Implement annotated let binding syntactic sugar

examples/pass/annotated_let_binding.uplp

@@ -0,0 +1,9 @@
+-- category = "value"
+--
+-- [metadata]
+-- type = "Num"
+-- value = 10
+let a: Num = 1 in
+let b: Num = 10 in
+let c: Bool = false in
+if c then a else b

src/ast.rs

@@ -30,9 +30,14 @@ impl RawExpr {
     pub fn make_let(
         rec: bool,
         ident: RawIdent,
+        ann: Option<Type>,
         binding: Box<RawExpr>,
         body: Box<RawExpr>,
     ) -> Box<Self> {
+        let binding = match ann {
+            Some(ann) => Box::new(RawExpr::Ascribed(binding, ann)),
+            None => binding,
+        };
         Box::new(Self::Let(rec, interner::Id::new(ident.0), binding, body))
     }
 

src/parser.lalrpop

@@ -12,8 +12,8 @@ pub Uplp = Expr;
 Expr: Box<RawExpr> = {
     InfixExpr,
     <InfixExpr> ":" <Type> => Box::new(RawExpr::Ascribed(<>)),
-    "let" <rec: "rec"?> <ident: Ident> "=" <bnd: Expr> "in" <body: Expr> =>
-        RawExpr::make_let(rec.is_some(), ident, bnd, body),
+    "let" <rec: "rec"?> <ident: Ident> <ann: Annotation?> "=" <bnd: Expr> "in" <body: Expr> =>
+        RawExpr::make_let(rec.is_some(), ident, ann, bnd, body),
     "|" <CommaSeparated<FnArg>> "|" <Expr> => RawExpr::make_lambda(<>),
     "if" <Expr> "then" <Expr> "else" <Expr> => Box::new(RawExpr::IfThenElse(<>)),
 }
@@ -125,4 +125,4 @@ match {
     // skip single-line comments beginning with "--"
     r"--[^\n\r]*[\n\r]*" => { },
     _
-}
+}