Auto merge of #88328 - fee1-dead:not-quite-const, r=oli-obk

Introduce `~const`

 - [x] Removed `?const` and change uses of `?const`
 - [x] Added `~const` to the AST. It is gated behind const_trait_impl.
 - [x] Validate `~const` in ast_validation.
 - [x] Update UI Tests
 - [x] Add enum `BoundConstness` (With variants `NotConst` and
 `ConstIfConst` allowing future extensions)
 - [x] Adjust trait selection and pre-existing code to use `BoundConstness`.
 - [ ] Optional steps for this PR
      - [x] Fix #88155
      - [x] ~~Do something with constness bounds in chalk~~ Must be done to rust-lang/chalk (just tried to refactor, there are a lot of errors to resolve :( )
      - [ ] Adjust Error messages for `~const` bounds that can't be satisfied.

r? `@oli-obk`

compiler/rustc_middle/src/traits/mod.rs

@@ -17,7 +17,6 @@ use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::Constness;
 use rustc_span::symbol::Symbol;
 use rustc_span::{Span, DUMMY_SP};
 use smallvec::SmallVec;
@@ -497,7 +496,7 @@ pub enum ImplSource<'tcx, N> {
     /// for some type parameter. The `Vec<N>` represents the
     /// obligations incurred from normalizing the where-clause (if
     /// any).
-    Param(Vec<N>, Constness),
+    Param(Vec<N>, ty::BoundConstness),
 
     /// Virtual calls through an object.
     Object(ImplSourceObjectData<'tcx, N>),

compiler/rustc_middle/src/ty/error.rs

@@ -33,7 +33,7 @@ impl<T> ExpectedFound<T> {
 #[derive(Clone, Debug, TypeFoldable)]
 pub enum TypeError<'tcx> {
     Mismatch,
-    ConstnessMismatch(ExpectedFound<hir::Constness>),
+    ConstnessMismatch(ExpectedFound<ty::BoundConstness>),
     UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
     AbiMismatch(ExpectedFound<abi::Abi>),
     Mutability,
@@ -102,7 +102,7 @@ impl<'tcx> fmt::Display for TypeError<'tcx> {
             CyclicConst(_) => write!(f, "encountered a self-referencing constant"),
             Mismatch => write!(f, "types differ"),
             ConstnessMismatch(values) => {
-                write!(f, "expected {} fn, found {} fn", values.expected, values.found)
+                write!(f, "expected {} bound, found {} bound", values.expected, values.found)
             }
             UnsafetyMismatch(values) => {
                 write!(f, "expected {} fn, found {} fn", values.expected, values.found)

compiler/rustc_middle/src/ty/mod.rs

@@ -37,7 +37,7 @@ use rustc_data_structures::tagged_ptr::CopyTaggedPtr;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res};
 use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, LocalDefIdMap, CRATE_DEF_INDEX};
-use rustc_hir::{Constness, Node};
+use rustc_hir::Node;
 use rustc_macros::HashStable;
 use rustc_span::symbol::{kw, Ident, Symbol};
 use rustc_span::Span;
@@ -181,6 +181,25 @@ pub enum Visibility {
     Invisible,
 }
 
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable, TyEncodable, TyDecodable)]
+pub enum BoundConstness {
+    /// `T: Trait`
+    NotConst,
+    /// `T: ~const Trait`
+    ///
+    /// Requires resolving to const only when we are in a const context.
+    ConstIfConst,
+}
+
+impl fmt::Display for BoundConstness {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::NotConst => f.write_str("normal"),
+            Self::ConstIfConst => f.write_str("`~const`"),
+        }
+    }
+}
+
 #[derive(
     Clone,
     Debug,
@@ -457,10 +476,6 @@ pub enum PredicateKind<'tcx> {
     /// Corresponds to `where Foo: Bar<A, B, C>`. `Foo` here would be
     /// the `Self` type of the trait reference and `A`, `B`, and `C`
     /// would be the type parameters.
-    ///
-    /// A trait predicate will have `Constness::Const` if it originates
-    /// from a bound on a `const fn` without the `?const` opt-out (e.g.,
-    /// `const fn foobar<Foo: Bar>() {}`).
     Trait(TraitPredicate<'tcx>),
 
     /// `where 'a: 'b`
@@ -632,10 +647,7 @@ impl<'tcx> Predicate<'tcx> {
 pub struct TraitPredicate<'tcx> {
     pub trait_ref: TraitRef<'tcx>,
 
-    /// A trait predicate will have `Constness::Const` if it originates
-    /// from a bound on a `const fn` without the `?const` opt-out (e.g.,
-    /// `const fn foobar<Foo: Bar>() {}`).
-    pub constness: hir::Constness,
+    pub constness: BoundConstness,
 }
 
 pub type PolyTraitPredicate<'tcx> = ty::Binder<'tcx, TraitPredicate<'tcx>>;
@@ -1304,7 +1316,7 @@ impl<'tcx> ParamEnv<'tcx> {
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TypeFoldable)]
 pub struct ConstnessAnd<T> {
-    pub constness: Constness,
+    pub constness: BoundConstness,
     pub value: T,
 }
 
@@ -1312,18 +1324,18 @@ pub struct ConstnessAnd<T> {
 // the constness of trait bounds is being propagated correctly.
 pub trait WithConstness: Sized {
     #[inline]
-    fn with_constness(self, constness: Constness) -> ConstnessAnd<Self> {
+    fn with_constness(self, constness: BoundConstness) -> ConstnessAnd<Self> {
         ConstnessAnd { constness, value: self }
     }
 
     #[inline]
-    fn with_const(self) -> ConstnessAnd<Self> {
-        self.with_constness(Constness::Const)
+    fn with_const_if_const(self) -> ConstnessAnd<Self> {
+        self.with_constness(BoundConstness::ConstIfConst)
     }
 
     #[inline]
     fn without_const(self) -> ConstnessAnd<Self> {
-        self.with_constness(Constness::NotConst)
+        self.with_constness(BoundConstness::NotConst)
     }
 }
 

compiler/rustc_middle/src/ty/relate.rs

@@ -200,12 +200,12 @@ impl<'tcx> Relate<'tcx> for ty::FnSig<'tcx> {
     }
 }
 
-impl<'tcx> Relate<'tcx> for ast::Constness {
+impl<'tcx> Relate<'tcx> for ty::BoundConstness {
     fn relate<R: TypeRelation<'tcx>>(
         relation: &mut R,
-        a: ast::Constness,
-        b: ast::Constness,
-    ) -> RelateResult<'tcx, ast::Constness> {
+        a: ty::BoundConstness,
+        b: ty::BoundConstness,
+    ) -> RelateResult<'tcx, ty::BoundConstness> {
         if a != b {
             Err(TypeError::ConstnessMismatch(expected_found(relation, a, b)))
         } else {

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -8,7 +8,6 @@ use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
 use crate::ty::print::{with_no_trimmed_paths, FmtPrinter, Printer};
 use crate::ty::{self, InferConst, Lift, Ty, TyCtxt};
 use rustc_data_structures::functor::IdFunctor;
-use rustc_hir as hir;
 use rustc_hir::def::Namespace;
 use rustc_hir::def_id::CRATE_DEF_INDEX;
 use rustc_index::vec::{Idx, IndexVec};
@@ -155,8 +154,8 @@ impl fmt::Debug for ty::ParamConst {
 
 impl fmt::Debug for ty::TraitPredicate<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        if let hir::Constness::Const = self.constness {
-            write!(f, "const ")?;
+        if let ty::BoundConstness::ConstIfConst = self.constness {
+            write!(f, "~const ")?;
         }
         write!(f, "TraitPredicate({:?})", self.trait_ref)
     }
@@ -241,6 +240,7 @@ TrivialTypeFoldableAndLiftImpls! {
     crate::traits::Reveal,
     crate::ty::adjustment::AutoBorrowMutability,
     crate::ty::AdtKind,
+    crate::ty::BoundConstness,
     // Including `BoundRegionKind` is a *bit* dubious, but direct
     // references to bound region appear in `ty::Error`, and aren't
     // really meant to be folded. In general, we can only fold a fully

compiler/rustc_middle/src/ty/sty.rs

@@ -878,7 +878,7 @@ impl<'tcx> PolyTraitRef<'tcx> {
     pub fn to_poly_trait_predicate(&self) -> ty::PolyTraitPredicate<'tcx> {
         self.map_bound(|trait_ref| ty::TraitPredicate {
             trait_ref,
-            constness: hir::Constness::NotConst,
+            constness: ty::BoundConstness::NotConst,
         })
     }
 }