Remove context dependant ! fallback

compiler/rustc_feature/src/removed.rs

@@ -189,6 +189,8 @@ declare_features! (
      Some("subsumed by `#![feature(allocator_internals)]`")),
     /// Allows use of unary negate on unsigned integers, e.g., -e for e: u8
     (removed, negate_unsigned, "1.0.0", Some(29645), None),
+    /// Allows diverging expressions to fall back to `!` rather than `()`.
+    (removed, never_type_fallback, "CURRENT_RUSTC_VERSION", Some(65992), Some("removed in favor of unconditional fallback"), 148871),
     /// Allows `#[no_coverage]` on functions.
     /// The feature was renamed to `coverage_attribute` and the attribute to `#[coverage(on|off)]`
     (removed, no_coverage, "1.74.0", Some(84605), Some("renamed to `coverage_attribute`"), 114656),

compiler/rustc_feature/src/unstable.rs

@@ -594,8 +594,6 @@ declare_features! (
     (incomplete, never_patterns, "1.76.0", Some(118155)),
     /// Allows the `!` type. Does not imply 'exhaustive_patterns' (below) any more.
     (unstable, never_type, "1.13.0", Some(35121)),
-    /// Allows diverging expressions to fall back to `!` rather than `()`.
-    (unstable, never_type_fallback, "1.41.0", Some(65992)),
     /// Switch `..` syntax to use the new (`Copy + IntoIterator`) range types.
     (unstable, new_range, "1.86.0", Some(123741)),
     /// Allows `#![no_core]`.

compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -265,7 +265,7 @@ pub(super) fn find_opaque_ty_constraints_for_rpit<'tcx>(
                 // so we can just make the hidden type be `!`.
                 // For backwards compatibility reasons, we fall back to
                 // `()` until we the diverging default is changed.
-                EarlyBinder::bind(Ty::new_diverging_default(tcx))
+                EarlyBinder::bind(tcx.types.unit)
             }
         }
         DefiningScopeKind::MirBorrowck => match tcx.mir_borrowck(owner_def_id) {

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -115,7 +115,9 @@ pub(crate) struct FnCtxt<'a, 'tcx> {
 
     pub(super) root_ctxt: &'a TypeckRootCtxt<'tcx>,
 
-    pub(super) fallback_has_occurred: Cell<bool>,
+    /// True if a divirging inference variable has been set to `()`/`!` because
+    /// of never type fallback. This is only used for diagnostics.
+    pub(super) diverging_fallback_has_occurred: Cell<bool>,
 
     pub(super) diverging_fallback_behavior: DivergingFallbackBehavior,
     pub(super) diverging_block_behavior: DivergingBlockBehavior,
@@ -153,7 +155,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 by_id: Default::default(),
             }),
             root_ctxt,
-            fallback_has_occurred: Cell::new(false),
+            diverging_fallback_has_occurred: Cell::new(false),
             diverging_fallback_behavior,
             diverging_block_behavior,
             trait_ascriptions: Default::default(),
@@ -190,7 +192,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         TypeErrCtxt {
             infcx: &self.infcx,
             typeck_results: Some(self.typeck_results.borrow()),
-            fallback_has_occurred: self.fallback_has_occurred.get(),
+            diverging_fallback_has_occurred: self.diverging_fallback_has_occurred.get(),
             normalize_fn_sig: Box::new(|fn_sig| {
                 if fn_sig.has_escaping_bound_vars() {
                     return fn_sig;
@@ -505,11 +507,6 @@ fn default_fallback(tcx: TyCtxt<'_>) -> DivergingFallbackBehavior {
         return DivergingFallbackBehavior::ToNever;
     }
 
-    // `feature(never_type_fallback)`: fallback to `!` or `()` trying to not break stuff
-    if tcx.features().never_type_fallback() {
-        return DivergingFallbackBehavior::ContextDependent;
-    }
-
     // Otherwise: fallback to `()`
     DivergingFallbackBehavior::ToUnit
 }
@@ -536,7 +533,6 @@ fn parse_never_type_options_attr(
             let mode = item.value_str().unwrap();
             match mode {
                 sym::unit => fallback = Some(DivergingFallbackBehavior::ToUnit),
-                sym::niko => fallback = Some(DivergingFallbackBehavior::ContextDependent),
                 sym::never => fallback = Some(DivergingFallbackBehavior::ToNever),
                 sym::no => fallback = Some(DivergingFallbackBehavior::NoFallback),
                 _ => {

compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs

@@ -1,35 +1,19 @@
 use std::cell::{Cell, RefCell};
 use std::ops::Deref;
 
-use rustc_data_structures::unord::{UnordMap, UnordSet};
+use rustc_data_structures::unord::UnordSet;
 use rustc_hir::def_id::LocalDefId;
-use rustc_hir::{self as hir, HirId, HirIdMap, LangItem};
+use rustc_hir::{self as hir, HirId, HirIdMap};
 use rustc_infer::infer::{InferCtxt, InferOk, OpaqueTypeStorageEntries, TyCtxtInferExt};
 use rustc_middle::span_bug;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt, TypingMode};
 use rustc_span::Span;
 use rustc_span::def_id::LocalDefIdMap;
-use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
-use rustc_trait_selection::traits::{
-    self, FulfillmentError, PredicateObligation, TraitEngine, TraitEngineExt as _,
-};
-use tracing::{debug, instrument};
+use rustc_trait_selection::traits::{self, FulfillmentError, TraitEngine, TraitEngineExt as _};
+use tracing::instrument;
 
 use super::callee::DeferredCallResolution;
 
-#[derive(Debug, Default, Copy, Clone)]
-pub(crate) struct InferVarInfo {
-    /// This is true if we identified that this Ty (`?T`) is found in a `?T: Foo`
-    /// obligation, where:
-    ///
-    ///  * `Foo` is not `Sized`
-    ///  * `(): Foo` may be satisfied
-    pub self_in_trait: bool,
-    /// This is true if we identified that this Ty (`?T`) is found in a `<_ as
-    /// _>::AssocType = ?T`
-    pub output: bool,
-}
-
 /// Data shared between a "typeck root" and its nested bodies,
 /// e.g. closures defined within the function. For example:
 /// ```ignore (illustrative)
@@ -83,8 +67,6 @@ pub(crate) struct TypeckRootCtxt<'tcx> {
     /// we record that type variable here. This is later used to inform
     /// fallback. See the `fallback` module for details.
     pub(super) diverging_type_vars: RefCell<UnordSet<Ty<'tcx>>>,
-
-    pub(super) infer_var_info: RefCell<UnordMap<ty::TyVid, InferVarInfo>>,
 }
 
 impl<'tcx> Deref for TypeckRootCtxt<'tcx> {
@@ -119,7 +101,6 @@ impl<'tcx> TypeckRootCtxt<'tcx> {
             deferred_asm_checks: RefCell::new(Vec::new()),
             deferred_repeat_expr_checks: RefCell::new(Vec::new()),
             diverging_type_vars: RefCell::new(Default::default()),
-            infer_var_info: RefCell::new(Default::default()),
         }
     }
 
@@ -129,8 +110,6 @@ impl<'tcx> TypeckRootCtxt<'tcx> {
             span_bug!(obligation.cause.span, "escaping bound vars in predicate {:?}", obligation);
         }
 
-        self.update_infer_var_info(&obligation);
-
         self.fulfillment_cx.borrow_mut().register_predicate_obligation(self, obligation);
     }
 
@@ -147,46 +126,4 @@ impl<'tcx> TypeckRootCtxt<'tcx> {
         self.register_predicates(infer_ok.obligations);
         infer_ok.value
     }
-
-    fn update_infer_var_info(&self, obligation: &PredicateObligation<'tcx>) {
-        let infer_var_info = &mut self.infer_var_info.borrow_mut();
-
-        // (*) binder skipped
-        if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(tpred)) =
-            obligation.predicate.kind().skip_binder()
-            && let Some(ty) =
-                self.shallow_resolve(tpred.self_ty()).ty_vid().map(|t| self.root_var(t))
-            && !self.tcx.is_lang_item(tpred.trait_ref.def_id, LangItem::Sized)
-        {
-            let new_self_ty = self.tcx.types.unit;
-
-            // Then construct a new obligation with Self = () added
-            // to the ParamEnv, and see if it holds.
-            let o = obligation.with(
-                self.tcx,
-                obligation.predicate.kind().rebind(
-                    // (*) binder moved here
-                    ty::PredicateKind::Clause(ty::ClauseKind::Trait(
-                        tpred.with_replaced_self_ty(self.tcx, new_self_ty),
-                    )),
-                ),
-            );
-            // Don't report overflow errors. Otherwise equivalent to may_hold.
-            if let Ok(result) = self.probe(|_| self.evaluate_obligation(&o))
-                && result.may_apply()
-            {
-                infer_var_info.entry(ty).or_default().self_in_trait = true;
-            }
-        }
-
-        if let ty::PredicateKind::Clause(ty::ClauseKind::Projection(predicate)) =
-            obligation.predicate.kind().skip_binder()
-            // If the projection predicate (Foo::Bar == X) has X as a non-TyVid,
-            // we need to make it into one.
-            && let Some(vid) = predicate.term.as_type().and_then(|ty| ty.ty_vid())
-        {
-            debug!("infer_var_info: {:?}.output = true", vid);
-            infer_var_info.entry(vid).or_default().output = true;
-        }
-    }
 }

compiler/rustc_middle/src/ty/sty.rs

@@ -878,11 +878,6 @@ impl<'tcx> Ty<'tcx> {
         Ty::new_imm_ref(tcx, tcx.lifetimes.re_static, tcx.types.str_)
     }
 
-    #[inline]
-    pub fn new_diverging_default(tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
-        if tcx.features().never_type_fallback() { tcx.types.never } else { tcx.types.unit }
-    }
-
     // lang and diagnostic tys
 
     fn new_generic_adt(tcx: TyCtxt<'tcx>, wrapper_def_id: DefId, ty_param: Ty<'tcx>) -> Ty<'tcx> {

compiler/rustc_trait_selection/src/error_reporting/mod.rs

@@ -21,7 +21,7 @@ pub struct TypeErrCtxt<'a, 'tcx> {
     pub infcx: &'a InferCtxt<'tcx>,
 
     pub typeck_results: Option<std::cell::Ref<'a, ty::TypeckResults<'tcx>>>,
-    pub fallback_has_occurred: bool,
+    pub diverging_fallback_has_occurred: bool,
 
     pub normalize_fn_sig: Box<dyn Fn(ty::PolyFnSig<'tcx>) -> ty::PolyFnSig<'tcx> + 'a>,
 
@@ -36,7 +36,7 @@ impl<'tcx> InferCtxt<'tcx> {
         TypeErrCtxt {
             infcx: self,
             typeck_results: None,
-            fallback_has_occurred: false,
+            diverging_fallback_has_occurred: false,
             normalize_fn_sig: Box::new(|fn_sig| fn_sig),
             autoderef_steps: Box::new(|ty| {
                 debug_assert!(false, "shouldn't be using autoderef_steps outside of typeck");

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -541,7 +541,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         // variable that used to fallback to `()` now falling back to `!`. Issue a
                         // note informing about the change in behaviour.
                         if leaf_trait_predicate.skip_binder().self_ty().is_never()
-                            && self.fallback_has_occurred
+                            && self.diverging_fallback_has_occurred
                         {
                             let predicate = leaf_trait_predicate.map_bound(|trait_pred| {
                                 trait_pred.with_replaced_self_ty(self.tcx, tcx.types.unit)
@@ -550,8 +550,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             if self.predicate_may_hold(&unit_obligation) {
                                 err.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> \
+                                    Rust's type-inference algorithm (see issue #148922 \
+                                    <https://github.com/rust-lang/rust/issues/148922> \
                                     for more information)",
                                 );
                                 err.help("you might have intended to use the type `()` here instead");

tests/ui/binding/empty-types-in-patterns.rs

@@ -1,6 +1,7 @@
 //@ run-pass
+//@ edition: 2024
 
-#![feature(never_type, never_type_fallback)]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 
 #![allow(unreachable_patterns)]

tests/ui/coercion/coerce-issue-49593-box-never.nofallback.stderr

@@ -1,13 +1,13 @@
 error[E0277]: the trait bound `(): std::error::Error` is not satisfied
-  --> $DIR/coerce-issue-49593-box-never.rs:18:5
+  --> $DIR/coerce-issue-49593-box-never.rs:17:5
    |
 LL |     Box::<_ /* ! */>::new(x)
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
    |
    = note: required for the cast from `Box<()>` to `Box<(dyn std::error::Error + 'static)>`
 
 error[E0277]: the trait bound `(): std::error::Error` is not satisfied
-  --> $DIR/coerce-issue-49593-box-never.rs:24:5
+  --> $DIR/coerce-issue-49593-box-never.rs:23:5
    |
 LL |     raw_ptr_box::<_ /* ! */>(x)
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`

tests/ui/coercion/coerce-issue-49593-box-never.rs

@@ -1,9 +1,8 @@
 //@ revisions: nofallback fallback
+//@[fallback] edition: 2024
 //@[fallback] check-pass
 
 #![feature(never_type)]
-#![cfg_attr(fallback, feature(never_type_fallback))]
-#![allow(unreachable_code)]
 
 use std::error::Error;
 use std::mem;

tests/ui/editions/never-type-fallback-breaking.e2024.stderr

@@ -4,7 +4,7 @@ error[E0277]: the trait bound `!: Default` is not satisfied
 LL |         true => Default::default(),
    |                 ^^^^^^^^^^^^^^^^^^ 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)
+   = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #148922 <https://github.com/rust-lang/rust/issues/148922> for more information)
    = help: you might have intended to use the type `()` here instead
 
 error[E0277]: the trait bound `!: Default` is not satisfied
@@ -13,7 +13,7 @@ error[E0277]: the trait bound `!: Default` is not satisfied
 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)
+   = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #148922 <https://github.com/rust-lang/rust/issues/148922> for more information)
    = help: you might have intended to use the type `()` here instead
 note: required by a bound in `deserialize`
   --> $DIR/never-type-fallback-breaking.rs:31:23
@@ -50,7 +50,7 @@ error[E0277]: the trait bound `!: Default` is not satisfied
 LL |     takes_apit(|| Default::default())?;
    |                   ^^^^^^^^^^^^^^^^^^ 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)
+   = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #148922 <https://github.com/rust-lang/rust/issues/148922> for more information)
    = help: you might have intended to use the type `()` here instead
 
 error[E0277]: the trait bound `!: Default` is not satisfied
@@ -61,7 +61,7 @@ LL |     takes_apit2(mk()?);
    |     |
    |     required by a bound introduced by this call
    |
-   = 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)
+   = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #148922 <https://github.com/rust-lang/rust/issues/148922> for more information)
    = help: you might have intended to use the type `()` here instead
 note: required by a bound in `takes_apit2`
   --> $DIR/never-type-fallback-breaking.rs:69:25

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

@@ -1,20 +1,20 @@
 error[E0277]: the trait bound `!: ImplementedForUnitButNotNever` is not satisfied
-  --> $DIR/defaulted-never-note.rs:31:9
+  --> $DIR/defaulted-never-note.rs:27:9
    |
 LL |     foo(_x);
    |     --- ^^ the trait `ImplementedForUnitButNotNever` is not implemented for `!`
    |     |
    |     required by a bound introduced by this call
    |
 help: the trait `ImplementedForUnitButNotNever` is implemented for `()`
-  --> $DIR/defaulted-never-note.rs:24:1
+  --> $DIR/defaulted-never-note.rs:20:1
    |
 LL | impl ImplementedForUnitButNotNever 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)
+   = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #148922 <https://github.com/rust-lang/rust/issues/148922> for more information)
    = help: you might have intended to use the type `()` here instead
 note: required by a bound in `foo`
-  --> $DIR/defaulted-never-note.rs:26:11
+  --> $DIR/defaulted-never-note.rs:22:11
    |
 LL | fn foo<T: ImplementedForUnitButNotNever>(_t: T) {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `foo`

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

@@ -1,13 +1,13 @@
 Future incompatibility report: Future breakage diagnostic:
 warning: this function depends on never type fallback being `()`
-  --> $DIR/defaulted-never-note.rs:29:1
+  --> $DIR/defaulted-never-note.rs:25:1
    |
 LL | fn smeg() {
    | ^^^^^^^^^
    |
    = help: specify the types explicitly
 note: in edition 2024, the requirement `!: ImplementedForUnitButNotNever` will fail
-  --> $DIR/defaulted-never-note.rs:31:9
+  --> $DIR/defaulted-never-note.rs:27:9
    |
 LL |     foo(_x);
    |         ^^

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

@@ -1,13 +1,9 @@
 //@ revisions: nofallback fallback
+//@[fallback] edition: 2024
 //@[nofallback] run-pass
 //@[fallback] check-fail
 
-// We need to opt into the `never_type_fallback` feature
-// to trigger the requirement that this is testing.
-#![cfg_attr(fallback, feature(never_type, never_type_fallback))]
-
-#![allow(unused)]
-#![expect(dependency_on_unit_never_type_fallback)]
+#![expect(dependency_on_unit_never_type_fallback, unused)]
 
 trait Deserialize: Sized {
     fn deserialize() -> Result<Self, String>;