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

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`

src/test/ui/never_type/diverging-fallback-control-flow.rs

@@ -11,6 +11,8 @@
 // These represent current behavior, but are pretty dubious.  I would
 // like to revisit these and potentially change them. --nmatsakis
 
+#![feature(never_type_fallback)]
+
 trait BadDefault {
     fn default() -> Self;
 }

src/test/ui/never_type/feature-gate-never_type_fallback.rs

@@ -0,0 +1,12 @@
+// This is a feature gate test for `never_type_fallback`.
+// It works by using a scenario where the type fall backs to `()` rather than ´!`
+// in the case where `#![feature(never_type_fallback)]` would change it to `!`.
+
+fn main() {}
+
+trait T {}
+
+fn should_ret_unit() -> impl T {
+    //~^ ERROR the trait bound `(): T` is not satisfied
+    panic!()
+}

src/test/ui/never_type/feature-gate-never_type_fallback.stderr

@@ -0,0 +1,11 @@
+error[E0277]: the trait bound `(): T` is not satisfied
+  --> $DIR/feature-gate-never_type_fallback.rs:9:25
+   |
+LL | fn should_ret_unit() -> impl T {
+   |                         ^^^^^^ the trait `T` is not implemented for `()`
+   |
+   = note: the return type of a function must have a statically known size
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/never_type/issue-13352.stderr

@@ -1,10 +1,10 @@
-error[E0277]: cannot add `!` to `usize`
+error[E0277]: cannot add `()` to `usize`
   --> $DIR/issue-13352.rs:9:13
    |
 LL |     2_usize + (loop {});
-   |             ^ no implementation for `usize + !`
+   |             ^ no implementation for `usize + ()`
    |
-   = help: the trait `std::ops::Add<!>` is not implemented for `usize`
+   = help: the trait `std::ops::Add<()>` is not implemented for `usize`
 
 error: aborting due to previous error
 

src/test/ui/never_type/issue-2149.stderr

@@ -1,10 +1,10 @@
-error[E0277]: cannot add `std::vec::Vec<B>` to `!`
+error[E0277]: cannot add `std::vec::Vec<B>` to `()`
   --> $DIR/issue-2149.rs:8:33
    |
 LL |         for elt in self { r = r + f(*elt); }
-   |                                 ^ no implementation for `! + std::vec::Vec<B>`
+   |                                 ^ no implementation for `() + std::vec::Vec<B>`
    |
-   = help: the trait `std::ops::Add<std::vec::Vec<B>>` is not implemented for `!`
+   = help: the trait `std::ops::Add<std::vec::Vec<B>>` is not implemented for `()`
 
 error[E0599]: no method named `bind` found for type `[&str; 1]` in the current scope
   --> $DIR/issue-2149.rs:13:12

src/test/ui/reachable/unreachable-loop-patterns.rs

@@ -1,3 +1,4 @@
+#![feature(never_type_fallback)]
 #![feature(exhaustive_patterns)]
 
 #![allow(unreachable_code)]

src/test/ui/reachable/unreachable-loop-patterns.stderr

@@ -1,17 +1,17 @@
 error: unreachable pattern
-  --> $DIR/unreachable-loop-patterns.rs:17:9
+  --> $DIR/unreachable-loop-patterns.rs:18:9
    |
 LL |     for _ in unimplemented!() as Void {}
    |         ^
    |
 note: lint level defined here
-  --> $DIR/unreachable-loop-patterns.rs:4:9
+  --> $DIR/unreachable-loop-patterns.rs:5:9
    |
 LL | #![deny(unreachable_patterns)]
    |         ^^^^^^^^^^^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/unreachable-loop-patterns.rs:17:14
+  --> $DIR/unreachable-loop-patterns.rs:18:14
    |
 LL |     for _ in unimplemented!() as Void {}
    |              ^^^^^^^^^^^^^^^^^^^^^^^^

src/test/ui/type-alias-impl-trait/different_defining_uses_never_type.stderr

@@ -4,7 +4,7 @@ error: concrete type differs from previous defining opaque type use
 LL | / fn bar() -> Foo {
 LL | |     panic!()
 LL | | }
-   | |_^ expected `&'static str`, got `!`
+   | |_^ expected `&'static str`, got `()`
    |
 note: previous use here
   --> $DIR/different_defining_uses_never_type.rs:8:1
@@ -20,7 +20,7 @@ error: concrete type differs from previous defining opaque type use
 LL | / fn boo() -> Foo {
 LL | |     loop {}
 LL | | }
-   | |_^ expected `&'static str`, got `!`
+   | |_^ expected `&'static str`, got `()`
    |
 note: previous use here
   --> $DIR/different_defining_uses_never_type.rs:8:1