From 2cff123416cf62648ddbe7aab387aabe1a01314f Mon Sep 17 00:00:00 2001
From: Manish Goregaokar <manishsmail@gmail.com>
Date: Sun, 4 Feb 2018 22:22:26 +0530
Subject: [PATCH] Add -Zepoch

---
 src/librustc/lib.rs            |  1 +
 src/librustc/session/config.rs | 45 ++++++++++++++++++++++++++++++++--
 src/librustc/session/mod.rs    |  7 +++++-
 3 files changed, 50 insertions(+), 3 deletions(-)

diff --git a/src/librustc/lib.rs b/src/librustc/lib.rs
index db6863d6dadc..a7a261950593 100644
--- a/src/librustc/lib.rs
+++ b/src/librustc/lib.rs
@@ -58,6 +58,7 @@
 #![feature(macro_vis_matcher)]
 #![feature(match_default_bindings)]
 #![feature(never_type)]
+#![feature(non_exhaustive)]
 #![feature(nonzero)]
 #![feature(quote)]
 #![feature(refcell_replace_swap)]
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 9543d01597d0..f1590f4aced4 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -112,6 +112,31 @@ pub enum OutputType {
     DepInfo,
 }
 
+/// The epoch of the compiler (RFC 2052)
+#[derive(Clone, Copy, Hash, PartialOrd, Ord, Eq, PartialEq)]
+#[non_exhaustive]
+pub enum Epoch {
+    // epochs must be kept in order, newest to oldest
+
+    /// The 2015 epoch
+    Epoch2015,
+    /// The 2018 epoch
+    Epoch2018,
+
+    // when adding new epochs, be sure to update:
+    //
+    // - the list in the `parse_epoch` static
+    // - the match in the `parse_epoch` function
+    // - add a `rust_####()` function to the session
+    // - update the enum in Cargo's sources as well
+    //
+    // When -Zepoch becomes --epoch, there will
+    // also be a check for the epoch being nightly-only
+    // somewhere. That will need to be updated
+    // whenever we're stabilizing/introducing a new epoch
+    // as well as changing the default Cargo template.
+}
+
 impl_stable_hash_for!(enum self::OutputType {
     Bitcode,
     Assembly,
@@ -802,11 +827,13 @@ macro_rules! options {
             Some("`string` or `string=string`");
         pub const parse_lto: Option<&'static str> =
             Some("one of `thin`, `fat`, or omitted");
+        pub const parse_epoch: Option<&'static str> =
+            Some("one of: `2015`, `2018`");
     }
 
     #[allow(dead_code)]
     mod $mod_set {
-        use super::{$struct_name, Passes, SomePasses, AllPasses, Sanitizer, Lto};
+        use super::{$struct_name, Passes, SomePasses, AllPasses, Sanitizer, Lto, Epoch};
         use rustc_back::{LinkerFlavor, PanicStrategy, RelroLevel};
         use std::path::PathBuf;
 
@@ -1010,6 +1037,15 @@ macro_rules! options {
             };
             true
         }
+
+        fn parse_epoch(slot: &mut Epoch, v: Option<&str>) -> bool {
+            match v {
+                Some("2015") => *slot = Epoch::Epoch2015,
+                Some("2018") => *slot = Epoch::Epoch2018,
+                _ => return false,
+            }
+            true
+        }
     }
 ) }
 
@@ -1297,6 +1333,10 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         `everybody_loops` (all function bodies replaced with `loop {}`),
         `hir` (the HIR), `hir,identified`, or
         `hir,typed` (HIR with types for each node)."),
+    epoch: Epoch = (Epoch::Epoch2015, parse_epoch, [TRACKED],
+        "The epoch to build Rust with. Newer epochs may include features
+         that require breaking changes. The default epoch is 2015 (the first
+         epoch). Crates compiled with different epochs can be linked together."),
 }
 
 pub fn default_lib_output() -> CrateType {
@@ -2088,7 +2128,7 @@ mod dep_tracking {
     use std::path::PathBuf;
     use std::collections::hash_map::DefaultHasher;
     use super::{Passes, CrateType, OptLevel, DebugInfoLevel, Lto,
-                OutputTypes, Externs, ErrorOutputType, Sanitizer};
+                OutputTypes, Externs, ErrorOutputType, Sanitizer, Epoch};
     use syntax::feature_gate::UnstableFeatures;
     use rustc_back::{PanicStrategy, RelroLevel};
 
@@ -2150,6 +2190,7 @@ mod dep_tracking {
     impl_dep_tracking_hash_via_hash!(cstore::NativeLibraryKind);
     impl_dep_tracking_hash_via_hash!(Sanitizer);
     impl_dep_tracking_hash_via_hash!(Option<Sanitizer>);
+    impl_dep_tracking_hash_via_hash!(Epoch);
 
     impl_dep_tracking_hash_for_sortable_vec_of!(String);
     impl_dep_tracking_hash_for_sortable_vec_of!(PathBuf);
diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index f4a00a43d8d9..9d7a9acc3d53 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -19,7 +19,7 @@ use lint;
 use middle::allocator::AllocatorKind;
 use middle::dependency_format;
 use session::search_paths::PathKind;
-use session::config::{BorrowckMode, DebugInfoLevel, OutputType};
+use session::config::{BorrowckMode, DebugInfoLevel, OutputType, Epoch};
 use ty::tls;
 use util::nodemap::{FxHashMap, FxHashSet};
 use util::common::{duration_to_secs_str, ErrorReported};
@@ -864,6 +864,11 @@ impl Session {
     pub fn teach(&self, code: &DiagnosticId) -> bool {
         self.opts.debugging_opts.teach && !self.parse_sess.span_diagnostic.code_emitted(code)
     }
+
+    /// Are we allowed to use features from the Rust 2018 epoch?
+    pub fn rust_2018(&self) -> bool {
+        self.opts.debugging_opts.epoch >= Epoch::Epoch2018
+    }
 }
 
 pub fn build_session(sopts: config::Options,