Unrolled build for #150823

Rollup merge of #150823 - camelid:ogca, r=BoxyUwU

Implement MVP for opaque generic const arguments

This is meant to be the interim successor to generic const expressions.
Essentially, const item RHS's will be allowed to do arbitrary const
operations using generics. The limitation is that these const items will
be treated opaquely, like ADTs in nominal typing, such that uses of them
will only be equal if the same const item is referenced. In other words,
two const items with the exact same RHS will not be considered equal.

I also added some logic to check feature gates that depend on others
being enabled (like oGCA depending on mGCA).

### Coherence

During coherence, OGCA consts should be normalized ambiguously because
they are opaque but eventually resolved to a real value. We don't want
two OGCAs that have the same value to be treated as distinct for
coherence purposes. (Just like opaque types.)

This actually doesn't work yet because there are pre-existing
fundamental issues with equate relations involving consts that need to
be normalized. The problem is that we normalize only one layer of the
const item and don't actually process the resulting anon const. Normally
the created inference variable should be handled, which in this case
would cause us to hit the anon const, but that's not happening.
Specifically, `visit_const` on `Generalizer` should be updated to be
similar to `visit_ty`.

r? @BoxyUwU

Author: rust-timer

compiler/rustc_ast_passes/src/errors.rs

@@ -950,6 +950,16 @@ pub(crate) struct IncompatibleFeatures {
     pub f2: Symbol,
 }
 
+#[derive(Diagnostic)]
+#[diag("`{$parent}` requires {$missing} to be enabled")]
+#[help("enable all of these features")]
+pub(crate) struct MissingDependentFeatures {
+    #[primary_span]
+    pub parent_span: Span,
+    pub parent: Symbol,
+    pub missing: String,
+}
+
 #[derive(Diagnostic)]
 #[diag("negative bounds are not supported")]
 pub(crate) struct NegativeBoundUnsupported {

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -441,6 +441,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     maybe_stage_features(sess, features, krate);
     check_incompatible_features(sess, features);
+    check_dependent_features(sess, features);
     check_new_solver_banned_features(sess, features);
 
     let mut visitor = PostExpansionVisitor { sess, features };
@@ -649,6 +650,27 @@ fn check_incompatible_features(sess: &Session, features: &Features) {
     }
 }
 
+fn check_dependent_features(sess: &Session, features: &Features) {
+    for &(parent, children) in
+        rustc_feature::DEPENDENT_FEATURES.iter().filter(|(parent, _)| features.enabled(*parent))
+    {
+        if children.iter().any(|f| !features.enabled(*f)) {
+            let parent_span = features
+                .enabled_features_iter_stable_order()
+                .find_map(|(name, span)| (name == parent).then_some(span))
+                .unwrap();
+            // FIXME: should probably format this in fluent instead of here
+            let missing = children
+                .iter()
+                .filter(|f| !features.enabled(**f))
+                .map(|s| format!("`{}`", s.as_str()))
+                .intersperse(String::from(", "))
+                .collect();
+            sess.dcx().emit_err(errors::MissingDependentFeatures { parent_span, parent, missing });
+        }
+    }
+}
+
 fn check_new_solver_banned_features(sess: &Session, features: &Features) {
     if !sess.opts.unstable_opts.next_solver.globally {
         return;

compiler/rustc_ast_passes/src/lib.rs

@@ -5,6 +5,7 @@
 // tidy-alphabetical-start
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
+#![feature(iter_intersperse)]
 #![feature(iter_is_partitioned)]
 // tidy-alphabetical-end
 

compiler/rustc_feature/src/lib.rs

@@ -136,5 +136,6 @@ pub use builtin_attrs::{
 };
 pub use removed::REMOVED_LANG_FEATURES;
 pub use unstable::{
-    EnabledLangFeature, EnabledLibFeature, Features, INCOMPATIBLE_FEATURES, UNSTABLE_LANG_FEATURES,
+    DEPENDENT_FEATURES, EnabledLangFeature, EnabledLibFeature, Features, INCOMPATIBLE_FEATURES,
+    UNSTABLE_LANG_FEATURES,
 };

compiler/rustc_feature/src/unstable.rs

@@ -605,6 +605,8 @@ declare_features! (
     (unstable, offset_of_enum, "1.75.0", Some(120141)),
     /// Allows using fields with slice type in offset_of!
     (unstable, offset_of_slice, "1.81.0", Some(126151)),
+    /// Allows using generics in more complex const expressions, based on definitional equality.
+    (unstable, opaque_generic_const_args, "CURRENT_RUSTC_VERSION", Some(151972)),
     /// Allows using `#[optimize(X)]`.
     (unstable, optimize_attribute, "1.34.0", Some(54882)),
     /// Allows specifying nop padding on functions for dynamic patching.
@@ -782,3 +784,9 @@ pub const INCOMPATIBLE_FEATURES: &[(Symbol, Symbol)] = &[
     // boolean logic required to tell which typing rules to use.
     (sym::ref_pat_eat_one_layer_2024, sym::ref_pat_eat_one_layer_2024_structural),
 ];
+
+/// Some features require one or more other features to be enabled.
+pub const DEPENDENT_FEATURES: &[(Symbol, &[Symbol])] = &[
+    (sym::opaque_generic_const_args, &[sym::min_generic_const_args]),
+    (sym::unsized_const_params, &[sym::adt_const_params]),
+];

compiler/rustc_hir_analysis/src/collect.rs

@@ -16,7 +16,7 @@
 
 use std::cell::Cell;
 use std::iter;
-use std::ops::Bound;
+use std::ops::{Bound, ControlFlow};
 
 use rustc_abi::{ExternAbi, Size};
 use rustc_ast::Recovered;
@@ -26,12 +26,13 @@ use rustc_errors::{
     Applicability, Diag, DiagCtxtHandle, E0228, ErrorGuaranteed, StashKey, struct_span_code_err,
 };
 use rustc_hir::attrs::AttributeKind;
-use rustc_hir::def::DefKind;
+use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::intravisit::{InferKind, Visitor, VisitorExt};
+use rustc_hir::intravisit::{self, InferKind, Visitor, VisitorExt};
 use rustc_hir::{self as hir, GenericParamKind, HirId, Node, PreciseCapturingArgKind, find_attr};
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::{DynCompatibilityViolation, ObligationCause};
+use rustc_middle::hir::nested_filter;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::util::{Discr, IntTypeExt};
 use rustc_middle::ty::{
@@ -1511,6 +1512,20 @@ fn anon_const_kind<'tcx>(tcx: TyCtxt<'tcx>, def: LocalDefId) -> ty::AnonConstKin
             let parent_hir_node = tcx.hir_node(tcx.parent_hir_id(const_arg_id));
             if tcx.features().generic_const_exprs() {
                 ty::AnonConstKind::GCE
+            } else if tcx.features().opaque_generic_const_args() {
+                // Only anon consts that are the RHS of a const item can be OGCA.
+                // Note: We can't just check tcx.parent because it needs to be EXACTLY
+                // the RHS, not just part of the RHS.
+                if !is_anon_const_rhs_of_const_item(tcx, def) {
+                    return ty::AnonConstKind::MCG;
+                }
+
+                let body = tcx.hir_body_owned_by(def);
+                let mut visitor = OGCAParamVisitor(tcx);
+                match visitor.visit_body(body) {
+                    ControlFlow::Break(UsesParam) => ty::AnonConstKind::OGCA,
+                    ControlFlow::Continue(()) => ty::AnonConstKind::MCG,
+                }
             } else if tcx.features().min_generic_const_args() {
                 ty::AnonConstKind::MCG
             } else if let hir::Node::Expr(hir::Expr {
@@ -1528,6 +1543,49 @@ fn anon_const_kind<'tcx>(tcx: TyCtxt<'tcx>, def: LocalDefId) -> ty::AnonConstKin
     }
 }
 
+fn is_anon_const_rhs_of_const_item<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> bool {
+    let hir_id = tcx.local_def_id_to_hir_id(def_id);
+    let Some((_, grandparent_node)) = tcx.hir_parent_iter(hir_id).nth(1) else { return false };
+    let (Node::Item(hir::Item { kind: hir::ItemKind::Const(_, _, _, ct_rhs), .. })
+    | Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Const(_, ct_rhs), .. })
+    | Node::TraitItem(hir::TraitItem {
+        kind: hir::TraitItemKind::Const(_, Some(ct_rhs)), ..
+    })) = grandparent_node
+    else {
+        return false;
+    };
+    let hir::ConstItemRhs::TypeConst(hir::ConstArg {
+        kind: hir::ConstArgKind::Anon(rhs_anon), ..
+    }) = ct_rhs
+    else {
+        return false;
+    };
+    def_id == rhs_anon.def_id
+}
+
+struct OGCAParamVisitor<'tcx>(TyCtxt<'tcx>);
+
+struct UsesParam;
+
+impl<'tcx> Visitor<'tcx> for OGCAParamVisitor<'tcx> {
+    type NestedFilter = nested_filter::OnlyBodies;
+    type Result = ControlFlow<UsesParam>;
+
+    fn maybe_tcx(&mut self) -> TyCtxt<'tcx> {
+        self.0
+    }
+
+    fn visit_path(&mut self, path: &hir::Path<'tcx>, _id: HirId) -> ControlFlow<UsesParam> {
+        if let Res::Def(DefKind::TyParam | DefKind::ConstParam | DefKind::LifetimeParam, _) =
+            path.res
+        {
+            return ControlFlow::Break(UsesParam);
+        }
+
+        intravisit::walk_path(self, path)
+    }
+}
+
 #[instrument(level = "debug", skip(tcx), ret)]
 fn const_of_item<'tcx>(
     tcx: TyCtxt<'tcx>,

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -92,6 +92,8 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
             match tcx.anon_const_kind(def_id) {
                 // Stable: anon consts are not able to use any generic parameters...
                 ty::AnonConstKind::MCG => None,
+                // OGCA anon consts inherit their parent's generics.
+                ty::AnonConstKind::OGCA => Some(parent_did),
                 // we provide generics to repeat expr counts as a backwards compatibility hack. #76200
                 ty::AnonConstKind::RepeatExprCount => Some(parent_did),
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -404,6 +404,11 @@ impl<'tcx> ForbidMCGParamUsesFolder<'tcx> {
                 diag.span_note(impl_.self_ty.span, "not a concrete type");
             }
         }
+        if self.tcx.features().min_generic_const_args()
+            && !self.tcx.features().opaque_generic_const_args()
+        {
+            diag.help("add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items");
+        }
         diag.emit()
     }
 }

compiler/rustc_middle/src/ty/consts.rs

@@ -2,7 +2,7 @@ use std::borrow::Cow;
 
 use rustc_data_structures::intern::Interned;
 use rustc_error_messages::MultiSpan;
-use rustc_macros::{HashStable, TyDecodable, TyEncodable};
+use rustc_macros::HashStable;
 use rustc_type_ir::walk::TypeWalker;
 use rustc_type_ir::{self as ir, TypeFlags, WithCachedTypeInfo};
 
@@ -335,16 +335,3 @@ impl<'tcx> Const<'tcx> {
         TypeWalker::new(self.into())
     }
 }
-
-#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, TyEncodable, TyDecodable, HashStable)]
-pub enum AnonConstKind {
-    /// `feature(generic_const_exprs)` anon consts are allowed to use arbitrary generic parameters in scope
-    GCE,
-    /// stable `min_const_generics` anon consts are not allowed to use any generic parameters
-    MCG,
-    /// anon consts used as the length of a repeat expr are syntactically allowed to use generic parameters
-    /// but must not depend on the actual instantiation. See #76200 for more information
-    RepeatExprCount,
-    /// anon consts outside of the type system, e.g. enum discriminants
-    NonTypeSystem,
-}

compiler/rustc_middle/src/ty/context.rs

@@ -238,6 +238,9 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     fn const_of_item(self, def_id: DefId) -> ty::EarlyBinder<'tcx, Const<'tcx>> {
         self.const_of_item(def_id)
     }
+    fn anon_const_kind(self, def_id: DefId) -> ty::AnonConstKind {
+        self.anon_const_kind(def_id)
+    }
 
     type AdtDef = ty::AdtDef<'tcx>;
     fn adt_def(self, adt_def_id: DefId) -> Self::AdtDef {