Improve error message on duplicate non-mergeable fields

dhall/src/operations/typecheck.rs

@@ -17,23 +17,20 @@ fn check_rectymerge(
     x: Nir<'_>,
     y: Nir<'_>,
 ) -> Result<(), TypeError> {
+    let not_record_err = || match span {
+        Span::DuplicateRecordFieldsSugar(_, r) => {
+            mk_span_err((**r).clone(), "DuplicateFieldName")
+        }
+        _ => mk_span_err(span.clone(), "RecordTypeMergeRequiresRecordType"),
+    };
+
     let kts_x = match x.kind() {
         NirKind::RecordType(kts) => kts,
-        _ => {
-            return mk_span_err(
-                span.clone(),
-                "RecordTypeMergeRequiresRecordType",
-            )
-        }
+        _ => return not_record_err(),
     };
     let kts_y = match y.kind() {
         NirKind::RecordType(kts) => kts,
-        _ => {
-            return mk_span_err(
-                span.clone(),
-                "RecordTypeMergeRequiresRecordType",
-            )
-        }
+        _ => return not_record_err(),
     };
     for (k, tx) in kts_x {
         if let Some(ty) = kts_y.get(k) {

dhall/src/syntax/ast/span.rs

@@ -19,7 +19,7 @@ pub enum Span {
     /// A location in the source text
     Parsed(ParsedSpan),
     /// Desugarings
-    DuplicateRecordFieldsSugar,
+    DuplicateRecordFieldsSugar(Box<Span>, Box<Span>),
     DottedFieldSugar,
     RecordPunSugar,
     /// For expressions obtained from decoding binary

dhall/src/syntax/text/parser.rs

@@ -97,9 +97,13 @@ fn insert_recordlit_entry(map: &mut BTreeMap<Label, Expr>, l: Label, e: Expr) {
         Entry::Occupied(mut entry) => {
             let dummy = Expr::new(Num(Bool(false)), Span::Artificial);
             let other = entry.insert(dummy);
+            let span = Span::DuplicateRecordFieldsSugar(
+                Box::new(other.span()),
+                Box::new(e.span()),
+            );
             entry.insert(Expr::new(
                 Op(BinOp(RecursiveRecordMerge, other, e)),
-                Span::DuplicateRecordFieldsSugar,
+                span,
             ));
         }
     }
@@ -133,11 +137,11 @@ lazy_static::lazy_static! {
     };
 }
 
-// Generate pest parser manually becaue otherwise we'd need to modify something outside of OUT_DIR
+// Generate pest parser manually because otherwise we'd need to modify something outside of OUT_DIR
 // and that's forbidden by docs.rs.
 // This is equivalent to:
 // ```
-// #[derive(Parser)
+// #[derive(Parser)]
 // #[grammar = "..."]
 // struct DhallParser;
 // ```

dhall/tests/type-inference/failure/unit/RecordLitDuplicateFieldsAbstract.txt

@@ -1 +1,12 @@
-Type error: error: RecordTypeMergeRequiresRecordType
+Type error: error: DuplicateFieldName
+  --> <current file>:6:47
+   |
+ 1 | {-  This test illustrates that duplicate fields need not be literals in order
+ 2 |     to be properly normalized.  One or both of the duplicate fields can be
+ 3 |     abstract because field duplication delegates its behavior to the `∧`
+ 4 |     operator
+...
+12 | -}
+13 | λ(r : { y : Natural }) → { x = { y = 1 }, x = r }
+   |                                               ^ DuplicateFieldName
+   |

dhall/tests/type-inference/failure/unit/RecordLitDuplicateFieldsCollidingRecords.txt

@@ -1 +1,7 @@
-Type error: error: RecordTypeMergeRequiresRecordType
+Type error: error: DuplicateFieldName
+ --> <current file>:1:22
+  |
+...
+8 | { x = { y = 0 }, x = { y = 0 } }
+  |                      ^^^^^^^^^ DuplicateFieldName
+  |

dhall/tests/type-inference/failure/unit/RecordLitDuplicateFieldsNotRecords.txt

@@ -1 +1,7 @@
-Type error: error: RecordTypeMergeRequiresRecordType
+Type error: error: DuplicateFieldName
+ --> <current file>:1:14
+  |
+...
+7 | { x = 0, x = 0 }
+  |              ^ DuplicateFieldName
+  |