Auto merge of #113210 - fee1-dead-contrib:effects-mvp, r=oli-obk

Effects/keyword generics MVP

This adds `feature(effects)`, which adds `const host: bool` to the generics of const functions, const traits and const impls. This will be used to replace the current logic around const traits.

r? `@oli-obk`

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -2652,6 +2652,15 @@ pub struct NormalAttr {
     pub tokens: Option<LazyAttrTokenStream>,
 }
 
+impl NormalAttr {
+    pub fn from_ident(ident: Ident) -> Self {
+        Self {
+            item: AttrItem { path: Path::from_ident(ident), args: AttrArgs::Empty, tokens: None },
+            tokens: None,
+        }
+    }
+}
+
 #[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
 pub struct AttrItem {
     pub path: Path,

compiler/rustc_ast_lowering/src/expr.rs

@@ -673,14 +673,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             self.lower_attrs(
                 inner_hir_id,
                 &[Attribute {
-                    kind: AttrKind::Normal(ptr::P(NormalAttr {
-                        item: AttrItem {
-                            path: Path::from_ident(Ident::new(sym::track_caller, span)),
-                            args: AttrArgs::Empty,
-                            tokens: None,
-                        },
-                        tokens: None,
-                    })),
+                    kind: AttrKind::Normal(ptr::P(NormalAttr::from_ident(Ident::new(sym::track_caller, span)))),
                     id: self.tcx.sess.parse_sess.attr_id_generator.mk_attr_id(),
                     style: AttrStyle::Outer,
                     span: unstable_span,

compiler/rustc_ast_lowering/src/item.rs

@@ -3,6 +3,7 @@ use super::ResolverAstLoweringExt;
 use super::{AstOwner, ImplTraitContext, ImplTraitPosition};
 use super::{FnDeclKind, LoweringContext, ParamMode};
 
+use hir::definitions::DefPathData;
 use rustc_ast::ptr::P;
 use rustc_ast::visit::AssocCtxt;
 use rustc_ast::*;
@@ -257,10 +258,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     );
 
                     let itctx = ImplTraitContext::Universal;
-                    let (generics, decl) = this.lower_generics(generics, id, &itctx, |this| {
-                        let ret_id = asyncness.opt_return_id();
-                        this.lower_fn_decl(&decl, id, *fn_sig_span, FnDeclKind::Fn, ret_id)
-                    });
+                    let (generics, decl) =
+                        this.lower_generics(generics, header.constness, id, &itctx, |this| {
+                            let ret_id = asyncness.opt_return_id();
+                            this.lower_fn_decl(&decl, id, *fn_sig_span, FnDeclKind::Fn, ret_id)
+                        });
                     let sig = hir::FnSig {
                         decl,
                         header: this.lower_fn_header(*header),
@@ -295,6 +297,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 add_ty_alias_where_clause(&mut generics, *where_clauses, true);
                 let (generics, ty) = self.lower_generics(
                     &generics,
+                    Const::No,
                     id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| match ty {
@@ -316,6 +319,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             ItemKind::Enum(enum_definition, generics) => {
                 let (generics, variants) = self.lower_generics(
                     generics,
+                    Const::No,
                     id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
@@ -329,6 +333,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             ItemKind::Struct(struct_def, generics) => {
                 let (generics, struct_def) = self.lower_generics(
                     generics,
+                    Const::No,
                     id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, struct_def),
@@ -338,6 +343,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             ItemKind::Union(vdata, generics) => {
                 let (generics, vdata) = self.lower_generics(
                     generics,
+                    Const::No,
                     id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, vdata),
@@ -369,7 +375,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 // parent lifetime.
                 let itctx = ImplTraitContext::Universal;
                 let (generics, (trait_ref, lowered_ty)) =
-                    self.lower_generics(ast_generics, id, &itctx, |this| {
+                    self.lower_generics(ast_generics, *constness, id, &itctx, |this| {
                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
                             this.lower_trait_ref(
                                 trait_ref,
@@ -410,8 +416,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }))
             }
             ItemKind::Trait(box Trait { is_auto, unsafety, generics, bounds, items }) => {
+                // FIXME(const_trait_impl, effects, fee1-dead) this should be simplified if possible
+                let constness = attrs
+                    .unwrap_or(&[])
+                    .iter()
+                    .find(|x| x.has_name(sym::const_trait))
+                    .map_or(Const::No, |x| Const::Yes(x.span));
                 let (generics, (unsafety, items, bounds)) = self.lower_generics(
                     generics,
+                    constness,
                     id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
@@ -431,6 +444,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             ItemKind::TraitAlias(generics, bounds) => {
                 let (generics, bounds) = self.lower_generics(
                     generics,
+                    Const::No,
                     id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
@@ -593,7 +607,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let fdec = &sig.decl;
                     let itctx = ImplTraitContext::Universal;
                     let (generics, (fn_dec, fn_args)) =
-                        self.lower_generics(generics, i.id, &itctx, |this| {
+                        self.lower_generics(generics, Const::No, i.id, &itctx, |this| {
                             (
                                 // Disallow `impl Trait` in foreign items.
                                 this.lower_fn_decl(
@@ -745,6 +759,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 add_ty_alias_where_clause(&mut generics, *where_clauses, false);
                 let (generics, kind) = self.lower_generics(
                     &generics,
+                    Const::No,
                     i.id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
@@ -843,6 +858,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 add_ty_alias_where_clause(&mut generics, *where_clauses, false);
                 self.lower_generics(
                     &generics,
+                    Const::No,
                     i.id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| match ty {
@@ -1201,9 +1217,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
         let header = self.lower_fn_header(sig.header);
         let itctx = ImplTraitContext::Universal;
-        let (generics, decl) = self.lower_generics(generics, id, &itctx, |this| {
-            this.lower_fn_decl(&sig.decl, id, sig.span, kind, is_async)
-        });
+        let (generics, decl) =
+            self.lower_generics(generics, sig.header.constness, id, &itctx, |this| {
+                this.lower_fn_decl(&sig.decl, id, sig.span, kind, is_async)
+            });
         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
     }
 
@@ -1275,6 +1292,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_generics<T>(
         &mut self,
         generics: &Generics,
+        constness: Const,
         parent_node_id: NodeId,
         itctx: &ImplTraitContext,
         f: impl FnOnce(&mut Self) -> T,
@@ -1372,6 +1390,87 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let impl_trait_bounds = std::mem::take(&mut self.impl_trait_bounds);
         predicates.extend(impl_trait_bounds.into_iter());
 
+        // Desugar `~const` bound in generics into an additional `const host: bool` param
+        // if the effects feature is enabled.
+        if let Const::Yes(span) = constness && self.tcx.features().effects
+            // Do not add host param if it already has it (manually specified)
+            && !params.iter().any(|x| {
+                self.attrs.get(&x.hir_id.local_id).map_or(false, |attrs| {
+                    attrs.iter().any(|x| x.has_name(sym::rustc_host))
+                })
+            })
+        {
+            let param_node_id = self.next_node_id();
+            let const_node_id = self.next_node_id();
+            let def_id = self.create_def(self.local_def_id(parent_node_id), param_node_id, DefPathData::TypeNs(sym::host), span);
+            let anon_const: LocalDefId = self.create_def(def_id, const_node_id, DefPathData::AnonConst, span);
+
+            let hir_id = self.next_id();
+            let const_id = self.next_id();
+            let const_expr_id = self.next_id();
+            let bool_id = self.next_id();
+
+            self.children.push((def_id, hir::MaybeOwner::NonOwner(hir_id)));
+            self.children.push((anon_const, hir::MaybeOwner::NonOwner(const_id)));
+
+            let attr_id = self.tcx.sess.parse_sess.attr_id_generator.mk_attr_id();
+
+            let attrs = self.arena.alloc_from_iter([
+                Attribute {
+                    kind: AttrKind::Normal(P(NormalAttr::from_ident(Ident::new(sym::rustc_host, span)))),
+                    span,
+                    id: attr_id,
+                    style: AttrStyle::Outer,
+                },
+            ]);
+            self.attrs.insert(hir_id.local_id, attrs);
+
+            let const_body = self.lower_body(|this| {
+                (
+                    &[],
+                    hir::Expr {
+                        hir_id: const_expr_id,
+                        kind: hir::ExprKind::Lit(
+                            this.arena.alloc(hir::Lit { node: LitKind::Bool(true), span }),
+                        ),
+                        span,
+                    },
+                )
+            });
+
+            let param = hir::GenericParam {
+                def_id,
+                hir_id,
+                name: hir::ParamName::Plain(Ident { name: sym::host, span }),
+                span,
+                kind: hir::GenericParamKind::Const {
+                    ty: self.arena.alloc(self.ty(
+                        span,
+                        hir::TyKind::Path(hir::QPath::Resolved(
+                            None,
+                            self.arena.alloc(hir::Path {
+                                res: Res::PrimTy(hir::PrimTy::Bool),
+                                span,
+                                segments: self.arena.alloc_from_iter([hir::PathSegment {
+                                    ident: Ident { name: sym::bool, span },
+                                    hir_id: bool_id,
+                                    res: Res::PrimTy(hir::PrimTy::Bool),
+                                    args: None,
+                                    infer_args: false,
+                                }]),
+                            }),
+                        )),
+                    )),
+                    default: Some(hir::AnonConst { def_id: anon_const, hir_id: const_id, body: const_body }),
+                },
+                colon_span: None,
+                pure_wrt_drop: false,
+                source: hir::GenericParamSource::Generics,
+            };
+
+            params.push(param);
+        }
+
         let lowered_generics = self.arena.alloc(hir::Generics {
             params: self.arena.alloc_from_iter(params),
             predicates: self.arena.alloc_from_iter(predicates),

compiler/rustc_feature/src/active.rs

@@ -391,6 +391,8 @@ declare_features! (
     (active, doc_masked, "1.21.0", Some(44027), None),
     /// Allows `dyn* Trait` objects.
     (incomplete, dyn_star, "1.65.0", Some(102425), None),
+    // Uses generic effect parameters for ~const bounds
+    (active, effects, "CURRENT_RUSTC_VERSION", Some(102090), None),
     /// Allows `X..Y` patterns.
     (active, exclusive_range_pattern, "1.11.0", Some(37854), None),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -723,6 +723,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         and it is only intended to be used in `alloc`."
     ),
 
+    rustc_attr!(
+        rustc_host, AttributeType::Normal, template!(Word), ErrorFollowing,
+        "#[rustc_host] annotates const generic parameters as the `host` effect param, \
+        and it is only intended for internal use and as a desugaring."
+    ),
+
     BuiltinAttribute {
         name: sym::rustc_diagnostic_item,
         // FIXME: This can be `true` once we always use `tcx.is_diagnostic_item`.

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -9,7 +9,7 @@ use rustc_hir::def_id::LocalDefId;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_session::lint;
 use rustc_span::symbol::{kw, Symbol};
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 
 pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
     use rustc_hir::*;
@@ -101,6 +101,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                         param_def_id_to_index,
                         has_self: generics.has_self,
                         has_late_bound_regions: generics.has_late_bound_regions,
+                        host_effect_index: None,
                     };
                 } else {
                     // HACK(eddyb) this provides the correct generics when
@@ -226,10 +227,12 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
     let has_self = opt_self.is_some();
     let mut parent_has_self = false;
     let mut own_start = has_self as u32;
+    let mut host_effect_index = None;
     let parent_count = parent_def_id.map_or(0, |def_id| {
         let generics = tcx.generics_of(def_id);
         assert!(!has_self);
         parent_has_self = generics.has_self;
+        host_effect_index = generics.host_effect_index;
         own_start = generics.count() as u32;
         generics.parent_count + generics.params.len()
     });
@@ -251,11 +254,11 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
 
     // Now create the real type and const parameters.
     let type_start = own_start - has_self as u32 + params.len() as u32;
-    let mut i = 0;
+    let mut i: u32 = 0;
     let mut next_index = || {
         let prev = i;
         i += 1;
-        prev as u32 + type_start
+        prev + type_start
     };
 
     const TYPE_DEFAULT_NOT_ALLOWED: &'static str = "defaults for type parameters are only allowed in \
@@ -295,16 +298,32 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
             })
         }
         GenericParamKind::Const { default, .. } => {
-            if !matches!(allow_defaults, Defaults::Allowed) && default.is_some() {
+            let is_host_param = tcx.has_attr(param.def_id, sym::rustc_host);
+
+            if !matches!(allow_defaults, Defaults::Allowed)
+                && default.is_some()
+                // `rustc_host` effect params are allowed to have defaults.
+                && !is_host_param
+            {
                 tcx.sess.span_err(
                     param.span,
                     "defaults for const parameters are only allowed in \
                     `struct`, `enum`, `type`, or `trait` definitions",
                 );
             }
 
+            let index = next_index();
+
+            if is_host_param {
+                if let Some(idx) = host_effect_index {
+                    bug!("parent also has host effect param? index: {idx}, def: {def_id:?}");
+                }
+
+                host_effect_index = Some(parent_count + index as usize);
+            }
+
             Some(ty::GenericParamDef {
-                index: next_index(),
+                index,
                 name: param.name.ident().name,
                 def_id: param.def_id.to_def_id(),
                 pure_wrt_drop: param.pure_wrt_drop,
@@ -356,6 +375,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
         param_def_id_to_index,
         has_self: has_self || parent_has_self,
         has_late_bound_regions: has_late_bound_regions(tcx, node),
+        host_effect_index,
     }
 }