Add proper allow(dead_code)

Author: mu001999

compiler/rustc_span/src/source_map/tests.rs

@@ -266,6 +266,7 @@ fn t10() {
 }
 
 /// Returns the span corresponding to the `n`th occurrence of `substring` in `source_text`.
+#[allow(dead_code)]
 trait SourceMapExtension {
     fn span_substr(
         &self,

library/core/tests/macros.rs

@@ -1,3 +1,4 @@
+#[allow(dead_code)]
 trait Trait {
     fn blah(&self);
 }