language: Add expression literals

Add expression literals, so you can set properties of type
Gtk.Expression.

blueprintcompiler/language/expression.py

@@ -81,6 +81,16 @@ class LiteralExpr(ExprBase):
             or self.root.is_legacy_template(self.literal.value.ident)
         )
 
+    @property
+    def is_this(self) -> bool:
+        from .values import IdentLiteral
+
+        return (
+            not self.is_object
+            and isinstance(self.literal.value, IdentLiteral)
+            and self.literal.value.ident == "item"
+        )
+
     @property
     def literal(self):
         from .values import Literal
@@ -91,6 +101,15 @@ class LiteralExpr(ExprBase):
     def type(self) -> T.Optional[GirType]:
         return self.literal.value.type
 
+    @validate()
+    def item_validations(self):
+        if self.is_this:
+            if not isinstance(self.rhs, CastExpr):
+                raise CompileError('"item" must be cast to its object type')
+
+            if not isinstance(self.rhs.rhs, LookupOp):
+                raise CompileError('"item" can only be used for looking up properties')
+
 
 class LookupOp(InfixExpr):
     grammar = [".", UseIdent("property")]
@@ -285,6 +304,9 @@ expr.children = [
 def decompile_lookup(
     ctx: DecompileCtx, gir: gir.GirContext, cdata: str, name: str, type: str
 ):
+    if ctx.parent_node is not None and ctx.parent_node.tag == "property":
+        ctx.print("expr ")
+
     if t := ctx.type_by_cname(type):
         type = decompile.full_name(t)
     else:
@@ -304,6 +326,8 @@ def decompile_lookup(
     if constant is not None:
         if constant == ctx.template_class:
             ctx.print("template." + name)
+        elif constant == "":
+            ctx.print("item as <" + type + ">." + name)
         else:
             ctx.print(constant + "." + name)
         return
@@ -318,6 +342,9 @@ def decompile_lookup(
 def decompile_constant(
     ctx: DecompileCtx, gir: gir.GirContext, cdata: str, type: T.Optional[str] = None
 ):
+    if ctx.parent_node is not None and ctx.parent_node.tag == "property":
+        ctx.print("expr ")
+
     if type is None:
         if cdata == ctx.template_class:
             ctx.print("template")
@@ -330,6 +357,9 @@ def decompile_constant(
 
 @decompiler("closure", skip_children=True)
 def decompile_closure(ctx: DecompileCtx, gir: gir.GirContext, function: str, type: str):
+    if ctx.parent_node is not None and ctx.parent_node.tag == "property":
+        ctx.print("expr ")
+
     if t := ctx.type_by_cname(type):
         type = decompile.full_name(t)
     else: