Auto merge of #83207 - oli-obk:valtree2, r=lcnr

normalize mir::Constant differently from ty::Const in preparation for valtrees

Valtrees are unable to represent many kind of constant values (this is on purpose). For constants that are used at runtime, we do not need a valtree representation and can thus use a different form of evaluation. In order to make this explicit and less fragile, I added a `fold_constant` method to `TypeFolder` and implemented it for normalization. Normalization can now, when it wants to eagerly evaluate a constant, normalize `mir::Constant` directly into a `mir::ConstantKind::Val` instead of relying on the `ty::Const` evaluation.

In the future we can get rid of the `ty::Const` in there entirely and add our own `Unevaluated` variant to `mir::ConstantKind`. This would allow us to remove the `promoted` field from `ty::ConstKind::Unevaluated`, as promoteds can never occur in the type system.

cc `@rust-lang/wg-const-eval`

r? `@lcnr`

compiler/rustc_middle/src/mir/mod.rs

@@ -2392,7 +2392,8 @@ pub struct Constant<'tcx> {
     pub literal: ConstantKind<'tcx>,
 }
 
-#[derive(Clone, Copy, PartialEq, PartialOrd, TyEncodable, TyDecodable, Hash, HashStable, Debug)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, TyEncodable, TyDecodable, Hash, HashStable, Debug)]
+#[derive(Lift)]
 pub enum ConstantKind<'tcx> {
     /// This constant came from the type system
     Ty(&'tcx ty::Const<'tcx>),
@@ -2691,7 +2692,13 @@ impl<'tcx> Display for Constant<'tcx> {
             ty::FnDef(..) => {}
             _ => write!(fmt, "const ")?,
         }
-        match self.literal {
+        Display::fmt(&self.literal, fmt)
+    }
+}
+
+impl<'tcx> Display for ConstantKind<'tcx> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        match *self {
             ConstantKind::Ty(c) => pretty_print_const(c, fmt, true),
             ConstantKind::Val(val, ty) => pretty_print_const_value(val, ty, fmt, true),
         }

compiler/rustc_middle/src/mir/type_foldable.rs

@@ -348,6 +348,11 @@ impl<'tcx> TypeFoldable<'tcx> for Constant<'tcx> {
 }
 
 impl<'tcx> TypeFoldable<'tcx> for ConstantKind<'tcx> {
+    #[inline(always)]
+    fn fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
+        folder.fold_mir_const(self)
+    }
+
     fn super_fold_with<F: TypeFolder<'tcx>>(self, folder: &mut F) -> Self {
         match self {
             ConstantKind::Ty(c) => ConstantKind::Ty(c.fold_with(folder)),

compiler/rustc_middle/src/query/mod.rs

@@ -1486,6 +1486,13 @@ rustc_queries! {
         desc { "normalizing `{}`", goal.value }
     }
 
+    /// Do not call this query directly: invoke `normalize_erasing_regions` instead.
+    query normalize_mir_const_after_erasing_regions(
+        goal: ParamEnvAnd<'tcx, mir::ConstantKind<'tcx>>
+    ) -> mir::ConstantKind<'tcx> {
+        desc { "normalizing `{}`", goal.value }
+    }
+
     query implied_outlives_bounds(
         goal: CanonicalTyGoal<'tcx>
     ) -> Result<

compiler/rustc_middle/src/ty/erase_regions.rs

@@ -1,3 +1,4 @@
+use crate::mir;
 use crate::ty::fold::{TypeFoldable, TypeFolder};
 use crate::ty::{self, Ty, TyCtxt, TypeFlags};
 
@@ -65,4 +66,8 @@ impl TypeFolder<'tcx> for RegionEraserVisitor<'tcx> {
             _ => self.tcx.lifetimes.re_erased,
         }
     }
+
+    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+        c.super_fold_with(self)
+    }
 }

compiler/rustc_middle/src/ty/fold.rs

@@ -30,6 +30,7 @@
 //!
 //! These methods return true to indicate that the visitor has found what it is
 //! looking for, and does not need to visit anything else.
+use crate::mir;
 use crate::ty::{self, flags::FlagComputation, Binder, Ty, TyCtxt, TypeFlags};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -180,6 +181,10 @@ pub trait TypeFolder<'tcx>: Sized {
     fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
         c.super_fold_with(self)
     }
+
+    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+        bug!("most type folders should not be folding MIR datastructures: {:?}", c)
+    }
 }
 
 pub trait TypeVisitor<'tcx>: Sized {

compiler/rustc_middle/src/ty/instance.rs

@@ -483,6 +483,7 @@ impl<'tcx> Instance<'tcx> {
         if let Some(substs) = self.substs_for_mir_body() { v.subst(tcx, substs) } else { *v }
     }
 
+    #[inline(always)]
     pub fn subst_mir_and_normalize_erasing_regions<T>(
         &self,
         tcx: TyCtxt<'tcx>,

compiler/rustc_middle/src/ty/normalize_erasing_regions.rs

@@ -7,6 +7,7 @@
 //! `normalize_generic_arg_after_erasing_regions` query for each type
 //! or constant found within. (This underlying query is what is cached.)
 
+use crate::mir;
 use crate::ty::fold::{TypeFoldable, TypeFolder};
 use crate::ty::subst::{Subst, SubstsRef};
 use crate::ty::{self, Ty, TyCtxt};
@@ -101,4 +102,10 @@ impl TypeFolder<'tcx> for NormalizeAfterErasingRegionsFolder<'tcx> {
         let arg = self.param_env.and(c.into());
         self.tcx.normalize_generic_arg_after_erasing_regions(arg).expect_const()
     }
+
+    #[inline]
+    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+        let arg = self.param_env.and(c);
+        self.tcx.normalize_mir_const_after_erasing_regions(arg)
+    }
 }

compiler/rustc_middle/src/ty/subst.rs

@@ -1,5 +1,6 @@
 // Type substitutions.
 
+use crate::mir;
 use crate::ty::codec::{TyDecoder, TyEncoder};
 use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
 use crate::ty::sty::{ClosureSubsts, GeneratorSubsts};
@@ -506,6 +507,11 @@ impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
             c.super_fold_with(self)
         }
     }
+
+    #[inline]
+    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
+        c.super_fold_with(self)
+    }
 }
 
 impl<'a, 'tcx> SubstFolder<'a, 'tcx> {