span: add a "future" edition

It's hard to implement edition migrations without having a perma-unstable
"future" edition to target.

compiler/rustc_span/src/edition.rs

@@ -23,11 +23,27 @@ pub enum Edition {
     Edition2021,
     /// The 2024 edition
     Edition2024,
+    /// The future edition - this variant will always exist and features associated with this
+    /// edition can be moved to the next 20XX edition when it is established and it is confirmed
+    /// that those features will be part of that edition.
+    ///
+    /// This variant allows edition changes to be implemented before being assigned to a concrete
+    /// edition - primarily when there are two different unstable behaviours that need tested across
+    /// an edition boundary.
+    ///
+    /// This edition will be permanently unstable and any features associated with this edition
+    /// must also be behind a feature gate.
+    EditionFuture,
 }
 
 // Must be in order from oldest to newest.
-pub const ALL_EDITIONS: &[Edition] =
-    &[Edition::Edition2015, Edition::Edition2018, Edition::Edition2021, Edition::Edition2024];
+pub const ALL_EDITIONS: &[Edition] = &[
+    Edition::Edition2015,
+    Edition::Edition2018,
+    Edition::Edition2021,
+    Edition::Edition2024,
+    Edition::EditionFuture,
+];
 
 pub const EDITION_NAME_LIST: &str = "2015|2018|2021|2024";
 
@@ -42,6 +58,7 @@ impl fmt::Display for Edition {
             Edition::Edition2018 => "2018",
             Edition::Edition2021 => "2021",
             Edition::Edition2024 => "2024",
+            Edition::EditionFuture => "future",
         };
         write!(f, "{s}")
     }
@@ -54,6 +71,7 @@ impl Edition {
             Edition::Edition2018 => "rust_2018_compatibility",
             Edition::Edition2021 => "rust_2021_compatibility",
             Edition::Edition2024 => "rust_2024_compatibility",
+            Edition::EditionFuture => "edition_future_compatibility",
         }
     }
 
@@ -63,6 +81,7 @@ impl Edition {
             Edition::Edition2018 => true,
             Edition::Edition2021 => true,
             Edition::Edition2024 => true,
+            Edition::EditionFuture => false,
         }
     }
 
@@ -85,6 +104,11 @@ impl Edition {
     pub fn at_least_rust_2024(self) -> bool {
         self >= Edition::Edition2024
     }
+
+    /// Are we allowed to use features from the future edition?
+    pub fn at_least_edition_future(self) -> bool {
+        self >= Edition::EditionFuture
+    }
 }
 
 impl FromStr for Edition {
@@ -95,6 +119,7 @@ impl FromStr for Edition {
             "2018" => Ok(Edition::Edition2018),
             "2021" => Ok(Edition::Edition2021),
             "2024" => Ok(Edition::Edition2024),
+            "future" => Ok(Edition::EditionFuture),
             _ => Err(()),
         }
     }

compiler/rustc_span/src/symbol.rs

@@ -1711,6 +1711,7 @@ symbols! {
         rust_cold_cc,
         rust_eh_catch_typeinfo,
         rust_eh_personality,
+        rust_future,
         rust_logo,
         rust_out,
         rustc,