Don't compress input universes

Author: compiler-errors

compiler/rustc_next_trait_solver/src/canonicalizer.rs

@@ -1,5 +1,3 @@
-use std::cmp::Ordering;
-
 use rustc_type_ir::data_structures::{HashMap, ensure_sufficient_stack};
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::solve::{Goal, QueryInput};
@@ -266,11 +264,11 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
         // See the rustc-dev-guide section about how we deal with universes
         // during canonicalization in the new solver.
         match self.canonicalize_mode {
-            // We try to deduplicate as many query calls as possible and hide
-            // all information which should not matter for the solver.
-            //
-            // For this we compress universes as much as possible.
-            CanonicalizeMode::Input { .. } => {}
+            // All placeholders and vars are canonicalized in the root universe.
+            CanonicalizeMode::Input { .. } => {
+                let var_kinds = self.delegate.cx().mk_canonical_var_kinds(&var_kinds);
+                (ty::UniverseIndex::ROOT, var_kinds)
+            }
             // When canonicalizing a response we map a universes already entered
             // by the caller to the root universe and only return useful universe
             // information for placeholders and inference variables created inside
@@ -288,113 +286,10 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
                     .map(|kind| kind.universe())
                     .max()
                     .unwrap_or(ty::UniverseIndex::ROOT);
-
                 let var_kinds = self.delegate.cx().mk_canonical_var_kinds(&var_kinds);
-                return (max_universe, var_kinds);
-            }
-        }
-
-        // Given a `var_kinds` with existentials `En` and universals `Un` in
-        // universes `n`, this algorithm compresses them in place so that:
-        //
-        // - the new universe indices are as small as possible
-        // - we create a new universe if we would otherwise
-        //   1. put existentials from a different universe into the same one
-        //   2. put a placeholder in the same universe as an existential which cannot name it
-        //
-        // Let's walk through an example:
-        // - var_kinds: [E0, U1, E5, U2, E2, E6, U6], curr_compressed_uv: 0, next_orig_uv: 0
-        // - var_kinds: [E0, U1, E5, U2, E2, E6, U6], curr_compressed_uv: 0, next_orig_uv: 1
-        // - var_kinds: [E0, U1, E5, U2, E2, E6, U6], curr_compressed_uv: 1, next_orig_uv: 2
-        // - var_kinds: [E0, U1, E5, U1, E1, E6, U6], curr_compressed_uv: 1, next_orig_uv: 5
-        // - var_kinds: [E0, U1, E2, U1, E1, E6, U6], curr_compressed_uv: 2, next_orig_uv: 6
-        // - var_kinds: [E0, U1, E1, U1, E1, E3, U3], curr_compressed_uv: 2, next_orig_uv: -
-        //
-        // This algorithm runs in `O(mn)` where `n` is the number of different universes and
-        // `m` the number of variables. This should be fine as both are expected to be small.
-        let mut curr_compressed_uv = ty::UniverseIndex::ROOT;
-        let mut existential_in_new_uv = None;
-        let mut next_orig_uv = Some(ty::UniverseIndex::ROOT);
-        while let Some(orig_uv) = next_orig_uv.take() {
-            let mut update_uv = |var: &mut CanonicalVarKind<I>, orig_uv, is_existential| {
-                let uv = var.universe();
-                match uv.cmp(&orig_uv) {
-                    Ordering::Less => (), // Already updated
-                    Ordering::Equal => {
-                        if is_existential {
-                            if existential_in_new_uv.is_some_and(|uv| uv < orig_uv) {
-                                // Condition 1.
-                                //
-                                // We already put an existential from a outer universe
-                                // into the current compressed universe, so we need to
-                                // create a new one.
-                                curr_compressed_uv = curr_compressed_uv.next_universe();
-                            }
-
-                            // `curr_compressed_uv` will now contain an existential from
-                            // `orig_uv`. Trying to canonicalizing an existential from
-                            // a higher universe has to therefore use a new compressed
-                            // universe.
-                            existential_in_new_uv = Some(orig_uv);
-                        } else if existential_in_new_uv.is_some() {
-                            // Condition 2.
-                            //
-                            //  `var` is a placeholder from a universe which is not nameable
-                            // by an existential which we already put into the compressed
-                            // universe `curr_compressed_uv`. We therefore have to create a
-                            // new universe for `var`.
-                            curr_compressed_uv = curr_compressed_uv.next_universe();
-                            existential_in_new_uv = None;
-                        }
-
-                        *var = var.with_updated_universe(curr_compressed_uv);
-                    }
-                    Ordering::Greater => {
-                        // We can ignore this variable in this iteration. We only look at
-                        // universes which actually occur in the input for performance.
-                        //
-                        // For this we set `next_orig_uv` to the next smallest, not yet compressed,
-                        // universe of the input.
-                        if next_orig_uv.is_none_or(|curr_next_uv| uv.cannot_name(curr_next_uv)) {
-                            next_orig_uv = Some(uv);
-                        }
-                    }
-                }
-            };
-
-            // For each universe which occurs in the input, we first iterate over all
-            // placeholders and then over all inference variables.
-            //
-            // Whenever we compress the universe of a placeholder, no existential with
-            // an already compressed universe can name that placeholder.
-            for is_existential in [false, true] {
-                for var in var_kinds.iter_mut() {
-                    // We simply put all regions from the input into the highest
-                    // compressed universe, so we only deal with them at the end.
-                    if !var.is_region() {
-                        if is_existential == var.is_existential() {
-                            update_uv(var, orig_uv, is_existential)
-                        }
-                    }
-                }
-            }
-        }
-
-        // We put all regions into a separate universe.
-        let mut first_region = true;
-        for var in var_kinds.iter_mut() {
-            if var.is_region() {
-                if first_region {
-                    first_region = false;
-                    curr_compressed_uv = curr_compressed_uv.next_universe();
-                }
-                debug_assert!(var.is_existential());
-                *var = var.with_updated_universe(curr_compressed_uv);
+                (max_universe, var_kinds)
             }
         }
-
-        let var_kinds = self.delegate.cx().mk_canonical_var_kinds(&var_kinds);
-        (curr_compressed_uv, var_kinds)
     }
 
     fn cached_fold_ty(&mut self, t: I::Ty) -> I::Ty {