Auto merge of #59008 - varkor:const-generics-infer, r=eddyb

Add const generics to infer (and transitive dependencies)

Split out from #53645. This work is a collaborative effort with @yodaldevoid.

There are a number of stubs. These are mainly to ensure we don't overlook them when completing the implementation, but are not necessary for the initial implementation. We plan to address these in follow up PRs.

r? @eddyb / @nikomatsakis

src/librustc/infer/canonical/canonicalizer.rs

@@ -10,10 +10,12 @@ use crate::infer::canonical::{
     OriginalQueryValues,
 };
 use crate::infer::InferCtxt;
+use crate::mir::interpret::ConstValue;
 use std::sync::atomic::Ordering;
 use crate::ty::fold::{TypeFoldable, TypeFolder};
 use crate::ty::subst::Kind;
-use crate::ty::{self, BoundVar, Lift, List, Ty, TyCtxt, TypeFlags};
+use crate::ty::{self, BoundVar, InferConst, Lift, List, Ty, TyCtxt, TypeFlags};
+use crate::ty::flags::FlagComputation;
 
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::indexed_vec::Idx;
@@ -432,6 +434,61 @@ impl<'cx, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for Canonicalizer<'cx, 'gcx, 'tcx>
             }
         }
     }
+
+    fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
+        match ct.val {
+            ConstValue::Infer(InferConst::Var(vid)) => {
+                debug!("canonical: const var found with vid {:?}", vid);
+                match self.infcx.unwrap().probe_const_var(vid) {
+                    Ok(c) => {
+                        debug!("(resolved to {:?})", c);
+                        return self.fold_const(c);
+                    }
+
+                    // `ConstVar(vid)` is unresolved, track its universe index in the
+                    // canonicalized result
+                    Err(mut ui) => {
+                        if !self.infcx.unwrap().tcx.sess.opts.debugging_opts.chalk {
+                            // FIXME: perf problem described in #55921.
+                            ui = ty::UniverseIndex::ROOT;
+                        }
+                        return self.canonicalize_const_var(
+                            CanonicalVarInfo {
+                                kind: CanonicalVarKind::Const(ui),
+                            },
+                            ct,
+                        );
+                    }
+                }
+            }
+            ConstValue::Infer(InferConst::Fresh(_)) => {
+                bug!("encountered a fresh const during canonicalization")
+            }
+            ConstValue::Infer(InferConst::Canonical(debruijn, _)) => {
+                if debruijn >= self.binder_index {
+                    bug!("escaping bound type during canonicalization")
+                } else {
+                    return ct;
+                }
+            }
+            ConstValue::Placeholder(placeholder) => {
+                return self.canonicalize_const_var(
+                    CanonicalVarInfo {
+                        kind: CanonicalVarKind::PlaceholderConst(placeholder),
+                    },
+                    ct,
+                );
+            }
+            _ => {}
+        }
+
+        let flags = FlagComputation::for_const(ct);
+        if flags.intersects(self.needs_canonical_flags) {
+            ct.super_fold_with(self)
+        } else {
+            ct
+        }
+    }
 }
 
 impl<'cx, 'gcx, 'tcx> Canonicalizer<'cx, 'gcx, 'tcx> {
@@ -450,11 +507,13 @@ impl<'cx, 'gcx, 'tcx> Canonicalizer<'cx, 'gcx, 'tcx> {
         let needs_canonical_flags = if canonicalize_region_mode.any() {
             TypeFlags::KEEP_IN_LOCAL_TCX |
             TypeFlags::HAS_FREE_REGIONS | // `HAS_RE_PLACEHOLDER` implies `HAS_FREE_REGIONS`
-            TypeFlags::HAS_TY_PLACEHOLDER
+            TypeFlags::HAS_TY_PLACEHOLDER |
+            TypeFlags::HAS_CT_PLACEHOLDER
         } else {
             TypeFlags::KEEP_IN_LOCAL_TCX |
             TypeFlags::HAS_RE_PLACEHOLDER |
-            TypeFlags::HAS_TY_PLACEHOLDER
+            TypeFlags::HAS_TY_PLACEHOLDER |
+            TypeFlags::HAS_CT_PLACEHOLDER
         };
 
         let gcx = tcx.global_tcx();
@@ -633,4 +692,28 @@ impl<'cx, 'gcx, 'tcx> Canonicalizer<'cx, 'gcx, 'tcx> {
             self.tcx().mk_ty(ty::Bound(self.binder_index, var.into()))
         }
     }
+
+    /// Given a type variable `const_var` of the given kind, first check
+    /// if `const_var` is bound to anything; if so, canonicalize
+    /// *that*. Otherwise, create a new canonical variable for
+    /// `const_var`.
+    fn canonicalize_const_var(
+        &mut self,
+        info: CanonicalVarInfo,
+        const_var: &'tcx ty::Const<'tcx>
+    ) -> &'tcx ty::Const<'tcx> {
+        let infcx = self.infcx.expect("encountered const-var without infcx");
+        let bound_to = infcx.resolve_const_var(const_var);
+        if bound_to != const_var {
+            self.fold_const(bound_to)
+        } else {
+            let var = self.canonical_var(info, const_var.into());
+            self.tcx().mk_const(
+                ty::Const {
+                    val: ConstValue::Infer(InferConst::Canonical(self.binder_index, var.into())),
+                    ty: const_var.ty,
+                }
+            )
+        }
+    }
 }

src/librustc/infer/canonical/mod.rs

@@ -21,7 +21,8 @@
 //!
 //! [c]: https://rust-lang.github.io/rustc-guide/traits/canonicalization.html
 
-use crate::infer::{InferCtxt, RegionVariableOrigin, TypeVariableOrigin};
+use crate::infer::{InferCtxt, RegionVariableOrigin, TypeVariableOrigin, ConstVariableOrigin};
+use crate::mir::interpret::ConstValue;
 use rustc_data_structures::indexed_vec::IndexVec;
 use rustc_macros::HashStable;
 use serialize::UseSpecializedDecodable;
@@ -30,7 +31,7 @@ use std::ops::Index;
 use syntax::source_map::Span;
 use crate::ty::fold::TypeFoldable;
 use crate::ty::subst::Kind;
-use crate::ty::{self, BoundVar, Lift, List, Region, TyCtxt};
+use crate::ty::{self, BoundVar, InferConst, Lift, List, Region, TyCtxt};
 
 mod canonicalizer;
 
@@ -115,6 +116,8 @@ impl CanonicalVarInfo {
             CanonicalVarKind::PlaceholderTy(_) => false,
             CanonicalVarKind::Region(_) => true,
             CanonicalVarKind::PlaceholderRegion(..) => false,
+            CanonicalVarKind::Const(_) => true,
+            CanonicalVarKind::PlaceholderConst(_) => false,
         }
     }
 }
@@ -137,6 +140,12 @@ pub enum CanonicalVarKind {
     /// are solving a goal like `for<'a> T: Foo<'a>` to represent the
     /// bound region `'a`.
     PlaceholderRegion(ty::PlaceholderRegion),
+
+    /// Some kind of const inference variable.
+    Const(ty::UniverseIndex),
+
+    /// A "placeholder" that represents "any const".
+    PlaceholderConst(ty::PlaceholderConst),
 }
 
 impl CanonicalVarKind {
@@ -150,6 +159,8 @@ impl CanonicalVarKind {
             CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.universe,
             CanonicalVarKind::Region(ui) => ui,
             CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.universe,
+            CanonicalVarKind::Const(ui) => ui,
+            CanonicalVarKind::PlaceholderConst(placeholder) => placeholder.universe,
         }
     }
 }
@@ -388,6 +399,33 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
                 };
                 self.tcx.mk_region(ty::RePlaceholder(placeholder_mapped)).into()
             }
+
+            CanonicalVarKind::Const(ui) => {
+                self.next_const_var_in_universe(
+                    self.next_ty_var_in_universe(
+                        TypeVariableOrigin::MiscVariable(span),
+                        universe_map(ui),
+                    ),
+                    ConstVariableOrigin::MiscVariable(span),
+                    universe_map(ui),
+                ).into()
+            }
+
+            CanonicalVarKind::PlaceholderConst(
+                ty::PlaceholderConst { universe, name },
+            ) => {
+                let universe_mapped = universe_map(universe);
+                let placeholder_mapped = ty::PlaceholderConst {
+                    universe: universe_mapped,
+                    name,
+                };
+                self.tcx.mk_const(
+                    ty::Const {
+                        val: ConstValue::Placeholder(placeholder_mapped),
+                        ty: self.tcx.types.err, // FIXME(const_generics)
+                    }
+                ).into()
+            }
         }
     }
 }
@@ -443,8 +481,13 @@ impl<'tcx> CanonicalVarValues<'tcx> {
                     UnpackedKind::Lifetime(..) => tcx.mk_region(
                         ty::ReLateBound(ty::INNERMOST, ty::BoundRegion::BrAnon(i))
                     ).into(),
-                    UnpackedKind::Const(..) => {
-                        unimplemented!() // FIXME(const_generics)
+                    UnpackedKind::Const(ct) => {
+                        tcx.mk_const(ty::Const {
+                            ty: ct.ty,
+                            val: ConstValue::Infer(
+                                InferConst::Canonical(ty::INNERMOST, ty::BoundVar::from_u32(i))
+                            ),
+                        }).into()
                     }
                 })
                 .collect()

src/librustc/infer/canonical/query_response.rs

@@ -16,6 +16,7 @@ use crate::infer::canonical::{
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
 use crate::infer::InferCtxtBuilder;
 use crate::infer::{InferCtxt, InferOk, InferResult};
+use crate::mir::interpret::ConstValue;
 use rustc_data_structures::indexed_vec::Idx;
 use rustc_data_structures::indexed_vec::IndexVec;
 use std::fmt::Debug;
@@ -25,7 +26,7 @@ use crate::traits::TraitEngine;
 use crate::traits::{Obligation, ObligationCause, PredicateObligation};
 use crate::ty::fold::TypeFoldable;
 use crate::ty::subst::{Kind, UnpackedKind};
-use crate::ty::{self, BoundVar, Lift, Ty, TyCtxt};
+use crate::ty::{self, BoundVar, InferConst, Lift, Ty, TyCtxt};
 use crate::util::captures::Captures;
 
 impl<'cx, 'gcx, 'tcx> InferCtxtBuilder<'cx, 'gcx, 'tcx> {
@@ -479,8 +480,17 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
                         opt_values[br.assert_bound_var()] = Some(*original_value);
                     }
                 }
-                UnpackedKind::Const(..) => {
-                    unimplemented!() // FIXME(const_generics)
+                UnpackedKind::Const(result_value) => {
+                    if let ty::Const {
+                        val: ConstValue::Infer(InferConst::Canonical(debrujin, b)),
+                        ..
+                    } = result_value {
+                        // ...in which case we would set `canonical_vars[0]` to `Some(const X)`.
+
+                        // We only allow a `ty::INNERMOST` index in substitutions.
+                        assert_eq!(*debrujin, ty::INNERMOST);
+                        opt_values[*b] = Some(*original_value);
+                    }
                 }
             }
         }

src/librustc/infer/canonical/substitute.rs

@@ -70,6 +70,13 @@ where
             }
         };
 
-        tcx.replace_escaping_bound_vars(value, fld_r, fld_t).0
+        let fld_c = |bound_ct: ty::BoundVar, _| {
+            match var_values.var_values[bound_ct].unpack() {
+                UnpackedKind::Const(ct) => ct,
+                c => bug!("{:?} is a const but value is {:?}", bound_ct, c),
+            }
+        };
+
+        tcx.replace_escaping_bound_vars(value, fld_r, fld_t, fld_c).0
     }
 }