feat: add record spread syntax { ...base, field: val }

Adds spread operator for records, allowing concise record updates:
  let p2 = { ...p, x: 5.0 }

Changes across the full pipeline:
- Lexer: new DotDotDot (...) token
- AST: optional spread field on Record variant
- Parser: detect ... at start of record expression
- Typechecker: merge spread record fields with explicit overrides
- Interpreter: evaluate spread, overlay explicit fields
- JS backend: emit native JS spread syntax
- C backend: copy spread into temp, assign overrides
- Formatter, linter, LSP, symbol table: propagate spread

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ast.rs

@@ -541,7 +541,9 @@ pub enum Expr {
         span: Span,
     },
     /// Record literal: { name: "Alice", age: 30 }
+    /// With optional spread: { ...base, name: "Bob" }
     Record {
+        spread: Option<Box<Expr>>,
         fields: Vec<(Ident, Expr)>,
         span: Span,
     },

src/formatter.rs

@@ -688,15 +688,17 @@ impl Formatter {
                         .join(", ")
                 )
             }
-            Expr::Record { fields, .. } => {
+            Expr::Record {
-                format!(
+                spread, fields, ..
-                    "{{ {} }}",
+            } => {
-                    fields
+                let mut parts = Vec::new();
-                        .iter()
+                if let Some(spread_expr) = spread {
-                        .map(|(name, val)| format!("{}: {}", name.name, self.format_expr(val)))
+                    parts.push(format!("...{}", self.format_expr(spread_expr)));
-                        .collect::<Vec<_>>()
+                }
-                        .join(", ")
+                for (name, val) in fields {
-                )
+                    parts.push(format!("{}: {}", name.name, self.format_expr(val)));
+                }
+                format!("{{ {} }}", parts.join(", "))
             }
             Expr::EffectOp { effect, operation, args, .. } => {
                 format!(

src/lexer.rs

@@ -90,6 +90,7 @@ pub enum TokenKind {
     Arrow,      // =>
     ThinArrow,  // ->
     Dot,        // .
+    DotDotDot,  // ...
     Colon,      // :
     ColonColon, // ::
     Comma,      // ,
@@ -181,6 +182,7 @@ impl fmt::Display for TokenKind {
             TokenKind::Arrow => write!(f, "=>"),
             TokenKind::ThinArrow => write!(f, "->"),
             TokenKind::Dot => write!(f, "."),
+            TokenKind::DotDotDot => write!(f, "..."),
             TokenKind::Colon => write!(f, ":"),
             TokenKind::ColonColon => write!(f, "::"),
             TokenKind::Comma => write!(f, ","),
@@ -373,7 +375,22 @@ impl<'a> Lexer<'a> {
                     TokenKind::Pipe
                 }
             }
-            '.' => TokenKind::Dot,
+            '.' => {
+                if self.peek() == Some('.') {
+                    // Check for ... (need to peek past second dot)
+                    // We look at source directly since we can only peek one ahead
+                    let next_next = self.source[self.pos..].chars().nth(1);
+                    if next_next == Some('.') {
+                        self.advance(); // consume second '.'
+                        self.advance(); // consume third '.'
+                        TokenKind::DotDotDot
+                    } else {
+                        TokenKind::Dot
+                    }
+                } else {
+                    TokenKind::Dot
+                }
+            }
             ':' => {
                 if self.peek() == Some(':') {
                     self.advance();

src/linter.rs

@@ -513,7 +513,10 @@ impl Linter {
             Expr::Field { object, .. } | Expr::TupleIndex { object, .. } => {
                 self.collect_refs_expr(object);
             }
-            Expr::Record { fields, .. } => {
+            Expr::Record { spread, fields, .. } => {
+                if let Some(spread_expr) = spread {
+                    self.collect_refs_expr(spread_expr);
+                }
                 for (_, val) in fields {
                     self.collect_refs_expr(val);
                 }

src/lsp.rs

@@ -1571,7 +1571,10 @@ fn collect_call_site_hints(
                 collect_call_site_hints(source, e, param_names, hints);
             }
         }
-        Expr::Record { fields, .. } => {
+        Expr::Record { spread, fields, .. } => {
+            if let Some(spread_expr) = spread {
+                collect_call_site_hints(source, spread_expr, param_names, hints);
+            }
             for (_, e) in fields {
                 collect_call_site_hints(source, e, param_names, hints);
             }

src/parser.rs

@@ -2208,6 +2208,11 @@ impl Parser {
             }));
         }
 
+        // Check for record spread: { ...expr, field: val }
+        if matches!(self.peek_kind(), TokenKind::DotDotDot) {
+            return self.parse_record_expr_rest(start);
+        }
+
         // Check if it's a record (ident: expr) or block
         if matches!(self.peek_kind(), TokenKind::Ident(_)) {
             let lookahead = self.tokens.get(self.pos + 1).map(|t| &t.kind);
@@ -2222,6 +2227,20 @@ impl Parser {
 
     fn parse_record_expr_rest(&mut self, start: Span) -> Result<Expr, ParseError> {
         let mut fields = Vec::new();
+        let mut spread = None;
+
+        // Check for spread: { ...expr, ... }
+        if self.check(TokenKind::DotDotDot) {
+            self.advance(); // consume ...
+            let spread_expr = self.parse_expr()?;
+            spread = Some(Box::new(spread_expr));
+
+            self.skip_newlines();
+            if self.check(TokenKind::Comma) {
+                self.advance();
+            }
+            self.skip_newlines();
+        }
 
         while !self.check(TokenKind::RBrace) {
             let name = self.parse_ident()?;
@@ -2238,7 +2257,11 @@ impl Parser {
 
         self.expect(TokenKind::RBrace)?;
         let span = start.merge(self.previous_span());
-        Ok(Expr::Record { fields, span })
+        Ok(Expr::Record {
+            spread,
+            fields,
+            span,
+        })
     }
 
     fn parse_block_rest(&mut self, start: Span) -> Result<Expr, ParseError> {

src/symbol_table.rs

@@ -527,7 +527,10 @@ impl SymbolTable {
                     self.visit_expr(e, scope_idx);
                 }
             }
-            Expr::Record { fields, .. } => {
+            Expr::Record { spread, fields, .. } => {
+                if let Some(spread_expr) = spread {
+                    self.visit_expr(spread_expr, scope_idx);
+                }
                 for (_, e) in fields {
                     self.visit_expr(e, scope_idx);
                 }

src/typechecker.rs

@@ -339,7 +339,10 @@ fn references_params(expr: &Expr, params: &[&str]) -> bool {
         Expr::Lambda { body, .. } => references_params(body, params),
         Expr::Tuple { elements, .. } => elements.iter().any(|e| references_params(e, params)),
         Expr::List { elements, .. } => elements.iter().any(|e| references_params(e, params)),
-        Expr::Record { fields, .. } => fields.iter().any(|(_, e)| references_params(e, params)),
+        Expr::Record { spread, fields, .. } => {
+            spread.as_ref().is_some_and(|s| references_params(s, params))
+                || fields.iter().any(|(_, e)| references_params(e, params))
+        }
         Expr::Match { scrutinee, arms, .. } => {
             references_params(scrutinee, params)
                 || arms.iter().any(|a| references_params(&a.body, params))
@@ -516,8 +519,9 @@ fn has_recursive_calls(func_name: &str, body: &Expr) -> bool {
         Expr::Tuple { elements, .. } | Expr::List { elements, .. } => {
             elements.iter().any(|e| has_recursive_calls(func_name, e))
         }
-        Expr::Record { fields, .. } => {
+        Expr::Record { spread, fields, .. } => {
-            fields.iter().any(|(_, e)| has_recursive_calls(func_name, e))
+            spread.as_ref().is_some_and(|s| has_recursive_calls(func_name, s))
+                || fields.iter().any(|(_, e)| has_recursive_calls(func_name, e))
         }
         Expr::Field { object, .. } | Expr::TupleIndex { object, .. } => has_recursive_calls(func_name, object),
         Expr::Let { value, body, .. } => {
@@ -672,6 +676,7 @@ fn generate_auto_migration_expr(
 
     // Build the record expression
     Some(Expr::Record {
+        spread: None,
         fields: field_exprs,
         span,
     })
@@ -1744,7 +1749,11 @@ impl TypeChecker {
                 span,
             } => self.infer_block(statements, result, *span),
 
-            Expr::Record { fields, span } => self.infer_record(fields, *span),
+            Expr::Record {
+                spread,
+                fields,
+                span,
+            } => self.infer_record(spread.as_deref(), fields, *span),
 
             Expr::Tuple { elements, span } => self.infer_tuple(elements, *span),
 
@@ -2551,12 +2560,46 @@ impl TypeChecker {
         self.infer_expr(result)
     }
 
-    fn infer_record(&mut self, fields: &[(Ident, Expr)], _span: Span) -> Type {
+    fn infer_record(
-        let field_types: Vec<(String, Type)> = fields
+        &mut self,
+        spread: Option<&Expr>,
+        fields: &[(Ident, Expr)],
+        span: Span,
+    ) -> Type {
+        // Start with spread fields if present
+        let mut field_types: Vec<(String, Type)> = if let Some(spread_expr) = spread {
+            let spread_type = self.infer_expr(spread_expr);
+            match spread_type {
+                Type::Record(spread_fields) => spread_fields,
+                _ => {
+                    self.errors.push(TypeError {
+                        message: format!(
+                            "Spread expression must be a record type, got {}",
+                            spread_type
+                        ),
+                        span,
+                    });
+                    Vec::new()
+                }
+            }
+        } else {
+            Vec::new()
+        };
+
+        // Apply explicit field overrides
+        let explicit_types: Vec<(String, Type)> = fields
             .iter()
             .map(|(name, expr)| (name.name.clone(), self.infer_expr(expr)))
             .collect();
 
+        for (name, typ) in explicit_types {
+            if let Some(existing) = field_types.iter_mut().find(|(n, _)| n == &name) {
+                existing.1 = typ;
+            } else {
+                field_types.push((name, typ));
+            }
+        }
+
         Type::Record(field_types)
     }