Rollup merge of rust-lang#145095 - tiif:unstable_const_param, r=BoxyUwU

Migrate `UnsizedConstParamTy`  to unstable impl of `ConstParamTy_`

Now that we have ``#[unstable_feature_bound]``, we can remove ``UnsizedConstParamTy`` that was meant to be an unstable impl of stable type and ``ConstParamTy_`` trait.

r? `@BoxyUwU`

Author: Zalathar

compiler/rustc_builtin_macros/src/deriving/bounds.rs

@@ -51,43 +51,4 @@ pub(crate) fn expand_deriving_const_param_ty(
     };
 
     trait_def.expand(cx, mitem, item, push);
-
-    let trait_def = TraitDef {
-        span,
-        path: path_std!(marker::UnsizedConstParamTy),
-        skip_path_as_bound: false,
-        needs_copy_as_bound_if_packed: false,
-        additional_bounds: vec![ty::Ty::Path(path_std!(cmp::Eq))],
-        supports_unions: false,
-        methods: Vec::new(),
-        associated_types: Vec::new(),
-        is_const,
-        is_staged_api_crate: cx.ecfg.features.staged_api(),
-    };
-
-    trait_def.expand(cx, mitem, item, push);
-}
-
-pub(crate) fn expand_deriving_unsized_const_param_ty(
-    cx: &ExtCtxt<'_>,
-    span: Span,
-    mitem: &MetaItem,
-    item: &Annotatable,
-    push: &mut dyn FnMut(Annotatable),
-    is_const: bool,
-) {
-    let trait_def = TraitDef {
-        span,
-        path: path_std!(marker::UnsizedConstParamTy),
-        skip_path_as_bound: false,
-        needs_copy_as_bound_if_packed: false,
-        additional_bounds: vec![ty::Ty::Path(path_std!(cmp::Eq))],
-        supports_unions: false,
-        methods: Vec::new(),
-        associated_types: Vec::new(),
-        is_const,
-        is_staged_api_crate: cx.ecfg.features.staged_api(),
-    };
-
-    trait_def.expand(cx, mitem, item, push);
 }

compiler/rustc_builtin_macros/src/lib.rs

@@ -129,7 +129,6 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         Clone: clone::expand_deriving_clone,
         Copy: bounds::expand_deriving_copy,
         ConstParamTy: bounds::expand_deriving_const_param_ty,
-        UnsizedConstParamTy: bounds::expand_deriving_unsized_const_param_ty,
         Debug: debug::expand_deriving_debug,
         Default: default::expand_deriving_default,
         Eq: eq::expand_deriving_eq,

compiler/rustc_hir/src/lang_items.rs

@@ -370,7 +370,6 @@ language_item_table! {
     CoercePointeeValidated, sym::coerce_pointee_validated, coerce_pointee_validated_trait, Target::Trait,     GenericRequirement::Exact(0);
 
     ConstParamTy,            sym::const_param_ty,      const_param_ty_trait,       Target::Trait,          GenericRequirement::Exact(0);
-    UnsizedConstParamTy,     sym::unsized_const_param_ty, unsized_const_param_ty_trait, Target::Trait, GenericRequirement::Exact(0);
 
     Poll,                    sym::Poll,                poll,                       Target::Enum,           GenericRequirement::None;
     PollReady,               sym::Ready,               poll_ready_variant,         Target::Variant,        GenericRequirement::None;

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -819,17 +819,7 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &ty::GenericParamDef) -> Result<(), Er
             let span = tcx.def_span(param.def_id);
             let def_id = param.def_id.expect_local();
 
-            if tcx.features().unsized_const_params() {
-                enter_wf_checking_ctxt(tcx, tcx.local_parent(def_id), |wfcx| {
-                    wfcx.register_bound(
-                        ObligationCause::new(span, def_id, ObligationCauseCode::ConstParam(ty)),
-                        wfcx.param_env,
-                        ty,
-                        tcx.require_lang_item(LangItem::UnsizedConstParamTy, span),
-                    );
-                    Ok(())
-                })
-            } else if tcx.features().adt_const_params() {
+            if tcx.features().adt_const_params() {
                 enter_wf_checking_ctxt(tcx, tcx.local_parent(def_id), |wfcx| {
                     wfcx.register_bound(
                         ObligationCause::new(span, def_id, ObligationCauseCode::ConstParam(ty)),
@@ -880,7 +870,6 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &ty::GenericParamDef) -> Result<(), Er
                     tcx,
                     tcx.param_env(param.def_id),
                     ty,
-                    LangItem::ConstParamTy,
                     cause,
                 ) {
                     // Can never implement `ConstParamTy`, don't suggest anything.

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -1,7 +1,6 @@
 //! Check properties that are required by built-in traits and set
 //! up data structures required by type-checking/codegen.
 
-use std::assert_matches::assert_matches;
 use std::collections::BTreeMap;
 
 use rustc_data_structures::fx::FxHashSet;
@@ -40,10 +39,7 @@ pub(super) fn check_trait<'tcx>(
     checker.check(lang_items.async_drop_trait(), visit_implementation_of_drop)?;
     checker.check(lang_items.copy_trait(), visit_implementation_of_copy)?;
     checker.check(lang_items.const_param_ty_trait(), |checker| {
-        visit_implementation_of_const_param_ty(checker, LangItem::ConstParamTy)
-    })?;
-    checker.check(lang_items.unsized_const_param_ty_trait(), |checker| {
-        visit_implementation_of_const_param_ty(checker, LangItem::UnsizedConstParamTy)
+        visit_implementation_of_const_param_ty(checker)
     })?;
     checker.check(lang_items.coerce_unsized_trait(), visit_implementation_of_coerce_unsized)?;
     checker
@@ -138,12 +134,7 @@ fn visit_implementation_of_copy(checker: &Checker<'_>) -> Result<(), ErrorGuaran
     }
 }
 
-fn visit_implementation_of_const_param_ty(
-    checker: &Checker<'_>,
-    kind: LangItem,
-) -> Result<(), ErrorGuaranteed> {
-    assert_matches!(kind, LangItem::ConstParamTy | LangItem::UnsizedConstParamTy);
-
+fn visit_implementation_of_const_param_ty(checker: &Checker<'_>) -> Result<(), ErrorGuaranteed> {
     let tcx = checker.tcx;
     let header = checker.impl_header;
     let impl_did = checker.impl_def_id;
@@ -157,7 +148,7 @@ fn visit_implementation_of_const_param_ty(
     }
 
     let cause = traits::ObligationCause::misc(DUMMY_SP, impl_did);
-    match type_allowed_to_implement_const_param_ty(tcx, param_env, self_type, kind, cause) {
+    match type_allowed_to_implement_const_param_ty(tcx, param_env, self_type, cause) {
         Ok(()) => Ok(()),
         Err(ConstParamTyImplementationError::InfrigingFields(fields)) => {
             let span = tcx.hir_expect_item(impl_did).expect_impl().self_ty.span;

compiler/rustc_infer/src/traits/engine.rs

@@ -72,12 +72,27 @@ pub trait TraitEngine<'tcx, E: 'tcx>: 'tcx {
             self.register_predicate_obligation(infcx, obligation);
         }
     }
-
+    /// Go over the list of pending obligations and try to evaluate them.
+    ///
+    /// For each result:
+    /// Ok: remove the obligation from the list
+    /// Ambiguous: leave the obligation in the list to be evaluated later
+    /// Err: remove the obligation from the list and return an error
+    ///
+    /// Returns a list of errors from obligations that evaluated to Err.
     #[must_use]
     fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E>;
 
     fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E>;
 
+    /// Evaluate all pending obligations, return error if they can't be evaluated.
+    ///
+    /// For each result:
+    /// Ok: remove the obligation from the list
+    /// Ambiguous: remove the obligation from the list and return an error
+    /// Err: remove the obligation from the list and return an error
+    ///
+    /// Returns a list of errors from obligations that evaluated to Ambiguous or Err.
     #[must_use]
     fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
         let errors = self.select_where_possible(infcx);

compiler/rustc_trait_selection/src/traits/misc.rs

@@ -1,15 +1,14 @@
 //! Miscellaneous type-system utilities that are too small to deserve their own modules.
 
-use std::assert_matches::assert_matches;
-
 use hir::LangItem;
 use rustc_ast::Mutability;
 use rustc_hir as hir;
 use rustc_infer::infer::{RegionResolutionError, TyCtxtInferExt};
 use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt, TypeVisitableExt, TypingMode};
+use rustc_span::sym;
 
 use crate::regions::InferCtxtRegionExt;
-use crate::traits::{self, FulfillmentError, ObligationCause};
+use crate::traits::{self, FulfillmentError, Obligation, ObligationCause};
 
 pub enum CopyImplementationError<'tcx> {
     InfringingFields(Vec<(&'tcx ty::FieldDef, Ty<'tcx>, InfringingFieldsReason<'tcx>)>),
@@ -98,10 +97,9 @@ pub fn type_allowed_to_implement_const_param_ty<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     self_type: Ty<'tcx>,
-    lang_item: LangItem,
     parent_cause: ObligationCause<'tcx>,
 ) -> Result<(), ConstParamTyImplementationError<'tcx>> {
-    assert_matches!(lang_item, LangItem::ConstParamTy | LangItem::UnsizedConstParamTy);
+    let mut need_unstable_feature_bound = false;
 
     let inner_tys: Vec<_> = match *self_type.kind() {
         // Trivially okay as these types are all:
@@ -112,18 +110,14 @@ pub fn type_allowed_to_implement_const_param_ty<'tcx>(
 
         // Handle types gated under `feature(unsized_const_params)`
         // FIXME(unsized_const_params): Make `const N: [u8]` work then forbid references
-        ty::Slice(inner_ty) | ty::Ref(_, inner_ty, Mutability::Not)
-            if lang_item == LangItem::UnsizedConstParamTy =>
-        {
+        ty::Slice(inner_ty) | ty::Ref(_, inner_ty, Mutability::Not) => {
+            need_unstable_feature_bound = true;
             vec![inner_ty]
         }
-        ty::Str if lang_item == LangItem::UnsizedConstParamTy => {
+        ty::Str => {
+            need_unstable_feature_bound = true;
             vec![Ty::new_slice(tcx, tcx.types.u8)]
         }
-        ty::Str | ty::Slice(..) | ty::Ref(_, _, Mutability::Not) => {
-            return Err(ConstParamTyImplementationError::UnsizedConstParamsFeatureRequired);
-        }
-
         ty::Array(inner_ty, _) => vec![inner_ty],
 
         // `str` morally acts like a newtype around `[u8]`
@@ -137,7 +131,7 @@ pub fn type_allowed_to_implement_const_param_ty<'tcx>(
                 adt,
                 args,
                 parent_cause.clone(),
-                lang_item,
+                LangItem::ConstParamTy,
             )
             .map_err(ConstParamTyImplementationError::InfrigingFields)?;
 
@@ -153,11 +147,25 @@ pub fn type_allowed_to_implement_const_param_ty<'tcx>(
         let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
         let ocx = traits::ObligationCtxt::new_with_diagnostics(&infcx);
 
+        // Make sure impls certain types are gated with #[unstable_feature_bound(unsized_const_params)]
+        if need_unstable_feature_bound {
+            ocx.register_obligation(Obligation::new(
+                tcx,
+                parent_cause.clone(),
+                param_env,
+                ty::ClauseKind::UnstableFeature(sym::unsized_const_params),
+            ));
+
+            if !ocx.select_all_or_error().is_empty() {
+                return Err(ConstParamTyImplementationError::UnsizedConstParamsFeatureRequired);
+            }
+        }
+
         ocx.register_bound(
             parent_cause.clone(),
             param_env,
             inner_ty,
-            tcx.require_lang_item(lang_item, parent_cause.span),
+            tcx.require_lang_item(LangItem::ConstParamTy, parent_cause.span),
         );
 
         let errors = ocx.select_all_or_error();

library/core/src/marker.rs

@@ -1083,7 +1083,7 @@ pub trait Tuple {}
 // We name this differently than the derive macro so that the `adt_const_params` can
 // be used independently of `unsized_const_params` without requiring a full path
 // to the derive macro every time it is used. This should be renamed on stabilization.
-pub trait ConstParamTy_: UnsizedConstParamTy + StructuralPartialEq + Eq {}
+pub trait ConstParamTy_: StructuralPartialEq + Eq {}
 
 /// Derive macro generating an impl of the trait `ConstParamTy`.
 #[rustc_builtin_macro]
@@ -1093,23 +1093,6 @@ pub macro ConstParamTy($item:item) {
     /* compiler built-in */
 }
 
-#[lang = "unsized_const_param_ty"]
-#[unstable(feature = "unsized_const_params", issue = "95174")]
-#[diagnostic::on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
-/// A marker for types which can be used as types of `const` generic parameters.
-///
-/// Equivalent to [`ConstParamTy_`] except that this is used by
-/// the `unsized_const_params` to allow for fake unstable impls.
-pub trait UnsizedConstParamTy: StructuralPartialEq + Eq {}
-
-/// Derive macro generating an impl of the trait `ConstParamTy`.
-#[rustc_builtin_macro]
-#[allow_internal_unstable(unsized_const_params)]
-#[unstable(feature = "unsized_const_params", issue = "95174")]
-pub macro UnsizedConstParamTy($item:item) {
-    /* compiler built-in */
-}
-
 // FIXME(adt_const_params): handle `ty::FnDef`/`ty::Closure`
 marker_impls! {
     #[unstable(feature = "adt_const_params", issue = "95174")]
@@ -1124,17 +1107,11 @@ marker_impls! {
 
 marker_impls! {
     #[unstable(feature = "unsized_const_params", issue = "95174")]
-    UnsizedConstParamTy for
-        usize, u8, u16, u32, u64, u128,
-        isize, i8, i16, i32, i64, i128,
-        bool,
-        char,
-        (),
-        {T: UnsizedConstParamTy, const N: usize} [T; N],
-
+    #[unstable_feature_bound(unsized_const_params)]
+    ConstParamTy_ for
         str,
-        {T: UnsizedConstParamTy} [T],
-        {T: UnsizedConstParamTy + ?Sized} &T,
+        {T: ConstParamTy_} [T],
+        {T: ConstParamTy_ + ?Sized} &T,
 }
 
 /// A common trait implemented by all function pointers.

library/core/src/mem/transmutability.rs

@@ -1,4 +1,4 @@
-use crate::marker::{ConstParamTy_, UnsizedConstParamTy};
+use crate::marker::ConstParamTy_;
 
 /// Marks that `Src` is transmutable into `Self`.
 ///
@@ -83,6 +83,7 @@ use crate::marker::{ConstParamTy_, UnsizedConstParamTy};
 /// Furthermore, stability does not imply portability. For example, the size of
 /// `usize` is stable, but not portable.
 #[unstable(feature = "transmutability", issue = "99571")]
+#[unstable_feature_bound(transmutability)]
 #[lang = "transmute_trait"]
 #[rustc_deny_explicit_impl]
 #[rustc_do_not_implement_via_object]
@@ -288,9 +289,8 @@ pub struct Assume {
 }
 
 #[unstable(feature = "transmutability", issue = "99571")]
+#[unstable_feature_bound(transmutability)]
 impl ConstParamTy_ for Assume {}
-#[unstable(feature = "transmutability", issue = "99571")]
-impl UnsizedConstParamTy for Assume {}
 
 impl Assume {
     /// With this, [`TransmuteFrom`] does not assume you have ensured any safety

library/core/src/tuple.rs

@@ -1,7 +1,7 @@
 // See core/src/primitive_docs.rs for documentation.
 
 use crate::cmp::Ordering::{self, *};
-use crate::marker::{ConstParamTy_, StructuralPartialEq, UnsizedConstParamTy};
+use crate::marker::{ConstParamTy_, StructuralPartialEq};
 use crate::ops::ControlFlow::{self, Break, Continue};
 
 // Recursive macro for implementing n-ary tuple functions and operations
@@ -47,17 +47,11 @@ macro_rules! tuple_impls {
         maybe_tuple_doc! {
             $($T)+ @
             #[unstable(feature = "adt_const_params", issue = "95174")]
+            #[unstable_feature_bound(unsized_const_params)]
             impl<$($T: ConstParamTy_),+> ConstParamTy_ for ($($T,)+)
             {}
         }
 
-        maybe_tuple_doc! {
-            $($T)+ @
-            #[unstable(feature = "unsized_const_params", issue = "95174")]
-            impl<$($T: UnsizedConstParamTy),+> UnsizedConstParamTy for ($($T,)+)
-            {}
-        }
-
         maybe_tuple_doc! {
             $($T)+ @
             #[unstable(feature = "structural_match", issue = "31434")]