Allow future-incompat lints to mention an epoch

src/librustc/session/config.rs

@@ -113,7 +113,7 @@ pub enum OutputType {
 }
 
 /// The epoch of the compiler (RFC 2052)
-#[derive(Clone, Copy, Hash, PartialOrd, Ord, Eq, PartialEq)]
+#[derive(Clone, Copy, Hash, PartialOrd, Ord, Eq, PartialEq, Debug)]
 #[non_exhaustive]
 pub enum Epoch {
     // epochs must be kept in order, newest to oldest
@@ -148,6 +148,15 @@ impl ToString for Epoch {
     }
 }
 
+impl Epoch {
+    pub fn lint_name(&self) -> &'static str {
+        match *self {
+            Epoch::Epoch2015 => "epoch_2015",
+            Epoch::Epoch2018 => "epoch_2018",
+        }
+    }
+}
+
 impl str::FromStr for Epoch {
     type Err = ();
     fn from_str(s: &str) -> Result<Self, ()> {

src/librustc/session/mod.rs

@@ -869,6 +869,10 @@ impl Session {
     pub fn rust_2018(&self) -> bool {
         self.opts.debugging_opts.epoch >= Epoch::Epoch2018
     }
+
+    pub fn epoch(&self) -> Epoch {
+        self.opts.debugging_opts.epoch
+    }
 }
 
 pub fn build_session(sopts: config::Options,