test: add test for in-macro double_negations warning

Author: anatawa12

tests/ui/lint/lint-double-negations-macro.rs

@@ -4,7 +4,13 @@ macro_rules! neg {
         -$e
     };
 }
+macro_rules! bad_macro {
+    ($e: expr) => {
+        --$e //~ WARN use of a double negation
+    };
+}
 
 fn main() {
     neg!(-1);
+    bad_macro!(1);
 }

tests/ui/lint/lint-double-negations-macro.stderr

@@ -0,0 +1,20 @@
+warning: use of a double negation
+  --> $DIR/lint-double-negations-macro.rs:9:9
+   |
+LL |         --$e
+   |         ^^^^
+...
+LL |     bad_macro!(1);
+   |     ------------- in this macro invocation
+   |
+   = note: the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+   = note: use `-= 1` if you meant to decrement the value
+   = note: `#[warn(double_negations)]` on by default
+   = note: this warning originates in the macro `bad_macro` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: add parentheses for clarity
+   |
+LL |         -(-$e)
+   |          +   +
+
+warning: 1 warning emitted
+