Implement lint for obligations broken by never type fallback change

compiler/rustc_hir_typeck/messages.ftl

@@ -44,6 +44,9 @@ hir_typeck_convert_using_method = try using `{$sugg}` to convert `{$found}` to `
 
 hir_typeck_ctor_is_private = tuple struct constructor `{$def}` is private
 
+hir_typeck_dependency_on_unit_never_type_fallback = this function depends on never type fallback being `()`
+    .help = specify the types explicitly
+
 hir_typeck_deref_is_empty = this expression `Deref`s to `{$deref_ty}` which implements `is_empty`
 
 hir_typeck_expected_default_return_type = expected `()` because of default return type

compiler/rustc_hir_typeck/src/errors.rs

@@ -183,6 +183,11 @@ pub enum NeverTypeFallbackFlowingIntoUnsafe {
     Deref,
 }
 
+#[derive(LintDiagnostic)]
+#[help]
+#[diag(hir_typeck_dependency_on_unit_never_type_fallback)]
+pub struct DependencyOnUnitNeverTypeFallback {}
+
 #[derive(Subdiagnostic)]
 #[multipart_suggestion(
     hir_typeck_add_missing_parentheses_in_range,

compiler/rustc_hir_typeck/src/fallback.rs

@@ -8,12 +8,17 @@
 use rustc_hir as hir;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::HirId;
-use rustc_infer::infer::{DefineOpaqueTypes, InferOk};
 use rustc_middle::bug;
+use rustc_infer::{
+    infer::{DefineOpaqueTypes, InferOk},
+    traits::ObligationCause,
+};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable};
 use rustc_session::lint;
 use rustc_span::DUMMY_SP;
 use rustc_span::{def_id::LocalDefId, Span};
+use rustc_trait_selection::traits::ObligationCtxt;
+use rustc_type_ir::TyVid;
 
 #[derive(Copy, Clone)]
 pub enum DivergingFallbackBehavior {
@@ -344,6 +349,9 @@ fn calculate_diverging_fallback(
         // `!`.
         let mut diverging_fallback = UnordMap::with_capacity(diverging_vids.len());
         let unsafe_infer_vars = OnceCell::new();
+
+        self.lint_obligations_broken_by_never_type_fallback_change(behavior, &diverging_vids);
+
         for &diverging_vid in &diverging_vids {
             let diverging_ty = Ty::new_var(self.tcx, diverging_vid);
             let root_vid = self.root_var(diverging_vid);
@@ -468,6 +476,46 @@ fn lint_never_type_fallback_flowing_into_unsafe_code(
         }
     }
 
+    fn lint_obligations_broken_by_never_type_fallback_change(
+        &self,
+        behavior: DivergingFallbackBehavior,
+        diverging_vids: &[TyVid],
+    ) {
+        let DivergingFallbackBehavior::FallbackToUnit = behavior else { return };
+
+        // Returns errors which happen if fallback is set to `fallback`
+        let try_out = |fallback| {
+            self.probe(|_| {
+                let obligations = self.fulfillment_cx.borrow().pending_obligations();
+                let ocx = ObligationCtxt::new(&self.infcx);
+                ocx.register_obligations(obligations.iter().cloned());
+
+                for &diverging_vid in diverging_vids {
+                    let diverging_ty = Ty::new_var(self.tcx, diverging_vid);
+
+                    _ = ocx.eq(&ObligationCause::dummy(), self.param_env, diverging_ty, fallback);
+                }
+
+                ocx.select_where_possible()
+            })
+        };
+
+        // If we have no errors with `fallback = ()`, but *do* have errors with `fallback = !`,
+        // then this code will be broken by the never type fallback change.qba
+        let unit_errors = try_out(self.tcx.types.unit);
+        if unit_errors.is_empty()
+            && let never_errors = try_out(self.tcx.types.never)
+            && !never_errors.is_empty()
+        {
+            self.tcx.emit_node_span_lint(
+                lint::builtin::DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK,
+                self.tcx.local_def_id_to_hir_id(self.body_id),
+                self.tcx.def_span(self.body_id),
+                errors::DependencyOnUnitNeverTypeFallback {},
+            )
+        }
+    }
+
     /// Returns a graph whose nodes are (unresolved) inference variables and where
     /// an edge `?A -> ?B` indicates that the variable `?A` is coerced to `?B`.
     fn create_coercion_graph(&self) -> VecGraph<ty::TyVid, true> {

compiler/rustc_lint_defs/src/builtin.rs

@@ -34,6 +34,7 @@
         CONST_EVALUATABLE_UNCHECKED,
         CONST_ITEM_MUTATION,
         DEAD_CODE,
+        DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK,
         DEPRECATED,
         DEPRECATED_CFG_ATTR_CRATE_TYPE_NAME,
         DEPRECATED_IN_FUTURE,
@@ -4199,6 +4200,62 @@
     report_in_external_macro
 }
 
+declare_lint! {
+    /// The `dependency_on_unit_never_type_fallback` lint detects cases where code compiles with
+    /// [never type fallback] being [`()`], but will stop compiling with fallback being [`!`].
+    ///
+    /// [never type fallback]: prim@never#never-type-fallback
+    /// [`()`]: prim@unit
+    /// [`!`]:
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(dependency_on_unit_never_type_fallback)]
+    /// fn main() {
+    ///     if true {
+    ///         // return has type `!` which, is some cases, causes never type fallback
+    ///         return
+    ///     } else {
+    ///         // the type produced by this call is not specified explicitly,
+    ///         // so it will be inferred from the previous branch
+    ///         Default::default()
+    ///     };
+    ///     // depending on the fallback, this may compile (because `()` implements `Default`),
+    ///     // or it may not (because `!` does not implement `Default`)
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Due to historic reasons never type fallback was `()`, meaning that `!` got spontaneously
+    /// coerced to `()`. There are plans to change that, but they may make the code such as above
+    /// not compile. Instead of depending on the fallback, you should specify the type explicitly:
+    /// ```
+    /// if true {
+    ///     return
+    /// } else {
+    ///     // type is explicitly specified, fallback can't hurt us no more
+    ///     <() as Default>::default()
+    /// };
+    /// ```
+    ///
+    /// See [Tracking Issue for making `!` fall back to `!`](https://github.com/rust-lang/rust/issues/123748).
+    ///
+    /// [`!`]: https://doc.rust-lang.org/core/primitive.never.html
+    /// [`()`]: https://doc.rust-lang.org/core/primitive.unit.html
+    pub DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK,
+    Warn,
+    "never type fallback affecting unsafe function calls",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps,
+        reference: "issue #123748 <https://github.com/rust-lang/rust/issues/123748>",
+    };
+    report_in_external_macro
+}
+
 declare_lint! {
     /// The `byte_slice_in_packed_struct_with_derive` lint detects cases where a byte slice field
     /// (`[u8]`) or string slice field (`str`) is used in a `packed` struct that derives one or

tests/ui/never_type/defaulted-never-note.fallback.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `!: ImplementedForUnitButNotNever` is not satisfied
-  --> $DIR/defaulted-never-note.rs:30:9
+  --> $DIR/defaulted-never-note.rs:32:9
    |
 LL |     foo(_x);
    |     --- ^^ the trait `ImplementedForUnitButNotNever` is not implemented for `!`

tests/ui/never_type/defaulted-never-note.nofallback.stderr

@@ -0,0 +1,13 @@
+warning: this function depends on never type fallback being `()`
+  --> $DIR/defaulted-never-note.rs:28:1
+   |
+LL | fn smeg() {
+   | ^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/never_type/defaulted-never-note.rs

@@ -26,6 +26,8 @@ fn foo<T: ImplementedForUnitButNotNever>(_t: T) {}
 //[fallback]~^ NOTE required by this bound in `foo`
 //[fallback]~| NOTE required by a bound in `foo`
 fn smeg() {
+    //[nofallback]~^ warn: this function depends on never type fallback being `()`
+    //[nofallback]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     let _x = return;
     foo(_x);
     //[fallback]~^ ERROR the trait bound

tests/ui/never_type/dependency-on-fallback-to-unit.rs

@@ -0,0 +1,28 @@
+//@ check-pass
+
+fn main() {
+    def();
+    _ = question_mark();
+}
+
+fn def() {
+    //~^ warn: this function depends on never type fallback being `()`
+    //~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    match true {
+        false => <_>::default(),
+        true => return,
+    };
+}
+
+// <https://github.com/rust-lang/rust/issues/51125>
+// <https://github.com/rust-lang/rust/issues/39216>
+fn question_mark() -> Result<(), ()> {
+    //~^ warn: this function depends on never type fallback being `()`
+    //~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    deserialize()?;
+    Ok(())
+}
+
+fn deserialize<T: Default>() -> Result<T, ()> {
+    Ok(T::default())
+}

tests/ui/never_type/dependency-on-fallback-to-unit.stderr

@@ -0,0 +1,23 @@
+warning: this function depends on never type fallback being `()`
+  --> $DIR/dependency-on-fallback-to-unit.rs:8:1
+   |
+LL | fn def() {
+   | ^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+
+warning: this function depends on never type fallback being `()`
+  --> $DIR/dependency-on-fallback-to-unit.rs:19:1
+   |
+LL | fn question_mark() -> Result<(), ()> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+
+warning: 2 warnings emitted
+

tests/ui/never_type/diverging-fallback-control-flow.nofallback.stderr

@@ -0,0 +1,23 @@
+warning: this function depends on never type fallback being `()`
+  --> $DIR/diverging-fallback-control-flow.rs:30:1
+   |
+LL | fn assignment() {
+   | ^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+
+warning: this function depends on never type fallback being `()`
+  --> $DIR/diverging-fallback-control-flow.rs:42:1
+   |
+LL | fn assignment_rev() {
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+
+warning: 2 warnings emitted
+

tests/ui/never_type/diverging-fallback-control-flow.rs

@@ -28,6 +28,8 @@ fn default() -> () {
 }
 
 fn assignment() {
+    //[nofallback]~^ warn: this function depends on never type fallback being `()`
+    //[nofallback]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     let x;
 
     if true {
@@ -38,6 +40,8 @@ fn assignment() {
 }
 
 fn assignment_rev() {
+    //[nofallback]~^ warn: this function depends on never type fallback being `()`
+    //[nofallback]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     let x;
 
     if true {

tests/ui/never_type/diverging-fallback-no-leak.fallback.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `!: Test` is not satisfied
-  --> $DIR/diverging-fallback-no-leak.rs:17:23
+  --> $DIR/diverging-fallback-no-leak.rs:20:23
    |
 LL |     unconstrained_arg(return);
    |     ----------------- ^^^^^^ the trait `Test` is not implemented for `!`

tests/ui/never_type/diverging-fallback-no-leak.nofallback.stderr

@@ -0,0 +1,13 @@
+warning: this function depends on never type fallback being `()`
+  --> $DIR/diverging-fallback-no-leak.rs:14:1
+   |
+LL | fn main() {
+   | ^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/never_type/diverging-fallback-no-leak.rs

@@ -12,6 +12,9 @@ impl Test for () {}
 fn unconstrained_arg<T: Test>(_: T) {}
 
 fn main() {
+    //[nofallback]~^ warn: this function depends on never type fallback being `()`
+    //[nofallback]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
     // Here the type variable falls back to `!`,
     // and hence we get a type error.
     unconstrained_arg(return);

tests/ui/never_type/diverging-fallback-unconstrained-return.nofallback.stderr

@@ -0,0 +1,13 @@
+warning: this function depends on never type fallback being `()`
+  --> $DIR/diverging-fallback-unconstrained-return.rs:28:1
+   |
+LL | fn main() {
+   | ^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/never_type/diverging-fallback-unconstrained-return.rs

@@ -26,6 +26,9 @@ fn unconstrained_return<T: UnitReturn>() -> T {
 }
 
 fn main() {
+    //[nofallback]~^ warn: this function depends on never type fallback being `()`
+    //[nofallback]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
     // In Ye Olde Days, the `T` parameter of `unconstrained_return`
     // winds up "entangled" with the `!` type that results from
     // `panic!`, and hence falls back to `()`. This is kind of unfortunate

tests/ui/never_type/fallback-closure-ret.nofallback.stderr

@@ -0,0 +1,13 @@
+warning: this function depends on never type fallback being `()`
+  --> $DIR/fallback-closure-ret.rs:21:1
+   |
+LL | fn main() {
+   | ^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/never_type/fallback-closure-ret.rs

@@ -12,12 +12,14 @@
 
 #![cfg_attr(fallback, feature(never_type_fallback))]
 
-trait Bar { }
-impl Bar for () {  }
-impl Bar for u32 {  }
+trait Bar {}
+impl Bar for () {}
+impl Bar for u32 {}
 
 fn foo<R: Bar>(_: impl Fn() -> R) {}
 
 fn main() {
+    //[nofallback]~^ warn: this function depends on never type fallback being `()`
+    //[nofallback]~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     foo(|| panic!());
 }

tests/ui/never_type/impl_trait_fallback.rs

@@ -6,5 +6,7 @@ trait T {}
 impl T for () {}
 
 fn should_ret_unit() -> impl T {
+    //~^ warn: this function depends on never type fallback being `()`
+    //~| warn: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     panic!()
 }

tests/ui/never_type/impl_trait_fallback.stderr

@@ -0,0 +1,13 @@
+warning: this function depends on never type fallback being `()`
+  --> $DIR/impl_trait_fallback.rs:8:1
+   |
+LL | fn should_ret_unit() -> impl T {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
+   = help: specify the types explicitly
+   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+
+warning: 1 warning emitted
+