Add cover grammar for 'await using'

[rbuckton]

Adds a cover grammar for await using.

Fixes #12

[github-actions]

A preview of this PR can be found at https://tc39.es/proposal-async-explicit-resource-management/pr/15.

[rbuckton]

This is a fairly early draft, but I'd like to get some feedback on the cover grammar. I'll continue to add some of the missing semantics rules needed to apply the supplemental syntax in later commits.

[rbuckton]

I still need to review Contains and the early errors for delete to determine if there is additional work to do to support the cover grammar. In the case of Contains, we do not currently explicitly handle PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList, though we do handle ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList, so it is unclear whether something similar is needed.

[rbuckton]

Here are some potential test cases for this grammar:

await using

is parsed as an ExpressionStatement containing an AwaitExpression (covered by CoverAwaitExpressionAndAwaitUsingDeclarationHead).

await using
x = y

is parsed as an ExpressionStatement containing an AwaitExpression (covered by CoverAwaitExpressionAndAwaitUsingDeclarationHead), followed by an ExpressionStatement containing an AssignmentExpression.

await using x

is a syntax error. CoverAwaitExpressionAndAwaitUsingDeclarationHead is parsed for await using, but an IdentifierName cannot follow a UnaryExpression. This is then parsed as an AwaitUsingDeclaration, but an early error raises a syntax error due to the missing initializer.

await
using x

is a syntax error. CoverAwaitExpressionAndAwaitUsingDeclarationHead is parsed for await using, but an IdentifierName cannot follow a UnaryExpression. When attempting to parse as an AwaitUsingDeclaration, this fails to meet the supplemental syntax for AwaitUsingDeclarationHead.

await
using
x

is parsed as an ExpressionStatement containing an AwaitExpression (covered by CoverAwaitExpressionAndAwaitUsingDeclarationHead), followed by an ExpressionStatement containing an IdentifierReference.

[rbuckton]

@waldemarhorwat: I'd appreciate if you're able to review sometime this week.

[waldemarhorwat]

Why is a second definition of AsyncFunctionDeclaration pasted in the middle of the runtime semantics section? There already is a definition of AsyncFunctionDeclaration in the spec and I don't see the existing one being removed in this pull request.

[rbuckton]

I'm not sure what you mean by this. There are no <ins> or <del> tags in the grammar below, so no changes were made. I included the grammar here for reference as AwaitExpression is referenced multiple times in the static semantics and runtime semantics.