From e77e53f37b5bff99e8b427ef8ac11a49b7802aa2 Mon Sep 17 00:00:00 2001
From: Ayaz Hafiz <ayaz@rwx.com>
Date: Tue, 9 Aug 2022 08:48:21 -0700
Subject: [PATCH] Enable optional record field underivable error

---
 crates/compiler/solve/src/ability.rs | 27 ++++++++++++++-------------
 crates/reporting/src/error/type.rs   |  4 +++-
 2 files changed, 17 insertions(+), 14 deletions(-)

diff --git a/crates/compiler/solve/src/ability.rs b/crates/compiler/solve/src/ability.rs
index bc4bfdb794..f63ebd414c 100644
--- a/crates/compiler/solve/src/ability.rs
+++ b/crates/compiler/solve/src/ability.rs
@@ -4,7 +4,9 @@ use roc_collections::{VecMap, VecSet};
 use roc_error_macros::{internal_error, todo_abilities};
 use roc_module::symbol::Symbol;
 use roc_region::all::{Loc, Region};
-use roc_solve_problem::{NotDerivableContext, TypeError, UnderivableReason, Unfulfilled};
+use roc_solve_problem::{
+    NotDerivableContext, NotDerivableDecode, TypeError, UnderivableReason, Unfulfilled,
+};
 use roc_types::num::NumericRange;
 use roc_types::subs::{
     instantiate_rigids, Content, FlatType, GetSubsSlice, Rank, RecordFields, Subs, Variable,
@@ -837,19 +839,18 @@ impl DerivableVisitor for DeriveDecoding {
         var: Variable,
         fields: RecordFields,
     ) -> Result<Descend, NotDerivable> {
-        let has_optional_field = subs
-            .get_subs_slice(fields.record_fields())
-            .iter()
-            .any(|field| matches!(field, RecordField::Optional(..)));
-
-        if has_optional_field {
-            Err(NotDerivable {
-                var,
-                context: NotDerivableContext::NoContext,
-            })
-        } else {
-            Ok(Descend(true))
+        for (field_name, _, field) in fields.iter_all() {
+            if matches!(subs[field], RecordField::Optional(..)) {
+                return Err(NotDerivable {
+                    var,
+                    context: NotDerivableContext::Decode(NotDerivableDecode::OptionalRecordField(
+                        subs[field_name].clone(),
+                    )),
+                });
+            }
         }
+
+        Ok(Descend(true))
     }
 
     #[inline(always)]
diff --git a/crates/reporting/src/error/type.rs b/crates/reporting/src/error/type.rs
index e51aea74a9..5f47d40762 100644
--- a/crates/reporting/src/error/type.rs
+++ b/crates/reporting/src/error/type.rs
@@ -425,7 +425,9 @@ fn underivable_hint<'b>(
                     alloc.reflow("Roc cannot derive decoding for a record with an optional field, which in this case is "),
                     alloc.record_field(field),
                     alloc.reflow(". Optional record fields are polymorphic over records that may or may not contain them at compile time, "),
-                    alloc.reflow("but are not a concept that extends to runtime! That means Roc cannot derive a decoder for a record with an optional value "),
+                    alloc.reflow("but are not a concept that extends to runtime!"),
+                    alloc.hardline(),
+                    alloc.reflow("That means Roc cannot derive a decoder for a record with an optional value "),
                     alloc.reflow("by way of an optional record field. If you want to model the idea that a field may or may not be present at runtime, "),
                     alloc.reflow("consider using a "),
                     alloc.symbol_unqualified(Symbol::RESULT_RESULT),