Gate fallback via #![feature(never_type_fallback)].

Author: Centril

src/librustc/ty/context.rs

@@ -2440,7 +2440,11 @@ impl<'tcx> TyCtxt<'tcx> {
 
     #[inline]
     pub fn mk_diverging_default(self) -> Ty<'tcx> {
-        self.types.never
+        if self.features().never_type_fallback {
+            self.types.never
+        } else {
+            self.types.unit
+        }
     }
 
     #[inline]

src/librustc_typeck/check/mod.rs

@@ -3129,8 +3129,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     // Tries to apply a fallback to `ty` if it is an unsolved variable.
-    // Non-numerics get replaced with `!`, unconstrained ints with `i32`,
-    // unconstrained floats with `f64`.
+    //
+    // - Unconstrained ints are replaced with `i32`.
+    //
+    // - Unconstrained floats are replaced with with `f64`.
+    //
+    // - Non-numerics get replaced with `!` when `#![feature(never_type_fallback)]`
+    //   is enabled. Otherwise, they are replaced with `()`.
+    //
     // Fallback becomes very dubious if we have encountered type-checking errors.
     // In that case, fallback to Error.
     // The return value indicates whether fallback has occurred.

src/libsyntax/feature_gate/active.rs

@@ -520,6 +520,9 @@ declare_features! (
     /// Allows using the `efiapi` ABI.
     (active, abi_efiapi, "1.40.0", Some(65815), None),
 
+    /// Allows diverging expressions to fall back to `!` rather than `()`.
+    (active, never_type_fallback, "1.41.0", Some(65992), None),
+
     /// Allows using the `#[register_attr]` attribute.
     (active, register_attr, "1.41.0", Some(66080), None),
 

src/libsyntax_pos/symbol.rs

@@ -444,6 +444,7 @@ symbols! {
         negate_unsigned,
         never,
         never_type,
+        never_type_fallback,
         new,
         next,
         __next,

src/test/ui/associated-types/associated-types-ICE-when-projecting-out-of-err.rs

@@ -21,5 +21,5 @@ trait Add<RHS=Self> {
 fn ice<A>(a: A) {
     let r = loop {};
     r = r + a;
-    //~^ ERROR the trait bound `!: Add<A>` is not satisfied
+    //~^ ERROR the trait bound `(): Add<A>` is not satisfied
 }

src/test/ui/associated-types/associated-types-ICE-when-projecting-out-of-err.stderr

@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `!: Add<A>` is not satisfied
+error[E0277]: the trait bound `(): Add<A>` is not satisfied
   --> $DIR/associated-types-ICE-when-projecting-out-of-err.rs:23:11
    |
 LL |     r = r + a;
-   |           ^ the trait `Add<A>` is not implemented for `!`
+   |           ^ the trait `Add<A>` is not implemented for `()`
 
 error: aborting due to previous error
 

src/test/ui/binding/empty-types-in-patterns.rs

@@ -1,4 +1,5 @@
 // run-pass
+#![feature(never_type_fallback)]
 #![feature(exhaustive_patterns)]
 #![feature(slice_patterns)]
 #![allow(unreachable_patterns)]

src/test/ui/coercion/coerce-issue-49593-box-never.rs

@@ -1,5 +1,5 @@
 // check-pass
-
+#![feature(never_type_fallback)]
 #![allow(unreachable_code)]
 
 use std::error::Error;

src/test/ui/never_type/defaulted-never-note.rs

@@ -1,3 +1,7 @@
+// We need to opt into the `never_type_fallback` feature
+// to trigger the requirement that this is testing.
+#![feature(never_type_fallback)]
+
 #![allow(unused)]
 
 trait Deserialize: Sized {

src/test/ui/never_type/defaulted-never-note.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `!: ImplementedForUnitButNotNever` is not satisfied
-  --> $DIR/defaulted-never-note.rs:23:5
+  --> $DIR/defaulted-never-note.rs:27:5
    |
 LL | fn foo<T: ImplementedForUnitButNotNever>(_t: T) {}
    |    ---    ----------------------------- required by this bound in `foo`