Account for macros in const generics

Author: estebank

compiler/rustc_lint/src/unused.rs

@@ -1138,6 +1138,7 @@ impl UnusedDelimLint for UnusedBraces {
                             && !cx.sess().source_map().is_multiline(value.span)
                             && value.attrs.is_empty()
                             && !value.span.from_expansion()
+                            && !inner.span.from_expansion()
                         {
                             self.emit_unused_delims_expr(cx, value, ctx, left_pos, right_pos)
                         }

src/test/ui/lint/unused_braces_macro.rs

@@ -0,0 +1,6 @@
+// build-pass
+pub fn foo<const BAR: bool> () {}
+
+fn main() {
+    foo::<{cfg!(feature = "foo")}>();
+}