Remove #[rustc_host], use internal desugaring

compiler/rustc_ast_lowering/src/item.rs

@@ -562,11 +562,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     .params
                     .iter()
                     .find(|param| {
-                        parent_hir
-                            .attrs
-                            .get(param.hir_id.local_id)
-                            .iter()
-                            .any(|attr| attr.has_name(sym::rustc_host))
+                        matches!(
+                            param.kind,
+                            hir::GenericParamKind::Const { is_host_effect: true, .. }
+                        )
                     })
                     .map(|param| param.def_id);
             }
@@ -1353,27 +1352,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let host_param_parts = if let Const::Yes(span) = constness
             && self.tcx.features().effects
         {
-            if let Some(param) =
-                generics.params.iter().find(|x| x.attrs.iter().any(|x| x.has_name(sym::rustc_host)))
-            {
-                // user has manually specified a `rustc_host` param, in this case, we set
-                // the param id so that lowering logic can use that. But we don't create
-                // another host param, so this gives `None`.
-                self.host_param_id = Some(self.local_def_id(param.id));
-                None
-            } else {
-                let param_node_id = self.next_node_id();
-                let hir_id = self.next_id();
-                let def_id = self.create_def(
-                    self.local_def_id(parent_node_id),
-                    param_node_id,
-                    DefPathData::TypeNs(sym::host),
-                    DefKind::ConstParam,
-                    span,
-                );
-                self.host_param_id = Some(def_id);
-                Some((span, hir_id, def_id))
-            }
+            let param_node_id = self.next_node_id();
+            let hir_id = self.next_id();
+            let def_id = self.create_def(
+                self.local_def_id(parent_node_id),
+                param_node_id,
+                DefPathData::TypeNs(sym::host),
+                DefKind::ConstParam,
+                span,
+            );
+            self.host_param_id = Some(def_id);
+            Some((span, hir_id, def_id))
         } else {
             None
         };
@@ -1442,19 +1431,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
             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| {
                 (
                     &[],
@@ -1496,6 +1472,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         hir_id: const_id,
                         body: const_body,
                     }),
+                    is_host_effect: true,
                 },
                 colon_span: None,
                 pure_wrt_drop: false,

compiler/rustc_ast_lowering/src/lib.rs

@@ -2109,7 +2109,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let default = default.as_ref().map(|def| self.lower_anon_const(def));
                 (
                     hir::ParamName::Plain(self.lower_ident(param.ident)),
-                    hir::GenericParamKind::Const { ty, default },
+                    hir::GenericParamKind::Const { ty, default, is_host_effect: false },
                 )
             }
         }
@@ -2537,22 +2537,14 @@ impl<'hir> GenericArgsCtor<'hir> {
             })
         });
 
-        let attr_id = lcx.tcx.sess.parse_sess.attr_id_generator.mk_attr_id();
-        let attr = lcx.arena.alloc(Attribute {
-            kind: AttrKind::Normal(P(NormalAttr::from_ident(Ident::new(sym::rustc_host, span)))),
-            span,
-            id: attr_id,
-            style: AttrStyle::Outer,
-        });
-        lcx.attrs.insert(hir_id.local_id, std::slice::from_ref(attr));
-
         let def_id = lcx.create_def(
             lcx.current_hir_id_owner.def_id,
             id,
             DefPathData::AnonConst,
             DefKind::AnonConst,
             span,
         );
+
         lcx.children.push((def_id, hir::MaybeOwner::NonOwner(hir_id)));
         self.args.push(hir::GenericArg::Const(hir::ConstArg {
             value: hir::AnonConst { def_id, hir_id, body },

compiler/rustc_feature/src/builtin_attrs.rs

@@ -719,12 +719,6 @@ 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/src/hir.rs

@@ -487,6 +487,7 @@ pub enum GenericParamKind<'hir> {
         ty: &'hir Ty<'hir>,
         /// Optional default value for the const generic param
         default: Option<AnonConst>,
+        is_host_effect: bool,
     },
 }
 

compiler/rustc_hir/src/intravisit.rs

@@ -874,7 +874,7 @@ pub fn walk_generic_param<'v, V: Visitor<'v>>(visitor: &mut V, param: &'v Generi
     match param.kind {
         GenericParamKind::Lifetime { .. } => {}
         GenericParamKind::Type { ref default, .. } => walk_list!(visitor, visit_ty, default),
-        GenericParamKind::Const { ref ty, ref default } => {
+        GenericParamKind::Const { ref ty, ref default, is_host_effect: _ } => {
             visitor.visit_ty(ty);
             if let Some(ref default) = default {
                 visitor.visit_const_param_default(param.hir_id, default);

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -859,7 +859,7 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) -> Result<(),
         hir::GenericParamKind::Lifetime { .. } | hir::GenericParamKind::Type { .. } => Ok(()),
 
         // Const parameters are well formed if their type is structural match.
-        hir::GenericParamKind::Const { ty: hir_ty, default: _ } => {
+        hir::GenericParamKind::Const { ty: hir_ty, default: _, is_host_effect: _ } => {
             let ty = tcx.type_of(param.def_id).instantiate_identity();
 
             if tcx.features().adt_const_params {

compiler/rustc_hir_analysis/src/collect.rs

@@ -1383,7 +1383,7 @@ fn impl_trait_ref(
                 let last_segment = path_segments.len() - 1;
                 let mut args = *path_segments[last_segment].args();
                 let last_arg = args.args.len() - 1;
-                assert!(matches!(args.args[last_arg], hir::GenericArg::Const(anon_const) if tcx.has_attr(anon_const.value.def_id, sym::rustc_host)));
+                assert!(matches!(args.args[last_arg], hir::GenericArg::Const(anon_const) if anon_const.is_desugared_from_effects));
                 args.args = &args.args[..args.args.len() - 1];
                 path_segments[last_segment].args = Some(&args);
                 let path = hir::Path {

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::{sym, Span};
+use rustc_span::Span;
 
 pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
     use rustc_hir::*;
@@ -298,13 +298,11 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                 kind,
             })
         }
-        GenericParamKind::Const { default, .. } => {
-            let is_host_param = tcx.has_attr(param.def_id, sym::rustc_host);
-
+        GenericParamKind::Const { ty: _, default, is_host_effect } => {
             if !matches!(allow_defaults, Defaults::Allowed)
                 && default.is_some()
-                // `rustc_host` effect params are allowed to have defaults.
-                && !is_host_param
+                // `host` effect params are allowed to have defaults.
+                && !is_host_effect
             {
                 tcx.sess.span_err(
                     param.span,
@@ -315,7 +313,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
 
             let index = next_index();
 
-            if is_host_param {
+            if is_host_effect {
                 if let Some(idx) = host_effect_index {
                     bug!("parent also has host effect param? index: {idx}, def: {def_id:?}");
                 }
@@ -330,7 +328,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                 pure_wrt_drop: param.pure_wrt_drop,
                 kind: ty::GenericParamDefKind::Const {
                     has_default: default.is_some(),
-                    is_host_effect: is_host_param,
+                    is_host_effect,
                 },
             })
         }
@@ -489,7 +487,7 @@ struct AnonConstInParamTyDetector {
 
 impl<'v> Visitor<'v> for AnonConstInParamTyDetector {
     fn visit_generic_param(&mut self, p: &'v hir::GenericParam<'v>) {
-        if let GenericParamKind::Const { ty, default: _ } = p.kind {
+        if let GenericParamKind::Const { ty, default: _, is_host_effect: _ } = p.kind {
             let prev = self.in_param_ty;
             self.in_param_ty = true;
             self.visit_ty(ty);

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -992,7 +992,7 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                     self.visit_ty(ty);
                 }
             }
-            GenericParamKind::Const { ty, default } => {
+            GenericParamKind::Const { ty, default, is_host_effect: _ } => {
                 self.visit_ty(ty);
                 if let Some(default) = default {
                     self.visit_body(self.tcx.hir().body(default.body));

compiler/rustc_hir_pretty/src/lib.rs

@@ -2126,7 +2126,7 @@ impl<'a> State<'a> {
                     self.print_type(default);
                 }
             }
-            GenericParamKind::Const { ty, ref default } => {
+            GenericParamKind::Const { ty, ref default, is_host_effect: _ } => {
                 self.word_space(":");
                 self.print_type(ty);
                 if let Some(default) = default {

compiler/rustc_lint/src/nonstandard_style.rs

@@ -534,9 +534,9 @@ impl<'tcx> LateLintPass<'tcx> for NonUpperCaseGlobals {
     }
 
     fn check_generic_param(&mut self, cx: &LateContext<'_>, param: &hir::GenericParam<'_>) {
-        if let GenericParamKind::Const { .. } = param.kind {
-            // `rustc_host` params are explicitly allowed to be lowercase.
-            if cx.tcx.has_attr(param.def_id, sym::rustc_host) {
+        if let GenericParamKind::Const { is_host_effect, .. } = param.kind {
+            // `host` params are explicitly allowed to be lowercase.
+            if is_host_effect {
                 return;
             }
             NonUpperCaseGlobals::check_upper_case(cx, "const parameter", &param.name.ident());

compiler/rustc_middle/src/ty/context.rs

@@ -2185,7 +2185,7 @@ impl<'tcx> TyCtxt<'tcx> {
             hir::Node::Item(hir::Item {
                 kind: hir::ItemKind::Impl(hir::Impl { generics, .. }),
                 ..
-            }) if generics.params.iter().any(|p| self.has_attr(p.def_id, sym::rustc_host))
+            }) if generics.params.iter().any(|p| matches!(p.kind, hir::GenericParamKind::Const { is_host_effect: true, .. }))
         )
     }
 

compiler/rustc_passes/src/lang_items.rs

@@ -21,7 +21,7 @@ use rustc_hir::{LangItem, LanguageItems, Target};
 use rustc_middle::ty::TyCtxt;
 use rustc_session::cstore::ExternCrate;
 use rustc_span::symbol::kw::Empty;
-use rustc_span::{sym, Span};
+use rustc_span::Span;
 
 use rustc_middle::query::Providers;
 
@@ -162,7 +162,12 @@ impl<'tcx> LanguageItemCollector<'tcx> {
                     generics
                         .params
                         .iter()
-                        .filter(|p| !self.tcx.has_attr(p.def_id, sym::rustc_host))
+                        .filter(|p| {
+                            !matches!(
+                                p.kind,
+                                hir::GenericParamKind::Const { is_host_effect: true, .. }
+                            )
+                        })
                         .count(),
                     generics.span,
                 ),

compiler/rustc_span/src/symbol.rs

@@ -1393,7 +1393,6 @@ symbols! {
         rustc_expected_cgu_reuse,
         rustc_has_incoherent_inherent_impls,
         rustc_hidden_type_of_opaques,
-        rustc_host,
         rustc_if_this_changed,
         rustc_inherit_overflow_checks,
         rustc_insignificant_dtor,

src/librustdoc/clean/mod.rs

@@ -595,13 +595,13 @@ fn clean_generic_param<'tcx>(
                 },
             )
         }
-        hir::GenericParamKind::Const { ty, default } => (
+        hir::GenericParamKind::Const { ty, default, is_host_effect } => (
             param.name.ident().name,
             GenericParamDefKind::Const {
                 ty: Box::new(clean_ty(ty, cx)),
                 default: default
                     .map(|ct| Box::new(ty::Const::from_anon_const(cx.tcx, ct.def_id).to_string())),
-                is_host_effect: cx.tcx.has_attr(param.def_id, sym::rustc_host),
+                is_host_effect,
             },
         ),
     };
@@ -2536,11 +2536,12 @@ fn clean_generic_args<'tcx>(
                     }
                     hir::GenericArg::Lifetime(_) => GenericArg::Lifetime(Lifetime::elided()),
                     hir::GenericArg::Type(ty) => GenericArg::Type(clean_ty(ty, cx)),
-                    // Checking for `#[rustc_host]` on the `AnonConst`  not only accounts for the case
+                    // Checking for `is_desugared_from_effects` on the `AnonConst` not only accounts for the case
                     // where the argument is `host` but for all possible cases (e.g., `true`, `false`).
-                    hir::GenericArg::Const(ct)
-                        if cx.tcx.has_attr(ct.value.def_id, sym::rustc_host) =>
-                    {
+                    hir::GenericArg::Const(hir::ConstArg {
+                        is_desugared_from_effects: true,
+                        ..
+                    }) => {
                         return None;
                     }
                     hir::GenericArg::Const(ct) => GenericArg::Const(Box::new(clean_const(ct, cx))),

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/helloworld.rs

@@ -3,15 +3,16 @@
 // gate-test-effects
 // ^ effects doesn't have a gate so we will trick tidy into thinking this is a gate test
 
-#![feature(const_trait_impl, effects, rustc_attrs)]
+#![feature(const_trait_impl, effects, core_intrinsics, const_eval_select)]
 
 // ensure we are passing in the correct host effect in always const contexts.
 
-pub const fn hmm<T, #[rustc_host] const host: bool = true>() -> usize {
-    if host {
-        1
-    } else {
-        0
+pub const fn hmm<T>() -> usize {
+    // FIXME(const_trait_impl): maybe we should have a way to refer to the (hidden) effect param
+    fn one() -> usize { 1 }
+    const fn zero() -> usize { 0 }
+    unsafe {
+        std::intrinsics::const_eval_select((), zero, one)
     }
 }