Improved suggestion on misrefactored_assign_op lint. Fixes #1239

Author: flip1995

clippy_lints/src/assign_ops.rs

@@ -87,19 +87,23 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for AssignOps {
                 });
                 if let hir::ExprBinary(binop, ref l, ref r) = rhs.node {
                     if op.node == binop.node {
-                        let lint = |assignee: &hir::Expr, rhs: &hir::Expr| {
+                        let lint = |assignee: &hir::Expr, rhs_other: &hir::Expr| {
                             span_lint_and_then(
                                 cx,
                                 MISREFACTORED_ASSIGN_OP,
                                 expr.span,
                                 "variable appears on both sides of an assignment operation",
                                 |db| if let (Some(snip_a), Some(snip_r)) =
-                                    (snippet_opt(cx, assignee.span), snippet_opt(cx, rhs.span))
+                                    (snippet_opt(cx, assignee.span), snippet_opt(cx, rhs_other.span))
                                 {
+                                    let a = &sugg::Sugg::hir(cx, assignee, "..");
+                                    let r = &sugg::Sugg::hir(cx, rhs, "..");
                                     db.span_suggestion(
                                         expr.span,
-                                        "replace it with",
-                                        format!("{} {}= {}", snip_a, op.node.as_str(), snip_r),
+                                        &format!("Did you mean {} = {} {} {} or {} = {}? Consider replacing it with",
+                                                 snip_a, snip_a, op.node.as_str(), snip_r,
+                                                 snip_a, sugg::make_binop(higher::binop(op.node), a, r)),
+                                        format!("{} {}= {}", snip_a, op.node.as_str(), snip_r)
                                     );
                                 },
                             );

tests/ui/assign_ops2.rs

@@ -13,6 +13,7 @@ fn main() {
     a /= a / 2;
     a %= a % 5;
     a &= a & 1;
+    a *= a * a;
     a -= 1 - a;
     a /= 5 / a;
     a %= 42 % a;

tests/ui/assign_ops2.stderr

@@ -2,51 +2,93 @@ error: variable appears on both sides of an assignment operation
  --> $DIR/assign_ops2.rs:8:5
   |
 8 |     a += a + 1;
-  |     ^^^^^^^^^^ help: replace it with: `a += 1`
+  |     ^^^^^^^^^^
   |
   = note: `-D misrefactored-assign-op` implied by `-D warnings`
+help: Did you mean a = a + 1 or a = a + a + 1? Consider replacing it with
+  |
+8 |     a += 1;
+  |     ^^^^^^
 
 error: variable appears on both sides of an assignment operation
  --> $DIR/assign_ops2.rs:9:5
   |
 9 |     a += 1 + a;
-  |     ^^^^^^^^^^ help: replace it with: `a += 1`
+  |     ^^^^^^^^^^
+help: Did you mean a = a + 1 or a = a + 1 + a? Consider replacing it with
+  |
+9 |     a += 1;
+  |     ^^^^^^
 
 error: variable appears on both sides of an assignment operation
   --> $DIR/assign_ops2.rs:10:5
    |
 10 |     a -= a - 1;
-   |     ^^^^^^^^^^ help: replace it with: `a -= 1`
+   |     ^^^^^^^^^^
+help: Did you mean a = a - 1 or a = a - (a - 1)? Consider replacing it with
+   |
+10 |     a -= 1;
+   |     ^^^^^^
 
 error: variable appears on both sides of an assignment operation
   --> $DIR/assign_ops2.rs:11:5
    |
 11 |     a *= a * 99;
-   |     ^^^^^^^^^^^ help: replace it with: `a *= 99`
+   |     ^^^^^^^^^^^
+help: Did you mean a = a * 99 or a = a * a * 99? Consider replacing it with
+   |
+11 |     a *= 99;
+   |     ^^^^^^^
 
 error: variable appears on both sides of an assignment operation
   --> $DIR/assign_ops2.rs:12:5
    |
 12 |     a *= 42 * a;
-   |     ^^^^^^^^^^^ help: replace it with: `a *= 42`
+   |     ^^^^^^^^^^^
+help: Did you mean a = a * 42 or a = a * 42 * a? Consider replacing it with
+   |
+12 |     a *= 42;
+   |     ^^^^^^^
 
 error: variable appears on both sides of an assignment operation
   --> $DIR/assign_ops2.rs:13:5
    |
 13 |     a /= a / 2;
-   |     ^^^^^^^^^^ help: replace it with: `a /= 2`
+   |     ^^^^^^^^^^
+help: Did you mean a = a / 2 or a = a / (a / 2)? Consider replacing it with
+   |
+13 |     a /= 2;
+   |     ^^^^^^
 
 error: variable appears on both sides of an assignment operation
   --> $DIR/assign_ops2.rs:14:5
    |
 14 |     a %= a % 5;
-   |     ^^^^^^^^^^ help: replace it with: `a %= 5`
+   |     ^^^^^^^^^^
+help: Did you mean a = a % 5 or a = a % (a % 5)? Consider replacing it with
+   |
+14 |     a %= 5;
+   |     ^^^^^^
 
 error: variable appears on both sides of an assignment operation
   --> $DIR/assign_ops2.rs:15:5
    |
 15 |     a &= a & 1;
-   |     ^^^^^^^^^^ help: replace it with: `a &= 1`
+   |     ^^^^^^^^^^
+help: Did you mean a = a & 1 or a = a & a & 1? Consider replacing it with
+   |
+15 |     a &= 1;
+   |     ^^^^^^
+
+error: variable appears on both sides of an assignment operation
+  --> $DIR/assign_ops2.rs:16:5
+   |
+16 |     a *= a * a;
+   |     ^^^^^^^^^^
+help: Did you mean a = a * a or a = a * a * a? Consider replacing it with
+   |
+16 |     a *= a;
+   |     ^^^^^^
 
-error: aborting due to 8 previous errors
+error: aborting due to 9 previous errors