From 212f0477f29ec27a3981a916de432fc9ef872ff3 Mon Sep 17 00:00:00 2001
From: Florian Diebold <flodiebold@gmail.com>
Date: Sat, 15 May 2021 19:28:58 +0200
Subject: Make diverging type variables work again

Chalk doesn't know about the `diverging` flag, so we need to instead
propagate it before fully resolving the variables.
---
 crates/hir_ty/src/infer.rs       |  3 +++
 crates/hir_ty/src/infer/unify.rs | 20 ++++++++++++++++++++
 2 files changed, 23 insertions(+)

(limited to 'crates')

diff --git a/crates/hir_ty/src/infer.rs b/crates/hir_ty/src/infer.rs
index f3cccca68..7898740be 100644
--- a/crates/hir_ty/src/infer.rs
+++ b/crates/hir_ty/src/infer.rs
@@ -284,6 +284,9 @@ impl<'a> InferenceContext<'a> {
 
     fn resolve_all(mut self) -> InferenceResult {
         // FIXME resolve obligations as well (use Guidance if necessary)
+
+        // make sure diverging type variables are marked as such
+        self.table.propagate_diverging_flag();
         let mut result = std::mem::take(&mut self.result);
         for ty in result.type_of_expr.values_mut() {
             let resolved = self.table.resolve_ty_completely(ty.clone());
diff --git a/crates/hir_ty/src/infer/unify.rs b/crates/hir_ty/src/infer/unify.rs
index e7718aa22..56e06b7d0 100644
--- a/crates/hir_ty/src/infer/unify.rs
+++ b/crates/hir_ty/src/infer/unify.rs
@@ -179,8 +179,28 @@ impl<'a> InferenceTable<'a> {
         }
     }
 
+    /// Chalk doesn't know about the `diverging` flag, so when it unifies two
+    /// type variables of which one is diverging, the chosen root might not be
+    /// diverging and we have no way of marking it as such at that time. This
+    /// function goes through all type variables and make sure their root is
+    /// marked as diverging if necessary, so that resolving them gives the right
+    /// result.
+    pub(super) fn propagate_diverging_flag(&mut self) {
+        for i in 0..self.type_variable_table.inner.len() {
+            if !self.type_variable_table.inner[i].diverging {
+                continue;
+            }
+            let v = InferenceVar::from(i as u32);
+            let root = self.var_unification_table.inference_var_root(v);
+            if let Some(data) = self.type_variable_table.inner.get_mut(root.index() as usize) {
+                data.diverging = true;
+            }
+        }
+    }
+
     fn new_var(&mut self, kind: TyVariableKind, diverging: bool) -> Ty {
         let var = self.var_unification_table.new_variable(UniverseIndex::ROOT);
+        // Chalk might have created some type variables for its own purposes that we don't know about...
         self.type_variable_table.inner.extend(
             (0..1 + var.index() as usize - self.type_variable_table.inner.len())
                 .map(|_| TypeVariableData { diverging: false }),
-- 
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