Fix false positive in string_add.

clippy_lints/src/strings.rs

@@ -1,6 +1,6 @@
 use rustc::declare_lint_pass;
 use rustc::hir::*;
-use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
+use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintContext, LintPass};
 use rustc_errors::Applicability;
 use rustc_session::declare_tool_lint;
 use syntax::source_map::Spanned;
@@ -80,6 +80,10 @@ 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_external_macro(cx.sess(), e.span) {
+            return;
+        }
+
         if let ExprKind::Binary(
             Spanned {
                 node: BinOpKind::Add, ..

tests/ui/auxiliary/macro_rules.rs

@@ -31,3 +31,11 @@ macro_rules! try_err {
         }
     };
 }
+
+#[macro_export]
+macro_rules! string_add {
+    () => {
+        let y = "".to_owned();
+        let z = y + "...";
+    };
+}

tests/ui/string_add.rs

@@ -1,3 +1,8 @@
+// aux-build:macro_rules.rs
+
+#[macro_use]
+extern crate macro_rules;
+
 #[warn(clippy::string_add)]
 #[allow(clippy::string_add_assign, unused)]
 fn main() {
@@ -16,4 +21,6 @@ fn main() {
     let mut x = 1;
     x = x + 1;
     assert_eq!(2, x);
+
+    string_add!();
 }

tests/ui/string_add.stderr

@@ -1,27 +1,27 @@
 error: manual implementation of an assign operation
-  --> $DIR/string_add.rs:8:9
+  --> $DIR/string_add.rs:13:9
    |
 LL |         x = x + ".";
    |         ^^^^^^^^^^^ help: replace it with: `x += "."`
    |
    = note: `-D clippy::assign-op-pattern` implied by `-D warnings`
 
 error: you added something to a string. Consider using `String::push_str()` instead
-  --> $DIR/string_add.rs:8:13
+  --> $DIR/string_add.rs:13:13
    |
 LL |         x = x + ".";
    |             ^^^^^^^
    |
    = note: `-D clippy::string-add` implied by `-D warnings`
 
 error: you added something to a string. Consider using `String::push_str()` instead
-  --> $DIR/string_add.rs:12:13
+  --> $DIR/string_add.rs:17:13
    |
 LL |     let z = y + "...";
    |             ^^^^^^^^^
 
 error: manual implementation of an assign operation
-  --> $DIR/string_add.rs:17:5
+  --> $DIR/string_add.rs:22:5
    |
 LL |     x = x + 1;
    |     ^^^^^^^^^ help: replace it with: `x += 1`