Clarify nonexhaustive match diagnostics

.llm/BETA_4_LANGUAGE_USABILITY.md

@@ -10,10 +10,12 @@ without changing the typed core or claiming new syntax stability.
 - Improve project/workspace build and run entry diagnostics.
 - Keep the accepted entry contract unchanged: `(fn main () -> i32 ...)`.
 - Use precise diagnostic codes for missing and invalid entry `main` functions.
+- Make non-exhaustive `match` diagnostics clearer and deterministic.
 
 ## Acceptance Gates
 
 - `cargo test --test project_mode entry_main`
+- `cargo test --test diagnostics_contract`
 - `cargo fmt --check`
 - `git diff --check`
 

compiler/src/check.rs

@@ -3690,10 +3690,18 @@ fn validate_match_arm_set(
         .collect::<Vec<_>>();
 
     if !missing.is_empty() {
+        let found = required
+            .iter()
+            .filter(|pattern| seen.contains_key(*pattern))
+            .map(lower::match_pattern_name)
+            .collect::<Vec<_>>();
         return Err(Diagnostic::new(
             file,
             "NonExhaustiveMatch",
-            format!("match is missing `{}` arm(s)", missing.join("`, `")),
+            format!(
+                "match is missing required arm(s): `{}`",
+                missing.join("`, `")
+            ),
         )
         .with_span(span)
         .expected(
@@ -3703,12 +3711,11 @@ fn validate_match_arm_set(
                 .collect::<Vec<_>>()
                 .join(" and "),
         )
-        .found(
+        .found(if found.is_empty() {
-            seen.keys()
+            "no valid arms".to_string()
-                .map(lower::match_pattern_name)
+        } else {
-                .collect::<Vec<_>>()
+            found.join(", ")
-                .join(", "),
+        }));
-        ));
     }
 
     Ok(())

docs/compiler/RELEASE_NOTES.md

@@ -15,6 +15,8 @@ integration/readiness release, not the first real beta.
   `WorkspaceEntryMainMissing`, and `WorkspaceEntryMainInvalidSignature`.
   Messages now spell out the required `(fn main () -> i32 ...)` contract and
   include the found parameter count and return type for invalid signatures.
+- Non-exhaustive `match` diagnostics now use clearer missing-arm wording and a
+  deterministic found-arm list.
 
 ## 1.0.0-beta.3
 

tests/enum-match-non-exhaustive.diag

@@ -3,7 +3,7 @@
   (version 1)
   (severity error)
   (code NonExhaustiveMatch)
-  (message "match is missing `Color.Blue` arm(s)")
+  (message "match is missing required arm(s): `Color.Blue`")
   (file "<fixture>")
   (span
     (bytes 60 99)

tests/non-exhaustive-match.diag

@@ -3,7 +3,7 @@
   (version 1)
   (severity error)
   (code NonExhaustiveMatch)
-  (message "match is missing `none` arm(s)")
+  (message "match is missing required arm(s): `none`")
   (file "<fixture>")
   (span
     (bytes 57 105)