Typechecker: support decorator declarations (#559)

This involves rudimentary typechecking of fields, and adding the backing
struct of the decorator into the scope like any ordinary `type`
declaration. Also, in order to get things to typecheck properly, I added
decorator declarations for the existing usages of decorators in the
compiler: `@noreturn`, `@external`, and `@intrinsic`.

projects/compiler/example.abra

@@ -1,3 +1,8 @@
+decorator bar {
+  a: String
+}
+
+@bar(a: "a")
 func foo(): Int {
   val a = 1 + []
   val (a, b) = foobar

projects/compiler/test/run-tests.js

@@ -726,6 +726,14 @@ const TYPECHECKER_TESTS = [
   { test: "typechecker/enumdecl/error_invalid_tostring_signature.abra", assertions: "typechecker/enumdecl/error_invalid_tostring_signature.out" },
   { test: "typechecker/enumdecl/error_method_bad_self_position.abra", assertions: "typechecker/enumdecl/error_method_bad_self_position.out" },
   { test: "typechecker/enumdecl/error_variant_field_initializer_type_mismatch.abra", assertions: "typechecker/enumdecl/error_variant_field_initializer_type_mismatch.out" },
+  // Decorator declaration
+  { test: "typechecker/decorators/error_enum_as_decorator.abra", assertions: "typechecker/decorators/error_enum_as_decorator.out" },
+  { test: "typechecker/decorators/error_noncomptime_field.abra", assertions: "typechecker/decorators/error_noncomptime_field.out" },
+  { test: "typechecker/decorators/error_too_few_args.abra", assertions: "typechecker/decorators/error_too_few_args.out" },
+  { test: "typechecker/decorators/error_too_many_args.abra", assertions: "typechecker/decorators/error_too_many_args.out" },
+  { test: "typechecker/decorators/error_type_as_decorator.abra", assertions: "typechecker/decorators/error_type_as_decorator.out" },
+  { test: "typechecker/decorators/error_unlabeled_args.abra", assertions: "typechecker/decorators/error_unlabeled_args.out" },
+  { test: "typechecker/decorators/function_decorator.abra", assertions: "typechecker/decorators/function_decorator.out.json" },
 
   // Returns
   { test: "typechecker/return/return.1.abra", assertions: "typechecker/return/return.1.out.json" },

projects/compiler/test/typechecker/decorators/_exports.abra

@@ -0,0 +1,8 @@
+pub decorator FromOtherModule1 {
+  pub a: Int
+  pub b: Int
+}
+
+pub decorator FromOtherModule2 {
+  pub a: Int
+}

projects/compiler/test/typechecker/decorators/error_enum_as_decorator.abra

@@ -0,0 +1,8 @@
+enum Foo {
+  Bar
+}
+
+@Foo.Bar(b: "asdf")
+func foo() {
+
+}

projects/compiler/test/typechecker/decorators/error_enum_as_decorator.out

@@ -0,0 +1,4 @@
+Error at %TEST_DIR%/typechecker/decorators/error_enum_as_decorator.abra:5:5
+Unexpected token '.', expected one of 'val', 'var', 'func', 'type', 'enum', 'decorator', '@', 'pub':
+  |  @Foo.Bar(b: "asdf")
+         ^

projects/compiler/test/typechecker/decorators/error_noncomptime_field.abra

@@ -0,0 +1,5 @@
+decorator Bar {
+  p: Person
+}
+
+type Person { name: String }

projects/compiler/test/typechecker/decorators/error_noncomptime_field.out

@@ -0,0 +1,6 @@
+Error at %TEST_DIR%/typechecker/decorators/error_noncomptime_field.abra:2:3
+Forbidden field type for decorator 'Bar'
+  |    p: Person
+       ^
+This field has type 'Person', which is not allowed for fields of decorators.
+Decorators' fields must be one of: Int, Float, Bool, String

projects/compiler/test/typechecker/decorators/error_too_few_args.abra

@@ -0,0 +1,6 @@
+decorator Bar {
+  a: String
+}
+
+@Bar
+func foo() {}

projects/compiler/test/typechecker/decorators/error_too_few_args.out

@@ -0,0 +1,5 @@
+Error at %TEST_DIR%/typechecker/decorators/error_too_few_args.abra:5:2
+Invalid instantiation
+  |  @Bar
+      ^
+The following required fields are missing: 'a'

projects/compiler/test/typechecker/decorators/error_too_many_args.abra

@@ -0,0 +1,6 @@
+decorator Bar {
+  a: String
+}
+
+@Bar("asdf", 123)
+func foo() {}

projects/compiler/test/typechecker/decorators/error_too_many_args.out

@@ -0,0 +1,5 @@
+Error at %TEST_DIR%/typechecker/decorators/error_too_many_args.abra:5:14
+Too many arguments for invocation
+  |  @Bar("asdf", 123)
+                  ^
+Expected no more than 1 argument, but 2 were passed

projects/compiler/test/typechecker/decorators/error_type_as_decorator.abra

@@ -0,0 +1,8 @@
+type Foo {
+  a: String
+}
+
+@Foo(b: "asdf")
+func foo() {
+
+}

projects/compiler/test/typechecker/decorators/error_type_as_decorator.out

@@ -0,0 +1,4 @@
+Error at %TEST_DIR%/typechecker/decorators/error_type_as_decorator.abra:5:2
+Cannot use non-decorator type as a decorator
+  |  @Foo(b: "asdf")
+      ^

projects/compiler/test/typechecker/decorators/error_unlabeled_args.abra

@@ -0,0 +1,7 @@
+decorator Bar {
+  a: String
+  b: Int
+}
+
+@Bar("asdf", 123)
+func foo() {}

projects/compiler/test/typechecker/decorators/error_unlabeled_args.out

@@ -0,0 +1,5 @@
+Error at %TEST_DIR%/typechecker/decorators/error_unlabeled_args.abra:6:6
+Invalid instantiation
+  |  @Bar("asdf", 123)
+          ^
+Calls of type constructors must include argument labels

projects/compiler/test/typechecker/decorators/function_decorator.abra

@@ -0,0 +1,18 @@
+import FromOtherModule1, FromOtherModule2 from "./_exports"
+
+decorator Bar {
+  a: String
+}
+
+// no necessary label if only one argument
+@Bar("asdf")
+func f1() {}
+
+@Bar(a: "asdf")
+func f2() {}
+
+@FromOtherModule1(a: 3, b: 1)
+func f3() {}
+
+@FromOtherModule2(4)
+func f4() {}