Rollup merge of #131081 - camelid:const-path-it-all, r=BoxyUwU

Use `ConstArgKind::Path` for all single-segment paths, not just params under `min_generic_const_args`

r? `@BoxyUwU`

edit by `@BoxyUwU:`

This PR introduces a `min_generic_const_args` feature gate and implements some preliminary work for it, representing all const arguments that are single segment paths as `ConstArg::Path` instead of only those that resolve to a const generic parameter. There are a few bits of follow up work after this lands:
- Figure out how to represent `Foo<{ STATIC }>`
- Figure out how to evaluate `Foo<{ EnumVariantConstructor }>`
- Make param env normalization handle non-anon-consts
- Move `try_from_lit` and `from_anon_const` to hir ty lowering too

3 files changed, 14 insertions, 99 deletions

diff --git a/compiler/rustc_middle/src/ty/consts.rs b/compiler/rustc_middle/src/ty/consts.rs
index 3bd09fc91c6..d853edb34c9 100644
--- a/compiler/rustc_middle/src/ty/consts.rs
+++ b/compiler/rustc_middle/src/ty/consts.rs
@@ -1,13 +1,14 @@
+use std::borrow::Cow;
+
 use rustc_data_structures::intern::Interned;
 use rustc_error_messages::MultiSpan;
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::{self as hir, HirId};
+use rustc_hir::def_id::LocalDefId;
+use rustc_hir::{self as hir};
 use rustc_macros::HashStable;
 use rustc_type_ir::{self as ir, TypeFlags, WithCachedTypeInfo};
 use tracing::{debug, instrument};
 
-use crate::middle::resolve_bound_vars as rbv;
 use crate::mir::interpret::{LitToConstInput, Scalar};
 use crate::ty::{self, GenericArgs, Ty, TyCtxt, TypeVisitableExt};
 
@@ -142,7 +143,7 @@ impl<'tcx> Const<'tcx> {
     pub fn new_error_with_message<S: Into<MultiSpan>>(
         tcx: TyCtxt<'tcx>,
         span: S,
-        msg: &'static str,
+        msg: impl Into<Cow<'static, str>>,
     ) -> Const<'tcx> {
         let reported = tcx.dcx().span_delayed_bug(span, msg);
         Const::new_error(tcx, reported)
@@ -183,46 +184,8 @@ impl<'tcx> rustc_type_ir::inherent::Const<TyCtxt<'tcx>> for Const<'tcx> {
     }
 }
 
-/// In some cases, [`hir::ConstArg`]s that are being used in the type system
-/// through const generics need to have their type "fed" to them
-/// using the query system.
-///
-/// Use this enum with [`Const::from_const_arg`] to instruct it with the
-/// desired behavior.
-#[derive(Debug, Clone, Copy)]
-pub enum FeedConstTy {
-    /// Feed the type.
-    ///
-    /// The `DefId` belongs to the const param that we are supplying
-    /// this (anon) const arg to.
-    Param(DefId),
-    /// Don't feed the type.
-    No,
-}
-
 impl<'tcx> Const<'tcx> {
-    /// Convert a [`hir::ConstArg`] to a [`ty::Const`](Self).
-    #[instrument(skip(tcx), level = "debug")]
-    pub fn from_const_arg(
-        tcx: TyCtxt<'tcx>,
-        const_arg: &'tcx hir::ConstArg<'tcx>,
-        feed: FeedConstTy,
-    ) -> Self {
-        if let FeedConstTy::Param(param_def_id) = feed
-            && let hir::ConstArgKind::Anon(anon) = &const_arg.kind
-        {
-            tcx.feed_anon_const_type(anon.def_id, tcx.type_of(param_def_id));
-        }
-
-        match const_arg.kind {
-            hir::ConstArgKind::Path(qpath) => {
-                // FIXME(min_generic_const_args): for now only params are lowered to ConstArgKind::Path
-                Self::from_param(tcx, qpath, const_arg.hir_id)
-            }
-            hir::ConstArgKind::Anon(anon) => Self::from_anon_const(tcx, anon.def_id),
-        }
-    }
-
+    // FIXME: move this and try_from_lit to hir_ty_lowering like lower_const_arg/from_const_arg
     /// Literals and const generic parameters are eagerly converted to a constant, everything else
     /// becomes `Unevaluated`.
     #[instrument(skip(tcx), level = "debug")]
@@ -240,7 +203,7 @@ impl<'tcx> Const<'tcx> {
 
         let ty = tcx.type_of(def).no_bound_vars().expect("const parameter types cannot be generic");
 
-        match Self::try_from_lit_or_param(tcx, ty, expr) {
+        match Self::try_from_lit(tcx, ty, expr) {
             Some(v) => v,
             None => ty::Const::new_unevaluated(tcx, ty::UnevaluatedConst {
                 def: def.to_def_id(),
@@ -249,40 +212,8 @@ impl<'tcx> Const<'tcx> {
         }
     }
 
-    /// Lower a const param to a [`Const`].
-    ///
-    /// IMPORTANT: `qpath` must be a const param, otherwise this will panic
-    fn from_param(tcx: TyCtxt<'tcx>, qpath: hir::QPath<'tcx>, hir_id: HirId) -> Self {
-        let hir::QPath::Resolved(_, &hir::Path { res: Res::Def(DefKind::ConstParam, def_id), .. }) =
-            qpath
-        else {
-            span_bug!(qpath.span(), "non-param {qpath:?} passed to Const::from_param")
-        };
-
-        match tcx.named_bound_var(hir_id) {
-            Some(rbv::ResolvedArg::EarlyBound(_)) => {
-                // Find the name and index of the const parameter by indexing the generics of
-                // the parent item and construct a `ParamConst`.
-                let item_def_id = tcx.parent(def_id);
-                let generics = tcx.generics_of(item_def_id);
-                let index = generics.param_def_id_to_index[&def_id];
-                let name = tcx.item_name(def_id);
-                ty::Const::new_param(tcx, ty::ParamConst::new(index, name))
-            }
-            Some(rbv::ResolvedArg::LateBound(debruijn, index, _)) => {
-                ty::Const::new_bound(tcx, debruijn, ty::BoundVar::from_u32(index))
-            }
-            Some(rbv::ResolvedArg::Error(guar)) => ty::Const::new_error(tcx, guar),
-            arg => bug!("unexpected bound var resolution for {:?}: {arg:?}", hir_id),
-        }
-    }
-
     #[instrument(skip(tcx), level = "debug")]
-    fn try_from_lit_or_param(
-        tcx: TyCtxt<'tcx>,
-        ty: Ty<'tcx>,
-        expr: &'tcx hir::Expr<'tcx>,
-    ) -> Option<Self> {
+    fn try_from_lit(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, expr: &'tcx hir::Expr<'tcx>) -> Option<Self> {
         // Unwrap a block, so that e.g. `{ P }` is recognised as a parameter. Const arguments
         // currently have to be wrapped in curly brackets, so it's necessary to special-case.
         let expr = match &expr.kind {
@@ -321,7 +252,7 @@ impl<'tcx> Const<'tcx> {
                 Err(e) => {
                     tcx.dcx().span_delayed_bug(
                         expr.span,
-                        format!("Const::from_anon_const: couldn't lit_to_const {e:?}"),
+                        format!("Const::try_from_lit: couldn't lit_to_const {e:?}"),
                     );
                 }
             }
@@ -414,20 +345,3 @@ impl<'tcx> Const<'tcx> {
         matches!(self.kind(), ty::ConstKind::Infer(_))
     }
 }
-
-pub fn const_param_default<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    def_id: LocalDefId,
-) -> ty::EarlyBinder<'tcx, Const<'tcx>> {
-    let default_ct = match tcx.hir_node_by_def_id(def_id) {
-        hir::Node::GenericParam(hir::GenericParam {
-            kind: hir::GenericParamKind::Const { default: Some(ct), .. },
-            ..
-        }) => ct,
-        _ => span_bug!(
-            tcx.def_span(def_id),
-            "`const_param_default` expected a generic parameter with a constant"
-        ),
-    };
-    ty::EarlyBinder::bind(Const::from_const_arg(tcx, default_ct, FeedConstTy::No))
-}
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 2ba1bf2822f..68c3b064eee 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -30,7 +30,7 @@ use rustc_errors::{
     Applicability, Diag, DiagCtxtHandle, ErrorGuaranteed, LintDiagnostic, MultiSpan,
 };
 use rustc_hir as hir;
-use rustc_hir::def::DefKind;
+use rustc_hir::def::{CtorKind, DefKind};
 use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE, LocalDefId};
 use rustc_hir::definitions::Definitions;
 use rustc_hir::intravisit::Visitor;
@@ -230,7 +230,9 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
             DefKind::OpaqueTy => ty::AliasTermKind::OpaqueTy,
             DefKind::TyAlias => ty::AliasTermKind::WeakTy,
             DefKind::AssocConst => ty::AliasTermKind::ProjectionConst,
-            DefKind::AnonConst => ty::AliasTermKind::UnevaluatedConst,
+            DefKind::AnonConst | DefKind::Const | DefKind::Ctor(_, CtorKind::Const) => {
+                ty::AliasTermKind::UnevaluatedConst
+            }
             kind => bug!("unexpected DefKind in AliasTy: {kind:?}"),
         }
     }
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index cddd6110c23..965a8c8c95e 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -62,7 +62,7 @@ pub use self::closure::{
     place_to_string_for_capture,
 };
 pub use self::consts::{
-    Const, ConstInt, ConstKind, Expr, ExprKind, FeedConstTy, ScalarInt, UnevaluatedConst, ValTree,
+    Const, ConstInt, ConstKind, Expr, ExprKind, ScalarInt, UnevaluatedConst, ValTree,
 };
 pub use self::context::{
     CtxtInterners, CurrentGcx, DeducedParamAttrs, Feed, FreeRegionInfo, GlobalCtxt, Lift, TyCtxt,
@@ -2249,7 +2249,6 @@ pub fn provide(providers: &mut Providers) {
         incoherent_impls: trait_def::incoherent_impls_provider,
         trait_impls_in_crate: trait_def::trait_impls_in_crate_provider,
         traits: trait_def::traits_provider,
-        const_param_default: consts::const_param_default,
         vtable_allocation: vtable::vtable_allocation_provider,
         ..*providers
     };