Don't trigger missing_const_for_fn in external macros

As reported in #3841. Only fixes the part where it triggers on the
`derive`.

Author: phansch

clippy_lints/src/missing_const_for_fn.rs

@@ -2,7 +2,7 @@ use crate::utils::{is_entrypoint_fn, span_lint};
 use rustc::hir;
 use rustc::hir::intravisit::FnKind;
 use rustc::hir::{Body, Constness, FnDecl, HirId};
-use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
+use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintPass};
 use rustc::{declare_tool_lint, lint_array};
 use rustc_mir::transform::qualify_min_const_fn::is_min_const_fn;
 use syntax_pos::Span;
@@ -82,7 +82,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
     ) {
         let def_id = cx.tcx.hir().local_def_id_from_hir_id(hir_id);
 
-        if is_entrypoint_fn(cx, def_id) {
+        if in_external_macro(cx.tcx.sess, span) || is_entrypoint_fn(cx, def_id) {
             return;
         }
 

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -55,3 +55,7 @@ trait Foo {
         33
     }
 }
+
+// Don't lint in external macros (derive)
+#[derive(PartialEq, Eq)]
+struct Point(isize, isize);