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

@@ -14,6 +14,7 @@ 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::{ObligationCause, ObligationCtxt};
 
 #[derive(Copy, Clone)]
 pub enum DivergingFallbackBehavior {
@@ -344,6 +345,9 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         // `!`.
         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 +472,52 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         }
     }
 
+    fn lint_obligations_broken_by_never_type_fallback_change(
+        &self,
+        behavior: DivergingFallbackBehavior,
+        diverging_vids: &[ty::TyVid],
+    ) {
+        let DivergingFallbackBehavior::ToUnit = behavior else { return };
+
+        // Fallback happens if and only if there are diverging variables
+        if diverging_vids.is_empty() {
+            return;
+        }
+
+        // Returns errors which happen if fallback is set to `fallback`
+        let remaining_errors_if_fallback_to = |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)
+                        .expect("expected diverging var to be unconstrained");
+                }
+
+                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 = remaining_errors_if_fallback_to(self.tcx.types.unit);
+        if unit_errors.is_empty()
+            && let never_errors = remaining_errors_if_fallback_to(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 @@ declare_lint_pass! {
         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 +4200,59 @@ declare_lint! {
     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]: https://doc.rust-lang.org/nightly/core/primitive.never.html#never-type-fallback
+    /// [`!`]: https://doc.rust-lang.org/core/primitive.never.html
+    /// [`()`]: https://doc.rust-lang.org/core/primitive.unit.html
+    ///
+    /// ### 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).
+    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

src/tools/clippy/tests/ui/new_ret_no_self.rs

@@ -390,9 +390,7 @@ mod issue7344 {
 
     impl<T> RetImplTraitSelf2<T> {
         // should not trigger lint
-        fn new(t: T) -> impl Trait2<(), Self> {
-            unimplemented!()
-        }
+        fn new(t: T) -> impl Trait2<(), Self> {}
     }
 
     struct RetImplTraitNoSelf2<T>(T);
@@ -401,7 +399,6 @@ mod issue7344 {
         // should trigger lint
         fn new(t: T) -> impl Trait2<(), i32> {
             //~^ ERROR: methods called `new` usually return `Self`
-            unimplemented!()
         }
     }
 

src/tools/clippy/tests/ui/new_ret_no_self.stderr

@@ -96,11 +96,10 @@ LL | |         }
    | |_________^
 
 error: methods called `new` usually return `Self`
-  --> tests/ui/new_ret_no_self.rs:402:9
+  --> tests/ui/new_ret_no_self.rs:400:9
    |
 LL | /         fn new(t: T) -> impl Trait2<(), i32> {
 LL | |
-LL | |             unimplemented!()
 LL | |         }
    | |_________^
 

tests/ui/delegation/not-supported.rs

@@ -70,12 +70,16 @@ mod opaque {
 
         pub fn opaque_arg(_: impl Trait) -> i32 { 0 }
         pub fn opaque_ret() -> impl Trait { unimplemented!() }
+        //~^ 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!
     }
     reuse to_reuse::opaque_arg;
     //~^ ERROR delegation with early bound generics is not supported yet
 
     trait ToReuse {
         fn opaque_ret() -> impl Trait { unimplemented!() }
+        //~^ 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!
     }
 
     // FIXME: Inherited `impl Trait`s create query cycles when used inside trait impls.

tests/ui/delegation/not-supported.stderr

@@ -107,62 +107,83 @@ LL |         reuse Trait::foo2 { &self.0 }
    |                      ^^^^
 
 error: delegation with early bound generics is not supported yet
-  --> $DIR/not-supported.rs:74:21
+  --> $DIR/not-supported.rs:76:21
    |
 LL |         pub fn opaque_arg(_: impl Trait) -> i32 { 0 }
    |         --------------------------------------- callee defined here
 ...
 LL |     reuse to_reuse::opaque_arg;
    |                     ^^^^^^^^^^
 
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:82:5: 82:24>::{synthetic#0}`
-  --> $DIR/not-supported.rs:83:25
+warning: this function depends on never type fallback being `()`
+  --> $DIR/not-supported.rs:80:9
+   |
+LL |         fn opaque_ret() -> impl Trait { unimplemented!() }
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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
+
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:86:5: 86:24>::{synthetic#0}`
+  --> $DIR/not-supported.rs:87:25
    |
 LL |         reuse to_reuse::opaque_ret;
    |                         ^^^^^^^^^^
    |
 note: ...which requires comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process...
-  --> $DIR/not-supported.rs:83:25
+  --> $DIR/not-supported.rs:87:25
    |
 LL |         reuse to_reuse::opaque_ret;
    |                         ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:82:5: 82:24>::{synthetic#0}`, completing the cycle
-note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:82:5: 82:24>` is well-formed
-  --> $DIR/not-supported.rs:82:5
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:86:5: 86:24>::{synthetic#0}`, completing the cycle
+note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:86:5: 86:24>` is well-formed
+  --> $DIR/not-supported.rs:86:5
    |
 LL |     impl ToReuse for u8 {
    |     ^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
-error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:85:5: 85:25>::{synthetic#0}`
-  --> $DIR/not-supported.rs:86:24
+warning: this function depends on never type fallback being `()`
+  --> $DIR/not-supported.rs:72:9
+   |
+LL |         pub fn opaque_ret() -> impl Trait { unimplemented!() }
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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
+
+error[E0391]: cycle detected when computing type of `opaque::<impl at $DIR/not-supported.rs:89:5: 89:25>::{synthetic#0}`
+  --> $DIR/not-supported.rs:90:24
    |
 LL |         reuse ToReuse::opaque_ret;
    |                        ^^^^^^^^^^
    |
 note: ...which requires comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process...
-  --> $DIR/not-supported.rs:86:24
+  --> $DIR/not-supported.rs:90:24
    |
 LL |         reuse ToReuse::opaque_ret;
    |                        ^^^^^^^^^^
-   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:85:5: 85:25>::{synthetic#0}`, completing the cycle
-note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:85:5: 85:25>` is well-formed
-  --> $DIR/not-supported.rs:85:5
+   = note: ...which again requires computing type of `opaque::<impl at $DIR/not-supported.rs:89:5: 89:25>::{synthetic#0}`, completing the cycle
+note: cycle used when checking that `opaque::<impl at $DIR/not-supported.rs:89:5: 89:25>` is well-formed
+  --> $DIR/not-supported.rs:89:5
    |
 LL |     impl ToReuse for u16 {
    |     ^^^^^^^^^^^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
 error: recursive delegation is not supported yet
-  --> $DIR/not-supported.rs:99:22
+  --> $DIR/not-supported.rs:103:22
    |
 LL |         pub reuse to_reuse2::foo;
    |                              --- callee defined here
 ...
 LL |     reuse to_reuse1::foo;
    |                      ^^^
 
-error: aborting due to 16 previous errors
+error: aborting due to 16 previous errors; 2 warnings emitted
 
 Some errors have detailed explanations: E0049, E0195, E0391.
 For more information about an error, try `rustc --explain E0049`.

tests/ui/editions/never-type-fallback-breaking.e2021.stderr

@@ -0,0 +1,23 @@
+warning: this function depends on never type fallback being `()`
+  --> $DIR/never-type-fallback-breaking.rs:15:1
+   |
+LL | fn m() {
+   | ^^^^^^
+   |
+   = 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/never-type-fallback-breaking.rs:27:1
+   |
+LL | fn q() -> Option<()> {
+   | ^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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/editions/never-type-fallback-breaking.e2024.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `!: Default` is not satisfied
-  --> $DIR/never-type-fallback-breaking.rs:17:17
+  --> $DIR/never-type-fallback-breaking.rs:19:17
    |
 LL |         true => Default::default(),
    |                 ^^^^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `!`
@@ -8,15 +8,15 @@ LL |         true => Default::default(),
    = help: did you intend to use the type `()` here instead?
 
 error[E0277]: the trait bound `!: Default` is not satisfied
-  --> $DIR/never-type-fallback-breaking.rs:30:5
+  --> $DIR/never-type-fallback-breaking.rs:34:5
    |
 LL |     deserialize()?;
    |     ^^^^^^^^^^^^^ the trait `Default` is not implemented for `!`
    |
    = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #48950 <https://github.com/rust-lang/rust/issues/48950> for more information)
    = help: did you intend to use the type `()` here instead?
 note: required by a bound in `deserialize`
-  --> $DIR/never-type-fallback-breaking.rs:26:23
+  --> $DIR/never-type-fallback-breaking.rs:30:23
    |
 LL |     fn deserialize<T: Default>() -> Option<T> {
    |                       ^^^^^^^ required by this bound in `deserialize`

tests/ui/editions/never-type-fallback-breaking.rs

@@ -13,6 +13,8 @@ fn main() {
 }
 
 fn m() {
+    //[e2021]~^ this function depends on never type fallback being `()`
+    //[e2021]~| this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     let x = match true {
         true => Default::default(),
         //[e2024]~^ error: the trait bound `!: Default` is not satisfied
@@ -23,6 +25,8 @@ fn m() {
 }
 
 fn q() -> Option<()> {
+    //[e2021]~^ this function depends on never type fallback being `()`
+    //[e2021]~| this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     fn deserialize<T: Default>() -> Option<T> {
         Some(T::default())
     }

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
+