0;
    |             ^^^^ help: change this to: `x`
 
-error: this expression creates a reference which is immediately dereferenced by the compiler
+error: this expression borrows a value the compiler would automatically borrow
   --> $DIR/needless_borrow.rs:74:22
    |
 LL |     let _ = unsafe { (&*x).0 };