[fine] Working on parser resilience

fine/src/parser.rs

@@ -395,6 +395,15 @@ impl<'a> CParser<'a> {
     //     eprintln!("{}: {}: {}", self.current.start, self.current, msg);
     // }
 
+    fn at_any(&self, kinds: &[TokenKind]) -> bool {
+        for kind in kinds {
+            if self.at(*kind) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     fn at(&self, kind: TokenKind) -> bool {
         self.peek() == kind
     }
@@ -586,6 +595,8 @@ fn field_decl(p: &mut CParser) {
     p.end(m, TreeKind::FieldDecl);
 }
 
+const PARAM_LIST_RECOVERY: &[TokenKind] = &[TokenKind::Arrow, TokenKind::LeftBrace, TokenKind::Fun];
+
 fn param_list(p: &mut CParser) {
     let m = p.start();
 
@@ -594,7 +605,10 @@ fn param_list(p: &mut CParser) {
         if p.at(TokenKind::Identifier) {
             parameter(p);
         } else {
-            break;
+            if p.at_any(PARAM_LIST_RECOVERY) {
+                break;
+            }
+            p.advance_with_error("expected parameter");
         }
     }
     p.expect(TokenKind::RightParen, "expect ')' to end a parameter list");

fine/tests/errors/resilience/README.md

@@ -0,0 +1,4 @@
+These tests all produce errors in their parse, but the point is that
+the trees are kinda as best as we can get.
+
+See e.g. https://matklad.github.io/2023/05/21/resilient-ll-parsing-tutorial.html

fine/tests/errors/resilience/function_extra_comma.fine

@@ -0,0 +1,53 @@
+fun f1(x: f64,
+
+fun f2(x: f64,, z: f64) {}
+
+fun f3() {}
+
+// @concrete:
+// | File
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"f1"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       Parameter
+// |         Identifier:'"x"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |         Comma:'","'
+// |       Error:'"Error at 'fun': expect ')' to end a parameter list"'
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"f2"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       Parameter
+// |         Identifier:'"x"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |         Comma:'","'
+// |       Error
+// |         Error:'"Error at ',': expected parameter"'
+// |         Comma:'","'
+// |       Parameter
+// |         Identifier:'"z"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |       RightParen:'")"'
+// |     Block
+// |       LeftBrace:'"{"'
+// |       RightBrace:'"}"'
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"f3"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       RightParen:'")"'
+// |     Block
+// |       LeftBrace:'"{"'
+// |       RightBrace:'"}"'
+//

fine/tests/errors/resilience/incomplete_function.fine

@@ -0,0 +1,57 @@
+fun fib_rec(f1: f64,
+
+fun fib(n: f64) -> f64 {
+  fib_rec(1, 1, n)
+}
+
+// @concrete:
+// | File
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"fib_rec"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       Parameter
+// |         Identifier:'"f1"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |         Comma:'","'
+// |       Error:'"Error at 'fun': expect ')' to end a parameter list"'
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"fib"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       Parameter
+// |         Identifier:'"n"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |       RightParen:'")"'
+// |     ReturnType
+// |       Arrow:'"->"'
+// |       TypeExpression
+// |         Identifier:'"f64"'
+// |     Block
+// |       LeftBrace:'"{"'
+// |       ExpressionStatement
+// |         CallExpression
+// |           Identifier
+// |             Identifier:'"fib_rec"'
+// |           ArgumentList
+// |             LeftParen:'"("'
+// |             Argument
+// |               LiteralExpression
+// |                 Number:'"1"'
+// |               Comma:'","'
+// |             Argument
+// |               LiteralExpression
+// |                 Number:'"1"'
+// |               Comma:'","'
+// |             Argument
+// |               Identifier
+// |                 Identifier:'"n"'
+// |             RightParen:'")"'
+// |       RightBrace:'"}"'
+//