feat(exhaustiveness): show both clauses in redundant error

crates/aiken-lang/src/tipo/environment.rs

@@ -1434,8 +1434,8 @@ impl<'a> Environment<'a> {
     }
 
     /// Checks that the given patterns are exhaustive for given type.
-    /// Currently only performs exhaustiveness checking for custom types,
-    /// only at the top level (without recursing into constructor arguments).
+    /// https://github.com/elm/compiler/blob/047d5026fe6547c842db65f7196fed3f0b4743ee/compiler/src/Nitpick/PatternMatches.hs#L397-L475
+    /// http://moscova.inria.fr/~maranget/papers/warn/index.html
     pub fn check_exhaustiveness(
         &mut self,
         unchecked_patterns: &[&TypedPattern],
@@ -1451,8 +1451,19 @@ impl<'a> Environment<'a> {
             if matrix.is_useful(&pattern_stack) {
                 matrix.push(pattern_stack);
             } else {
+                let index = matrix
+                    .flatten()
+                    .into_iter()
+                    .enumerate()
+                    .find(|(_, p)| p == pattern_stack.head())
+                    .map(|(i, _)| i)
+                    .expect("should find index");
+
+                let typed_pattern = unchecked_patterns[index];
+
                 return Err(Error::RedundantMatchClause {
-                    location: unchecked_pattern.location(),
+                    original: typed_pattern.location(),
+                    redundant: unchecked_pattern.location(),
                 });
             }
         }

crates/aiken-lang/src/tipo/error.rs

@@ -543,17 +543,19 @@ Maybe you meant to turn it public using the '{keyword_pub}' keyword?"#
     #[error(
         "{}\n",
         format!(
-            "I discovered a '{keyword_when}/{keyword_is}' expression with a redundant clause.",
+            "I discovered a '{keyword_when}/{keyword_is}' expression with a redundant pattern.",
             keyword_is = "is".if_supports_color(Stdout, |s| s.purple()),
             keyword_when = "when".if_supports_color(Stdout, |s| s.purple())
         )
     )]
     #[diagnostic(url("https://aiken-lang.org/language-tour/control-flow#matching"))]
     #[diagnostic(code("redundant_pattern_match"))]
-    #[diagnostic(help("You can safely remove this clause."))]
+    #[diagnostic(help("Double check these patterns and then remove one of the clauses."))]
     RedundantMatchClause {
-        #[label]
-        location: Span,
+        #[label("first found here")]
+        original: Span,
+        #[label("redundant")]
+        redundant: Span,
     },
 
     #[error("I couldn't figure out the type of a record you're trying to access.\n")]

crates/aiken-lang/src/tipo/exhaustive.rs

@@ -41,7 +41,7 @@ impl PatternStack {
         self.0.insert(index, element);
     }
 
-    fn head(&self) -> &Pattern {
+    pub(super) fn head(&self) -> &Pattern {
         &self.0[0]
     }
 
@@ -385,7 +385,7 @@ pub(crate) enum Complete {
     No,
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq)]
 pub(crate) enum Pattern {
     Wildcard,
     Literal(Literal),