Add support for parsing the 'foreign' keyword in let-declarations

src/cst.ts

@@ -102,6 +102,7 @@ export const enum SyntaxKind {
   TypeKeyword,
   ReturnKeyword,
   MatchKeyword,
+  ForeignKeyword,
   IfKeyword,
   ElifKeyword,
   ElseKeyword,
@@ -864,6 +865,16 @@ export class MatchKeyword extends TokenBase {
 
 }
 
+export class ForeignKeyword extends TokenBase {
+
+  public readonly kind = SyntaxKind.ForeignKeyword;
+
+  public get text(): string {
+    return 'foreign';
+  }
+
+}
+
 export class ModKeyword extends TokenBase {
 
   public readonly kind = SyntaxKind.ModKeyword;
@@ -992,6 +1003,7 @@ export type Token
   | ElseKeyword
   | ElifKeyword
   | EnumKeyword
+  | ForeignKeyword
 
 export type TokenKind
   = Token['kind']
@@ -2101,6 +2113,7 @@ export class LetDeclaration extends SyntaxBase {
   public constructor(
     public pubKeyword: PubKeyword | null,
     public letKeyword: LetKeyword,
+    public foreignKeyword: ForeignKeyword | null,
     public mutKeyword: MutKeyword | null,
     public pattern: Pattern | WrappedOperator,
     public params: Param[],

src/parser.ts

@@ -784,6 +784,7 @@ export class Parser {
     let t0 = this.getToken();
     let pubKeyword = null;
     let mutKeyword = null;
+    let foreignKeyword = null;
     if (t0.kind === SyntaxKind.PubKeyword) {
       pubKeyword = t0;
       t0 = this.getToken();
@@ -791,20 +792,25 @@ export class Parser {
     if (t0.kind !== SyntaxKind.LetKeyword) {
       this.raiseParseError(t0, [ SyntaxKind.LetKeyword ]);
     }
-    const t1 = this.peekToken();
+    let t1 = this.peekToken();
+    if (t1.kind === SyntaxKind.ForeignKeyword) {
+      this.getToken();
+      foreignKeyword = t1;
+      t1 = this.peekToken();
+    }
     if (t1.kind === SyntaxKind.MutKeyword) {
       this.getToken();
       mutKeyword = t1;
+      t1 = this.peekToken();
     }
-    const t2 = this.peekToken();
-    const t3 = this.peekToken(2);
-    const t4 = this.peekToken(3);
+    const t2 = this.peekToken(2);
+    const t3 = this.peekToken(3);
     let pattern;
-    if (t2.kind === SyntaxKind.LParen && t3.kind === SyntaxKind.CustomOperator && t4.kind === SyntaxKind.RParen) {
+    if (t1.kind === SyntaxKind.LParen && t2.kind === SyntaxKind.CustomOperator && t3.kind === SyntaxKind.RParen) {
       this.getToken()
       this.getToken();
       this.getToken();
-      pattern = new WrappedOperator(t2, t3, t4);
+      pattern = new WrappedOperator(t1, t2, t3);
     } else {
       pattern = this.parsePattern();
     }
@@ -859,6 +865,7 @@ export class Parser {
     return new LetDeclaration(
       pubKeyword,
       t0,
+      foreignKeyword,
       mutKeyword,
       pattern,
       params,

src/scanner.ts

@@ -40,6 +40,7 @@ import {
   MatchKeyword,
   RArrowAlt,
   VBar,
+  ForeignKeyword,
 } from "./cst"
 import { Diagnostics, UnexpectedCharDiagnostic } from "./diagnostics"
 import { Stream, BufferedStream, assert } from "./util";
@@ -366,6 +367,7 @@ export class Scanner extends BufferedStream<Token> {
             case 'struct': return new StructKeyword(startPos);
             case 'enum': return new EnumKeyword(startPos);
             case 'match': return new MatchKeyword(startPos);
+            case 'foreign': return new ForeignKeyword(startPos);
             default:
               if (isUpper(text[0])) {
                 return new IdentifierAlt(text, startPos);