Fix false positive in string_add.

Author: daxpedda

clippy_lints/src/strings.rs

@@ -8,7 +8,9 @@ use syntax::source_map::Spanned;
 use if_chain::if_chain;
 
 use crate::utils::SpanlessEq;
-use crate::utils::{get_parent_expr, is_allowed, match_type, paths, span_lint, span_lint_and_sugg, walk_ptrs_ty};
+use crate::utils::{
+    get_parent_expr, in_macro, is_allowed, match_type, paths, span_lint, span_lint_and_sugg, walk_ptrs_ty,
+};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for string appends of the form `x = x + y` (without
@@ -80,6 +82,10 @@ declare_lint_pass!(StringAdd => [STRING_ADD, STRING_ADD_ASSIGN]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for StringAdd {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
+        if in_macro(e.span) {
+            return;
+        }
+
         if let ExprKind::Binary(
             Spanned {
                 node: BinOpKind::Add, ..

tests/ui/string_add.rs

@@ -16,4 +16,13 @@ fn main() {
     let mut x = 1;
     x = x + 1;
     assert_eq!(2, x);
+
+    macro_rules! mac {
+        () => {
+            let y = "".to_owned();
+            let z = y + "...";
+        };
+    }
+
+    mac!();
 }