Remove #[macro_use] extern crate tracing from rustc_span`.

Because explicit macro imports are better than implicit macro imports.

compiler/rustc_span/src/hygiene.rs

@@ -40,6 +40,7 @@
 use std::collections::hash_map::Entry;
 use std::fmt;
 use std::hash::Hash;
+use tracing::{debug, trace};
 
 /// A `SyntaxContext` represents a chain of pairs `(ExpnId, Transparency)` named "marks".
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]

compiler/rustc_span/src/lib.rs

@@ -38,13 +38,11 @@
 // this crate without this line making `rustc_span` available.
 extern crate self as rustc_span;
 
-#[macro_use]
-extern crate tracing;
-
 use rustc_data_structures::{outline, AtomicRef};
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_serialize::opaque::{FileEncoder, MemDecoder};
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+use tracing::debug;
 
 mod caching_source_map_view;
 pub mod source_map;

compiler/rustc_span/src/source_map.rs

@@ -16,6 +16,7 @@
 use std::fs;
 use std::io::{self, BorrowedBuf, Read};
 use std::path;
+use tracing::{debug, instrument, trace};
 
 #[cfg(test)]
 mod tests;