Proposal: disallow bare return error union

[fengb]

One of the things I like about zig is the explicit control flow. But a slight confusion point arises because current semantics treats return and try as "equals":

fn getUser(postId: u32) !u32 {
    const post = try getPost(postId);
    const author = try getAuthor(post.authorId);
    return author.getUser();
}

We can clearly tell there are 2 lines can error, but the third is ambiguous and requires digging into the getUser definition to confidently say it cannot error. If we disallow bare union returns, this must be converted to return try author.getUser(); which eliminates the guessing game.

At a basic level, requiring an extra 4 characters feels a little silly, but I think the benefits can add up, especially in larger projects with many "tail call" errors.

fn dispatch(action: Action) !Response {
    return switch (action) {
        .Go => doGo(),
        .Stop => doStop(),
        .Jump => doJump(),
        .Stomp => doStomp(),
        .Die => doDie(),
    };
}

// vs

fn dispatch(action: Action) !Response {
    return switch (action) {
        .Go => try doGo(),
        .Stop => try doStop(),
        .Jump => try doJump(),
        .Stomp => doStomp(),
        .Die => doDie(),
    };
}

I don't think this would affect any other language semantics.