From 4f8953642a11ac5f994aa369a5450c5f35ff3b7d Mon Sep 17 00:00:00 2001
From: Craig Roy <craig.roy@cambridgequantum.com>
Date: Tue, 29 Aug 2023 16:57:14 +0100
Subject: [PATCH 1/2] feat: Location information in extension inference error

---
 src/extension/infer.rs | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/src/extension/infer.rs b/src/extension/infer.rs
index bae062066..c3ffa0f14 100644
--- a/src/extension/infer.rs
+++ b/src/extension/infer.rs
@@ -76,6 +76,18 @@ pub enum InferExtensionError {
         /// The incompatible solution that we found was already there
         actual: ExtensionSet,
     },
+    #[error("Solved extensions {expected} at {expected_loc:?} and {actual} at {actual_loc:?} should be equal.")]
+    /// A version of the above with info about which nodes failed to unify
+    MismatchedConcreteWithLocations {
+        /// Where the solution we want to insert came from
+        expected_loc: (Node, Direction),
+        /// The solution we were trying to insert for this meta
+        expected: ExtensionSet,
+        /// Which node we're trying to add a solution for
+        actual_loc: (Node, Direction),
+        /// The incompatible solution that we found was already there
+        actual: ExtensionSet,
+    },
     /// A variable went unsolved that wasn't related to a parameter
     #[error("Unsolved variable at location {:?}", location)]
     Unsolved {
@@ -375,10 +387,19 @@ impl UnificationContext {
         } else {
             None
         };
-        err.unwrap_or(InferExtensionError::MismatchedConcrete {
-            expected: rs1,
-            actual: rs2,
-        })
+        if let (Some(loc1), Some(loc2)) = (loc1, loc2) {
+            err.unwrap_or(InferExtensionError::MismatchedConcreteWithLocations {
+                expected_loc: *loc1,
+                expected: rs1,
+                actual_loc: *loc2,
+                actual: rs2,
+            })
+        } else {
+            err.unwrap_or(InferExtensionError::MismatchedConcrete {
+                expected: rs1,
+                actual: rs2,
+            })
+        }
     }
 
     /// Take a group of equal metas and merge them into a new, single meta.

From 5fb19afa769578058b38247bd1df4ae241c4d0b6 Mon Sep 17 00:00:00 2001
From: Craig Roy <craig.roy@cambridgequantum.com>
Date: Tue, 29 Aug 2023 17:14:32 +0100
Subject: [PATCH 2/2] doc: Update function docstring

---
 src/extension/infer.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/extension/infer.rs b/src/extension/infer.rs
index c3ffa0f14..457b463b7 100644
--- a/src/extension/infer.rs
+++ b/src/extension/infer.rs
@@ -326,7 +326,10 @@ impl UnificationContext {
         }
     }
 
-    /// Try to turn mismatches into `ExtensionError` when possible
+    /// When trying to unify two metas, check if they both correspond to
+    /// different ends of the same wire. If so, return an `ExtensionError`.
+    /// Otherwise check whether they both correspond to *some* location on the
+    /// graph and include that info the otherwise generic `MismatchedConcrete`.
     fn report_mismatch(
         &self,
         m1: Meta,