Fix useless_format false positive with macros

Clippy was issuing a warning when `format!` was used inside a macro.
That's a problem because macros have different syntax and can be outside
the control of the user.

This skips the `useless_format` check if the `format!` call is inside a
macro.

clippy_lints/src/format.rs

@@ -3,7 +3,7 @@ use rustc::lint::*;
 use rustc::ty;
 use syntax::ast::LitKind;
 use utils::paths;
-use utils::{is_expn_of, match_def_path, match_type, opt_def_id, resolve_node, snippet, span_lint_and_then, walk_ptrs_ty};
+use utils::{in_macro, is_expn_of, match_def_path, match_type, opt_def_id, resolve_node, snippet, span_lint_and_then, walk_ptrs_ty};
 
 /// **What it does:** Checks for the use of `format!("string literal with no
 /// argument")` and `format!("{}", foo)` where `foo` is a string.
@@ -39,6 +39,9 @@ impl LintPass for Pass {
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
         if let Some(span) = is_expn_of(expr.span, "format") {
+            if in_macro(span) {
+                return;
+            }
             match expr.node {
                 // `format!("{}", foo)` expansion
                 ExprCall(ref fun, ref args) => {

tests/ui/format.rs

@@ -2,6 +2,12 @@
 
 #![warn(useless_format)]
 
+struct Foo(pub String);
+
+macro_rules! foo {
+  ($($t:tt)*) => (Foo(format!($($t)*)))
+}
+
 fn main() {
     format!("foo");
 
@@ -31,4 +37,7 @@ fn main() {
     println!("foo {}", "foo");
     println!("{}", 42);
     println!("foo {}", 42);
+
+    // A format! inside a macro should not trigger a warning
+    foo!("should not warn");
 }

tests/ui/format.stderr

@@ -1,7 +1,7 @@
 error: useless use of `format!`
- --> $DIR/format.rs:6:5
+  --> $DIR/format.rs:12:5
    |
-6 |     format!("foo");
+12 |     format!("foo");
    |     ^^^^^^^^^^^^^^^ help: consider using .to_string(): `"foo".to_string()`
    |
    = note: `-D useless-format` implied by `-D warnings`