fix: `identity_op` suggestions use correct parenthesis

The `identity_op` lint was suggesting code fixes that resulted
in incorrect or broken code, due to missing parenthesis in the fix
that changed the semantics of the code.

For a binary expression, `left op right`, if the `left` was redundant,
it would check if the right side needed parenthesis, but if the `right`
was redundant, it would just assume that the left side did not need
parenthesis.

This can result in either rustfix generating broken code and failing,
or code that has different behavior than before the fix.
e.g. `-(x + y + 0)` would turn into `-x + y`, changing the behavior,
and `1u64 + (x + y + 0i32) as u64` where `x: i32` and `y: i32` would
turn into `1u64 + x + y as u64`, creating broken code where `x` cannot
be added to the other values, as it was never cast to `u64`.

This commit fixes both of these cases by always checking the
non-redundant child of a binary expression for needed parenthesis, and
makes it so if we need parenthesis, but they already exist, we don't add
any redundant ones.

Fixes #13470

4 files changed, 252 insertions, 105 deletions

diff --git a/clippy_lints/src/operators/identity_op.rs b/clippy_lints/src/operators/identity_op.rs
index 830be50c8ba..d8430f6492d 100644
--- a/clippy_lints/src/operators/identity_op.rs
+++ b/clippy_lints/src/operators/identity_op.rs
@@ -42,83 +42,53 @@ pub(crate) fn check<'tcx>(
 
     match op {
         BinOpKind::Add | BinOpKind::BitOr | BinOpKind::BitXor => {
-            let _ = check_op(
-                cx,
-                left,
-                0,
-                expr.span,
-                peeled_right_span,
-                needs_parenthesis(cx, expr, right),
-                right_is_coerced_to_value,
-            ) || check_op(
-                cx,
-                right,
-                0,
-                expr.span,
-                peeled_left_span,
-                Parens::Unneeded,
-                left_is_coerced_to_value,
-            );
+            if is_redundant_op(cx, left, 0) {
+                let paren = needs_parenthesis(cx, expr, right);
+                span_ineffective_operation(cx, expr.span, peeled_right_span, paren, right_is_coerced_to_value);
+            } else if is_redundant_op(cx, right, 0) {
+                let paren = needs_parenthesis(cx, expr, left);
+                span_ineffective_operation(cx, expr.span, peeled_left_span, paren, left_is_coerced_to_value);
+            }
         },
         BinOpKind::Shl | BinOpKind::Shr | BinOpKind::Sub => {
-            let _ = check_op(
-                cx,
-                right,
-                0,
-                expr.span,
-                peeled_left_span,
-                Parens::Unneeded,
-                left_is_coerced_to_value,
-            );
+            if is_redundant_op(cx, right, 0) {
+                span_ineffective_operation(
+                    cx,
+                    expr.span,
+                    peeled_left_span,
+                    Parens::Unneeded,
+                    left_is_coerced_to_value,
+                );
+            }
         },
         BinOpKind::Mul => {
-            let _ = check_op(
-                cx,
-                left,
-                1,
-                expr.span,
-                peeled_right_span,
-                needs_parenthesis(cx, expr, right),
-                right_is_coerced_to_value,
-            ) || check_op(
-                cx,
-                right,
-                1,
-                expr.span,
-                peeled_left_span,
-                Parens::Unneeded,
-                left_is_coerced_to_value,
-            );
+            if is_redundant_op(cx, left, 1) {
+                let paren = needs_parenthesis(cx, expr, right);
+                span_ineffective_operation(cx, expr.span, peeled_right_span, paren, right_is_coerced_to_value);
+            } else if is_redundant_op(cx, right, 1) {
+                let paren = needs_parenthesis(cx, expr, left);
+                span_ineffective_operation(cx, expr.span, peeled_left_span, paren, left_is_coerced_to_value);
+            }
         },
         BinOpKind::Div => {
-            let _ = check_op(
-                cx,
-                right,
-                1,
-                expr.span,
-                peeled_left_span,
-                Parens::Unneeded,
-                left_is_coerced_to_value,
-            );
+            if is_redundant_op(cx, right, 1) {
+                span_ineffective_operation(
+                    cx,
+                    expr.span,
+                    peeled_left_span,
+                    Parens::Unneeded,
+                    left_is_coerced_to_value,
+                );
+            }
         },
         BinOpKind::BitAnd => {
-            let _ = check_op(
-                cx,
-                left,
-                -1,
-                expr.span,
-                peeled_right_span,
-                needs_parenthesis(cx, expr, right),
-                right_is_coerced_to_value,
-            ) || check_op(
-                cx,
-                right,
-                -1,
-                expr.span,
-                peeled_left_span,
-                Parens::Unneeded,
-                left_is_coerced_to_value,
-            );
+            if is_redundant_op(cx, left, -1) {
+                let paren = needs_parenthesis(cx, expr, right);
+                span_ineffective_operation(cx, expr.span, peeled_right_span, paren, right_is_coerced_to_value);
+            } else if is_redundant_op(cx, right, -1) {
+                let paren = needs_parenthesis(cx, expr, left);
+                span_ineffective_operation(cx, expr.span, peeled_left_span, paren, left_is_coerced_to_value);
+            }
         },
         BinOpKind::Rem => check_remainder(cx, left, right, expr.span, left.span),
         _ => (),
@@ -138,43 +108,75 @@ enum Parens {
     Unneeded,
 }
 
-/// Checks if `left op right` needs parenthesis when reduced to `right`
+/// Checks if a binary expression needs parenthesis when reduced to just its
+/// right or left child.
+///
+/// e.g. `-(x + y + 0)` cannot be reduced to `-x + y`, as the behavior changes silently.
+/// e.g. `1u64 + ((x + y + 0i32) as u64)` cannot be reduced to `1u64 + x + y as u64`, since
+/// the the cast expression will not apply to the same expression.
 /// e.g. `0 + if b { 1 } else { 2 } + if b { 3 } else { 4 }` cannot be reduced
 /// to `if b { 1 } else { 2 } + if b { 3 } else { 4 }` where the `if` could be
-/// interpreted as a statement
+/// interpreted as a statement. The same behavior happens for `match`, `loop`,
+/// and blocks.
+/// e.g.  `2 * (0 + { a })` can be reduced to `2 * { a }` without the need for parenthesis,
+/// but `1 * ({ a } + 4)` cannot be reduced to `{ a } + 4`, as a block at the start of a line
+/// will be interpreted as a statement instead of an expression.
 ///
-/// See #8724
-fn needs_parenthesis(cx: &LateContext<'_>, binary: &Expr<'_>, right: &Expr<'_>) -> Parens {
-    match right.kind {
+/// See #8724, #13470
+fn needs_parenthesis(cx: &LateContext<'_>, binary: &Expr<'_>, child: &Expr<'_>) -> Parens {
+    match child.kind {
         ExprKind::Binary(_, lhs, _) | ExprKind::Cast(lhs, _) => {
-            // ensure we're checking against the leftmost expression of `right`
-            //
-            //     ~~~ `lhs`
-            // 0 + {4} * 2
-            //     ~~~~~~~ `right`
-            return needs_parenthesis(cx, binary, lhs);
+            // For casts and binary expressions, we want to add parenthesis if
+            // the parent HIR node is an expression with a different precedence,
+            // or if the parent HIR node is a Block or Stmt, and the new left hand side
+            // would be treated as a statement rather than an expression.
+            if let Some((_, parent)) = cx.tcx.hir().parent_iter(binary.hir_id).next() {
+                if let Node::Expr(expr) = parent {
+                    if child.precedence().order() != expr.precedence().order() {
+                        return Parens::Needed;
+                    }
+                    return Parens::Unneeded;
+                }
+                match parent {
+                    Node::Block(_) | Node::Stmt(_) => {
+                        // ensure we're checking against the leftmost expression of `child`
+                        //
+                        // ~~~~~~~~~~~ `binary`
+                        //     ~~~ `lhs`
+                        // 0 + {4} * 2
+                        //     ~~~~~~~ `child`
+                        return needs_parenthesis(cx, binary, lhs);
+                    },
+                    _ => return Parens::Unneeded,
+                }
+            }
         },
-        ExprKind::If(..) | ExprKind::Match(..) | ExprKind::Block(..) | ExprKind::Loop(..) => {},
-        _ => return Parens::Unneeded,
-    }
+        ExprKind::If(..) | ExprKind::Match(..) | ExprKind::Block(..) | ExprKind::Loop(..) => {
+            // For if, match, block, and loop expressions, we want to add parenthesis if
+            // the closest ancestor node that is not an expression is a block or statement.
+            // This would mean that the rustfix suggestion will appear at the start of a line, which causes
+            // these expressions to be interpreted as statements if they do not have parenthesis.
+            let mut prev_id = binary.hir_id;
+            for (_, parent) in cx.tcx.hir().parent_iter(binary.hir_id) {
+                if let Node::Expr(expr) = parent
+                    && let ExprKind::Binary(_, lhs, _) | ExprKind::Cast(lhs, _) | ExprKind::Unary(_, lhs) = expr.kind
+                    && lhs.hir_id == prev_id
+                {
+                    // keep going until we find a node that encompasses left of `binary`
+                    prev_id = expr.hir_id;
+                    continue;
+                }
 
-    let mut prev_id = binary.hir_id;
-    for (_, node) in cx.tcx.hir().parent_iter(binary.hir_id) {
-        if let Node::Expr(expr) = node
-            && let ExprKind::Binary(_, lhs, _) | ExprKind::Cast(lhs, _) = expr.kind
-            && lhs.hir_id == prev_id
-        {
-            // keep going until we find a node that encompasses left of `binary`
-            prev_id = expr.hir_id;
-            continue;
-        }
-
-        match node {
-            Node::Block(_) | Node::Stmt(_) => break,
-            _ => return Parens::Unneeded,
-        };
+                match parent {
+                    Node::Block(_) | Node::Stmt(_) => return Parens::Needed,
+                    _ => return Parens::Unneeded,
+                };
+            }
+        },
+        _ => {
+            return Parens::Unneeded;
+        },
     }
-
     Parens::Needed
 }
 
@@ -199,7 +201,7 @@ fn check_remainder(cx: &LateContext<'_>, left: &Expr<'_>, right: &Expr<'_>, span
     }
 }
 
-fn check_op(cx: &LateContext<'_>, e: &Expr<'_>, m: i8, span: Span, arg: Span, parens: Parens, is_erased: bool) -> bool {
+fn is_redundant_op(cx: &LateContext<'_>, e: &Expr<'_>, m: i8) -> bool {
     if let Some(Constant::Int(v)) = ConstEvalCtxt::new(cx).eval_simple(e).map(Constant::peel_refs) {
         let check = match *cx.typeck_results().expr_ty(e).peel_refs().kind() {
             ty::Int(ity) => unsext(cx.tcx, -1_i128, ity),
@@ -212,7 +214,6 @@ fn check_op(cx: &LateContext<'_>, e: &Expr<'_>, m: i8, span: Span, arg: Span, pa
             1 => v == 1,
             _ => unreachable!(),
         } {
-            span_ineffective_operation(cx, span, arg, parens, is_erased);
             return true;
         }
     }
@@ -234,7 +235,13 @@ fn span_ineffective_operation(
         expr_snippet.into_owned()
     };
     let suggestion = match parens {
-        Parens::Needed => format!("({expr_snippet})"),
+        Parens::Needed => {
+            if !expr_snippet.starts_with('(') && !expr_snippet.ends_with(')') {
+                format!("({expr_snippet})")
+            } else {
+                expr_snippet
+            }
+        },
         Parens::Unneeded => expr_snippet,
     };
 
diff --git a/tests/ui/identity_op.fixed b/tests/ui/identity_op.fixed
index b18d8560f6f..18b7401768d 100644
--- a/tests/ui/identity_op.fixed
+++ b/tests/ui/identity_op.fixed
@@ -149,7 +149,7 @@ fn main() {
 
     2 * { a };
     //~^ ERROR: this operation has no effect
-    (({ a } + 4));
+    ({ a } + 4);
     //~^ ERROR: this operation has no effect
     1;
     //~^ ERROR: this operation has no effect
@@ -212,3 +212,43 @@ fn issue_12050() {
         //~^ ERROR: this operation has no effect
     }
 }
+
+fn issue_13470() {
+    let x = 1i32;
+    let y = 1i32;
+    // Removes the + 0i32 while keeping the parentheses around x + y so the cast operation works
+    let _: u64 = (x + y) as u64;
+    //~^ ERROR: this operation has no effect
+    // both of the next two lines should look the same after rustfix
+    let _: u64 = 1u64 & (x + y) as u64;
+    //~^ ERROR: this operation has no effect
+    // Same as above, but with extra redundant parenthesis
+    let _: u64 = 1u64 & (x + y) as u64;
+    //~^ ERROR: this operation has no effect
+    // Should maintain parenthesis even if the surrounding expr has the same precedence
+    let _: u64 = 5u64 + (x + y) as u64;
+    //~^ ERROR: this operation has no effect
+
+    // If we don't maintain the parens here, the behavior changes
+    let _ = -(x + y);
+    //~^ ERROR: this operation has no effect
+    // Maintain parenthesis if the parent expr is of higher precedence
+    let _ = 2i32 * (x + y);
+    //~^ ERROR: this operation has no effect
+    // No need for parenthesis if the parent expr is of equal precedence
+    let _ = 2i32 + x + y;
+    //~^ ERROR: this operation has no effect
+    // But make sure that inner parens still exist
+    let z = 1i32;
+    let _ = 2 + x + (y * z);
+    //~^ ERROR: this operation has no effect
+    // Maintain parenthesis if the parent expr is of lower precedence
+    // This is for clarity, and clippy will not warn on these being unnecessary
+    let _ = 2i32 + (x * y);
+    //~^ ERROR: this operation has no effect
+
+    let x = 1i16;
+    let y = 1i16;
+    let _: u64 = 1u64 + (x as i32 + y as i32) as u64;
+    //~^ ERROR: this operation has no effect
+}
diff --git a/tests/ui/identity_op.rs b/tests/ui/identity_op.rs
index f1f01b42447..55483f4e5fb 100644
--- a/tests/ui/identity_op.rs
+++ b/tests/ui/identity_op.rs
@@ -212,3 +212,43 @@ fn issue_12050() {
         //~^ ERROR: this operation has no effect
     }
 }
+
+fn issue_13470() {
+    let x = 1i32;
+    let y = 1i32;
+    // Removes the + 0i32 while keeping the parentheses around x + y so the cast operation works
+    let _: u64 = (x + y + 0i32) as u64;
+    //~^ ERROR: this operation has no effect
+    // both of the next two lines should look the same after rustfix
+    let _: u64 = 1u64 & (x + y + 0i32) as u64;
+    //~^ ERROR: this operation has no effect
+    // Same as above, but with extra redundant parenthesis
+    let _: u64 = 1u64 & ((x + y) + 0i32) as u64;
+    //~^ ERROR: this operation has no effect
+    // Should maintain parenthesis even if the surrounding expr has the same precedence
+    let _: u64 = 5u64 + ((x + y) + 0i32) as u64;
+    //~^ ERROR: this operation has no effect
+
+    // If we don't maintain the parens here, the behavior changes
+    let _ = -(x + y + 0i32);
+    //~^ ERROR: this operation has no effect
+    // Maintain parenthesis if the parent expr is of higher precedence
+    let _ = 2i32 * (x + y + 0i32);
+    //~^ ERROR: this operation has no effect
+    // No need for parenthesis if the parent expr is of equal precedence
+    let _ = 2i32 + (x + y + 0i32);
+    //~^ ERROR: this operation has no effect
+    // But make sure that inner parens still exist
+    let z = 1i32;
+    let _ = 2 + (x + (y * z) + 0);
+    //~^ ERROR: this operation has no effect
+    // Maintain parenthesis if the parent expr is of lower precedence
+    // This is for clarity, and clippy will not warn on these being unnecessary
+    let _ = 2i32 + (x * y * 1i32);
+    //~^ ERROR: this operation has no effect
+
+    let x = 1i16;
+    let y = 1i16;
+    let _: u64 = 1u64 + ((x as i32 + y as i32) as u64 + 0u64);
+    //~^ ERROR: this operation has no effect
+}
diff --git a/tests/ui/identity_op.stderr b/tests/ui/identity_op.stderr
index 9fff86b86f9..856bebfa1ff 100644
--- a/tests/ui/identity_op.stderr
+++ b/tests/ui/identity_op.stderr
@@ -221,7 +221,7 @@ error: this operation has no effect
   --> tests/ui/identity_op.rs:152:5
    |
 LL |     1 * ({ a } + 4);
-   |     ^^^^^^^^^^^^^^^ help: consider reducing it to: `(({ a } + 4))`
+   |     ^^^^^^^^^^^^^^^ help: consider reducing it to: `({ a } + 4)`
 
 error: this operation has no effect
   --> tests/ui/identity_op.rs:154:5
@@ -313,5 +313,65 @@ error: this operation has no effect
 LL |         let _: i32 = **&&*&x + 0;
    |                      ^^^^^^^^^^^ help: consider reducing it to: `***&&*&x`
 
-error: aborting due to 52 previous errors
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:220:18
+   |
+LL |     let _: u64 = (x + y + 0i32) as u64;
+   |                  ^^^^^^^^^^^^^^ help: consider reducing it to: `(x + y)`
+
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:223:25
+   |
+LL |     let _: u64 = 1u64 & (x + y + 0i32) as u64;
+   |                         ^^^^^^^^^^^^^^ help: consider reducing it to: `(x + y)`
+
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:226:25
+   |
+LL |     let _: u64 = 1u64 & ((x + y) + 0i32) as u64;
+   |                         ^^^^^^^^^^^^^^^^ help: consider reducing it to: `(x + y)`
+
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:229:25
+   |
+LL |     let _: u64 = 5u64 + ((x + y) + 0i32) as u64;
+   |                         ^^^^^^^^^^^^^^^^ help: consider reducing it to: `(x + y)`
+
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:233:14
+   |
+LL |     let _ = -(x + y + 0i32);
+   |              ^^^^^^^^^^^^^^ help: consider reducing it to: `(x + y)`
+
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:236:20
+   |
+LL |     let _ = 2i32 * (x + y + 0i32);
+   |                    ^^^^^^^^^^^^^^ help: consider reducing it to: `(x + y)`
+
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:239:20
+   |
+LL |     let _ = 2i32 + (x + y + 0i32);
+   |                    ^^^^^^^^^^^^^^ help: consider reducing it to: `x + y`
+
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:243:17
+   |
+LL |     let _ = 2 + (x + (y * z) + 0);
+   |                 ^^^^^^^^^^^^^^^^^ help: consider reducing it to: `x + (y * z)`
+
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:247:20
+   |
+LL |     let _ = 2i32 + (x * y * 1i32);
+   |                    ^^^^^^^^^^^^^^ help: consider reducing it to: `(x * y)`
+
+error: this operation has no effect
+  --> tests/ui/identity_op.rs:252:25
+   |
+LL |     let _: u64 = 1u64 + ((x as i32 + y as i32) as u64 + 0u64);
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider reducing it to: `(x as i32 + y as i32) as u64`
+
+error: aborting due to 62 previous errors