Better help message for comparison_chain lint

Author: samueltardieu

clippy_lints/src/comparison_chain.rs

@@ -1,4 +1,5 @@
 use clippy_utils::diagnostics::span_lint_and_help;
+use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::implements_trait;
 use clippy_utils::{SpanlessEq, if_sequence, is_else_clause, is_in_const_context};
 use rustc_hir::{BinOpKind, Expr, ExprKind};
@@ -120,13 +121,18 @@ impl<'tcx> LateLintPass<'tcx> for ComparisonChain {
                 return;
             }
         }
+        let ExprKind::Binary(_, lhs, rhs) = conds[0].kind else {
+            unreachable!();
+        };
+        let lhs = Sugg::hir(cx, lhs, "..").maybe_par();
+        let rhs = Sugg::hir(cx, rhs, "..").addr();
         span_lint_and_help(
             cx,
             COMPARISON_CHAIN,
             expr.span,
             "`if` chain can be rewritten with `match`",
             None,
-            "consider rewriting the `if` chain to use `cmp` and `match`",
+            format!("consider rewriting the `if` chain with: `match {lhs}.cmp({rhs}) {{...}}`"),
         );
     }
 }

tests/ui/comparison_chain.rs

@@ -238,4 +238,16 @@ const fn sign_i8(n: i8) -> Sign {
     }
 }
 
+fn needs_parens() -> &'static str {
+    let (x, y) = (1, 2);
+    if x + 1 > y * 2 {
+        //~^ ERROR: `if` chain can be rewritten with `match`
+        "aa"
+    } else if x + 1 < y * 2 {
+        "bb"
+    } else {
+        "cc"
+    }
+}
+
 fn main() {}

tests/ui/comparison_chain.stderr

@@ -9,7 +9,7 @@ LL | |         b()
 LL | |     }
    | |_____^
    |
-   = help: consider rewriting the `if` chain to use `cmp` and `match`
+   = help: consider rewriting the `if` chain with: `match x.cmp(&y) {...}`
    = note: `-D clippy::comparison-chain` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::comparison_chain)]`
 
@@ -25,7 +25,7 @@ LL | |         c()
 LL | |     }
    | |_____^
    |
-   = help: consider rewriting the `if` chain to use `cmp` and `match`
+   = help: consider rewriting the `if` chain with: `match x.cmp(&y) {...}`
 
 error: `if` chain can be rewritten with `match`
   --> tests/ui/comparison_chain.rs:37:5
@@ -39,7 +39,7 @@ LL | |         c()
 LL | |     }
    | |_____^
    |
-   = help: consider rewriting the `if` chain to use `cmp` and `match`
+   = help: consider rewriting the `if` chain with: `match x.cmp(&y) {...}`
 
 error: `if` chain can be rewritten with `match`
   --> tests/ui/comparison_chain.rs:46:5
@@ -53,7 +53,7 @@ LL | |         c()
 LL | |     }
    | |_____^
    |
-   = help: consider rewriting the `if` chain to use `cmp` and `match`
+   = help: consider rewriting the `if` chain with: `match x.cmp(&1) {...}`
 
 error: `if` chain can be rewritten with `match`
   --> tests/ui/comparison_chain.rs:121:5
@@ -66,7 +66,7 @@ LL | |         b()
 LL | |     }
    | |_____^
    |
-   = help: consider rewriting the `if` chain to use `cmp` and `match`
+   = help: consider rewriting the `if` chain with: `match x.cmp(&y) {...}`
 
 error: `if` chain can be rewritten with `match`
   --> tests/ui/comparison_chain.rs:128:5
@@ -80,7 +80,7 @@ LL | |         c()
 LL | |     }
    | |_____^
    |
-   = help: consider rewriting the `if` chain to use `cmp` and `match`
+   = help: consider rewriting the `if` chain with: `match x.cmp(&y) {...}`
 
 error: `if` chain can be rewritten with `match`
   --> tests/ui/comparison_chain.rs:137:5
@@ -94,7 +94,21 @@ LL | |         c()
 LL | |     }
    | |_____^
    |
-   = help: consider rewriting the `if` chain to use `cmp` and `match`
+   = help: consider rewriting the `if` chain with: `match x.cmp(&y) {...}`
 
-error: aborting due to 7 previous errors
+error: `if` chain can be rewritten with `match`
+  --> tests/ui/comparison_chain.rs:243:5
+   |
+LL | /     if x + 1 > y * 2 {
+LL | |
+LL | |         "aa"
+LL | |     } else if x + 1 < y * 2 {
+...  |
+LL | |         "cc"
+LL | |     }
+   | |_____^
+   |
+   = help: consider rewriting the `if` chain with: `match (x + 1).cmp(&(y * 2)) {...}`
+
+error: aborting due to 8 previous errors