Don't emit useless_attribute lint in external macros

clippy_lints/src/attrs.rs

@@ -8,7 +8,8 @@ use crate::utils::{
 use if_chain::if_chain;
 use rustc::hir::*;
 use rustc::lint::{
-    CheckLintNameResult, EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintArray, LintContext, LintPass,
+    in_external_macro, CheckLintNameResult, EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintArray,
+    LintContext, LintPass,
 };
 use rustc::ty::{self, TyCtxt};
 use rustc::{declare_tool_lint, lint_array};
@@ -241,6 +242,9 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for AttrPass {
                 let skip_unused_imports = item.attrs.iter().any(|attr| attr.check_name("macro_use"));
 
                 for attr in &item.attrs {
+                    if in_external_macro(cx.sess(), attr.span) {
+                        return;
+                    }
                     if let Some(lint_list) = &attr.meta_item_list() {
                         if let Some(ident) = attr.ident() {
                             match &*ident.as_str() {

tests/ui/auxiliary/proc_macro_derive.rs

@@ -0,0 +1,17 @@
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+#![feature(repr128, proc_macro_hygiene, proc_macro_quote)]
+
+extern crate proc_macro;
+
+use proc_macro::{quote, TokenStream};
+
+#[proc_macro_derive(DeriveSomething)]
+pub fn derive(_: TokenStream) -> TokenStream {
+    let output = quote! {
+        #[allow(dead_code)]
+        extern crate clippy_lints;
+    };
+    output
+}

tests/ui/useless_attribute.rs

@@ -1,3 +1,5 @@
+// aux-build:proc_macro_derive.rs
+
 #![warn(clippy::useless_attribute)]
 
 #[allow(dead_code)]
@@ -10,6 +12,9 @@
 #[macro_use]
 extern crate clippy_lints;
 
+#[macro_use]
+extern crate proc_macro_derive;
+
 // don't lint on unused_import for `use` items
 #[allow(unused_imports)]
 use std::collections;
@@ -22,4 +27,9 @@ mod foo {
 #[allow(deprecated)]
 pub use foo::Bar;
 
+// This should not trigger the lint. There's lint level definitions inside the external derive
+// that would trigger the useless_attribute lint.
+#[derive(DeriveSomething)]
+struct Baz;
+
 fn main() {}

tests/ui/useless_attribute.stderr

@@ -1,13 +1,13 @@
 error: useless lint attribute
-  --> $DIR/useless_attribute.rs:3:1
+  --> $DIR/useless_attribute.rs:5:1
    |
 LL | #[allow(dead_code)]
    | ^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![allow(dead_code)]`
    |
    = note: `-D clippy::useless-attribute` implied by `-D warnings`
 
 error: useless lint attribute
-  --> $DIR/useless_attribute.rs:4:1
+  --> $DIR/useless_attribute.rs:6:1
    |
 LL | #[cfg_attr(feature = "cargo-clippy", allow(dead_code))]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![cfg_attr(feature = "cargo-clippy", allow(dead_code)`