[unnecessary_cast]: Avoid breaking precedence

If the whole cast expression is a unary expression (`(*x as T)`) or an
addressof expression (`(&x as T)`), then not surrounding the suggestion
into a block risks us changing the precedence of operators if the cast
expression is followed by an operation with higher precedence than the
unary operator (`(*x as T).foo()` would become `*x.foo()`, which changes
what the `*` applies on).
The same is true if the expression encompassing the cast expression is a
unary expression or an addressof expression.

The lint supports the latter case, but missed the former one. This PR
fixes that.

Fixes #11968

Author: Ethiraric

clippy_lints/src/casts/unnecessary_cast.rs

@@ -151,13 +151,24 @@ pub(super) fn check<'tcx>(
             return false;
         }
 
+        // If the whole cast expression is a unary expression (`(*x as T)`) or an addressof
+        // expression (`(&x as T)`), then not surrounding the suggestion into a block risks us
+        // changing the precedence of operators if the cast expression is followed by an operation
+        // with higher precedence than the unary operator (`(*x as T).foo()` would become
+        // `*x.foo()`, which changes what the `*` applies on).
+        // The same is true if the expression encompassing the cast expression is a unary
+        // expression or an addressof expression.
+        let needs_block = matches!(cast_expr.kind, ExprKind::Unary(..) | ExprKind::AddrOf(..))
+            || get_parent_expr(cx, expr)
+                .map_or(false, |e| matches!(e.kind, ExprKind::Unary(..) | ExprKind::AddrOf(..)));
+
         span_lint_and_sugg(
             cx,
             UNNECESSARY_CAST,
             expr.span,
             &format!("casting to the same type is unnecessary (`{cast_from}` -> `{cast_to}`)"),
             "try",
-            if get_parent_expr(cx, expr).map_or(false, |e| matches!(e.kind, ExprKind::AddrOf(..))) {
+            if needs_block {
                 format!("{{ {cast_str} }}")
             } else {
                 cast_str

tests/ui/unnecessary_cast.fixed

@@ -221,4 +221,10 @@ mod fixable {
     fn issue_9603() {
         let _: f32 = -0x400 as f32;
     }
+
+    // Issue #11968: The suggestion for this lint removes the parentheses and leave the code as
+    // `*x.pow(2)` which tries to dereference the return value rather than `x`.
+    fn issue_11968(x: &usize) -> usize {
+        { *x }.pow(2)
+    }
 }

tests/ui/unnecessary_cast.rs

@@ -221,4 +221,10 @@ mod fixable {
     fn issue_9603() {
         let _: f32 = -0x400 as f32;
     }
+
+    // Issue #11968: The suggestion for this lint removes the parentheses and leave the code as
+    // `*x.pow(2)` which tries to dereference the return value rather than `x`.
+    fn issue_11968(x: &usize) -> usize {
+        (*x as usize).pow(2)
+    }
 }

tests/ui/unnecessary_cast.stderr

@@ -241,5 +241,11 @@ error: casting to the same type is unnecessary (`f32` -> `f32`)
 LL |         let _num = foo() as f32;
    |                    ^^^^^^^^^^^^ help: try: `foo()`
 
-error: aborting due to 40 previous errors
+error: casting to the same type is unnecessary (`usize` -> `usize`)
+  --> tests/ui/unnecessary_cast.rs:228:9
+   |
+LL |         (*x as usize).pow(2)
+   |         ^^^^^^^^^^^^^ help: try: `{ *x }`
+
+error: aborting due to 41 previous errors