Trailing comma support in calls and function definitions

Summary

Luau will support trailing commas in the following places:

local function definition(
    x: number, -- <-- Here, previously a syntax error
)

call(
    a,
    b, -- <-- Here, previously a syntax error
)

type Fn = (
    x: number,
) -> ()

Motivation

Currently, trailing commas are supported in several places in Luau.

local t = {
    a,
    b,
    c,
}

-- Which even means...
f {
    a,
    b,
    c,
}

type Type = {
    a: number,
    b: number,
}

However, they are not supported in function definitions, types, or calls. This is an unintuitive syntax error and it is obvious what the user intended to do.

Design

We will make these adjustments to the grammar:

funcargs =
-   '(' [explist] ')'
+   '(' {exp ','} [','] ')'
    | tableconstructor
    | STRING

This allows f(a, b, c,).

There is no difference between f(a(),) and f(a()). Meaning, if a() returns 1, 2, 3, then f(a(),) will be equivalent to f(1, 2, 3). This is consistent with tables and { a(), }.

parlist =
-    bindinglist [',' '...']
+    (bindinglist [',' | ',' '...']
    | '...' [':' (Type | GenericTypePack)]

This results in:

function f(
    a,
    b, -- New!
) end

function f(
    a,
    ... -- The usual
) end

function f(
    a,
    ..., -- NOT allowed
)

Trailing comma is not allowed after ... as it is never correct to put more arguments after the ellipsis. This is consistent with:

JavaScript - Supports function f(a, b,), but function f(...a,) {} results in "Rest parameter must be last formal parameter".

Though this is not universally agreed upon:

Rust's unstable C variadics support unsafe extern "C" fn f(a: u32, ...,) {}, though this syntax is not finalized.
PHP supports function f($a, ...$args,) {}
Go requires a trailing comma after every argument, including variadics, if split over multiple lines.

BoundTypeList =
    [NAME ':'] Type
        [',' BoundTypeList]
+       [',']
    | '...' Type

This results in:

type Fn = (x: T,) -> () -- Allowed, new!
type Fn = (x: T, ...U) -> () -- Allowed
type Fn = (x: T, ...U,) -> () -- Not allowed

Drawbacks

There are no interesting drawbacks.

Alternatives

Supporting function f(...,) is easily doable if the rules seems unintuitive.

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