fix: Preserve error order when deduplicating errors

Previously when deduplicating errors, the errors were first sorted and
then deduplicated. This lost the ordering of errors which can be useful
for understanding why a value is not compatible with a declaration.

Signed-off-by: Graham Dennis <[email protected]>

Author: GrahamDennis

Diff for: cue/errors/errors.go

@@ -396,9 +396,7 @@ func (p list) sanitize() list {
 	if p == nil {
 		return p
 	}
-	a := slices.Clone(p)
-	a.RemoveMultiples()
-	return a
+	return p.RemoveMultiples()
 }
 
 // Sort sorts an List. *posError entries are sorted by position,
@@ -417,19 +415,22 @@ func (p list) Sort() {
 	})
 }
 
-// RemoveMultiples sorts an List and removes all but the first error per line.
-func (p *list) RemoveMultiples() {
-	p.Sort()
-	var last Error
-	i := 0
-	for _, e := range *p {
-		if last == nil || !approximateEqual(last, e) {
-			last = e
-			(*p)[i] = e
-			i++
+// RemoveMultiples removes all but the first instance of an error message per line.
+func (p list) RemoveMultiples() list {
+	deduplicated := slices.Clone(p)
+
+	deduplicatedIdx := 0
+OUTER:
+	for _, element := range p {
+		for _, deduplicatedElement := range deduplicated[0:deduplicatedIdx] {
+			if approximateEqual(deduplicatedElement, element) {
+				continue OUTER
+			}
 		}
+		deduplicated[deduplicatedIdx] = element
+		deduplicatedIdx++
 	}
-	(*p) = (*p)[0:i]
+	return deduplicated[0:deduplicatedIdx]
 }
 
 func approximateEqual(a, b Error) bool {