Note the future epoch for epoch lints

src/librustc/lint/mod.rs

@@ -493,9 +493,14 @@ pub fn struct_lint_level<'a>(sess: &'a Session,
     // Check for future incompatibility lints and issue a stronger warning.
     let lints = sess.lint_store.borrow();
     if let Some(future_incompatible) = lints.future_incompatible(LintId::of(lint)) {
+        let future = if let Some(epoch) = future_incompatible.epoch {
+            format!("the {} epoch", epoch)
+        } else {
+            "a future release".to_owned()
+        };
         let explanation = format!("this was previously accepted by the compiler \
                                    but is being phased out; \
-                                   it will become a hard error in a future release!");
+                                   it will become a hard error in {}!", future);
         let citation = format!("for more information, see {}",
                                future_incompatible.reference);
         err.warn(&explanation);

src/libsyntax/epoch.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::fmt;
 use std::str::FromStr;
 
 /// The epoch of the compiler (RFC 2052)
@@ -37,12 +38,13 @@ pub enum Epoch {
 // must be in order from oldest to newest
 pub const ALL_EPOCHS: &[Epoch] = &[Epoch::Epoch2015, Epoch::Epoch2018];
 
-impl ToString for Epoch {
-    fn to_string(&self) -> String {
-        match *self {
-            Epoch::Epoch2015 => "2015".into(),
-            Epoch::Epoch2018 => "2018".into(),
-        }
+impl fmt::Display for Epoch {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let s = match *self {
+            Epoch::Epoch2015 => "2015",
+            Epoch::Epoch2018 => "2018",
+        };
+        write!(f, "{}", s)
     }
 }