Sugg: do not parenthesize a double unary operator

For example, adding `*` in front of `*expression` is best shown as
`**expression` rather than `*(*expression)`.

Author: samueltardieu

clippy_utils/src/sugg.rs

@@ -494,7 +494,17 @@ impl<T: Display> Display for ParenHelper<T> {
 /// operators have the same
 /// precedence.
 pub fn make_unop(op: &str, expr: Sugg<'_>) -> Sugg<'static> {
-    Sugg::MaybeParen(format!("{op}{}", expr.maybe_paren()).into())
+    // If the `expr` starts with `op` already, do not add wrap it in
+    // parentheses.
+    let expr = if let Sugg::MaybeParen(ref sugg) = expr
+        && !has_enclosing_paren(sugg)
+        && sugg.starts_with(op)
+    {
+        expr
+    } else {
+        expr.maybe_paren()
+    };
+    Sugg::MaybeParen(format!("{op}{expr}").into())
 }
 
 /// Builds the string for `<lhs> <op> <rhs>` adding parenthesis when necessary.

tests/ui/nonminimal_bool.stderr

@@ -179,7 +179,7 @@ error: inequality checks against true can be replaced by a negation
   --> tests/ui/nonminimal_bool.rs:186:8
    |
 LL |     if !b != true {}
-   |        ^^^^^^^^^^ help: try simplifying it as shown: `!(!b)`
+   |        ^^^^^^^^^^ help: try simplifying it as shown: `!!b`
 
 error: this boolean expression can be simplified
   --> tests/ui/nonminimal_bool.rs:189:8
@@ -209,7 +209,7 @@ error: inequality checks against true can be replaced by a negation
   --> tests/ui/nonminimal_bool.rs:193:8
    |
 LL |     if true != !b {}
-   |        ^^^^^^^^^^ help: try simplifying it as shown: `!(!b)`
+   |        ^^^^^^^^^^ help: try simplifying it as shown: `!!b`
 
 error: this boolean expression can be simplified
   --> tests/ui/nonminimal_bool.rs:196:8