RFC for trailing commas in calls

docs/trailing-commas-in-calls.md

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+# Trailing comma support in calls and function definitions
+
+## Summary
+
+Luau will support trailing commas in the following places:
+
+```lua
+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.
+
+```lua
+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:
+
+```patch
+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(), }`.
+
+```patch
+parlist =
+-    bindinglist [',' '...']
++    (bindinglist [',' | ',' '...']
+    | '...' [':' (Type | GenericTypePack)]
+```
+
+This results in:
+
+```lua
+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.
+
+```patch
+BoundTypeList =
+    [NAME ':'] Type
+        [',' BoundTypeList]
++       [',']
+    | '...' Type
+```
+
+This results in:
+
+```lua
+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.