RFC: Support `__len` metamethod for tables and `rawlen` function (#536)

rfcs/len-metamethod-rawlen.md

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+# Support `__len` metamethod for tables and `rawlen` function
+
+## Summary
+
+`__len` metamethod will be called by `#` operator on tables, matching Lua 5.2
+
+## Motivation
+
+Lua 5.1 invokes `__len` only on userdata objects, whereas Lua 5.2 extends this to tables. In addition to making `__len` metamethod more uniform and making Luau
+more compatible with later versions of Lua, this has the important advantage which is that it makes it possible to implement an index based container.
+
+Before `__iter` and `__len` it was possible to implement a custom container using `__index`/`__newindex`, but to iterate through the container a custom function was
+necessary, because Luau didn't support generalized iteration, `__pairs`/`__ipairs` from Lua 5.2, or `#` override.
+
+With generalized iteration, a custom container can implement its own iteration behavior so as long as code uses `for k,v in obj` iteration style, the container can
+be interfaced with the same way as a table. However, when the container uses integer indices, manual iteration via `#` would still not work - which is required for some
+more complicated algorithms, or even to simply iterate through the container backwards.
+
+Supporting `__len` would make it possible to implement a custom integer based container that exposes the same interface as a table does.
+
+## Design
+
+`#v` will call `__len` metamethod if the object is a table and the metamethod exists; the result of the metamethod will be returned if it's a number (an error will be raised otherwise).
+
+`table.` functions that implicitly compute table length, such as `table.getn`, `table.insert`, will continue using the actual table length. This is consistent with the
+general policy that Luau doesn't support metamethods in `table.` functions.
+
+A new function, `rawlen(v)`, will be added to the standard library; given a string or a table, it will return the length of the object without calling any metamethods.
+The new function has the previous behavior of `#` operator with the exception of not supporting userdata inputs, as userdata doesn't have an inherent definition of length.
+
+## Drawbacks
+
+`#` is an operator that is used frequently and as such an extra metatable check here may impact performance. However, `#` is usually called on tables without metatables,
+and even when it is, using the existing metamethod-absence-caching approach we use for many other metamethods a test version of the change to support `__len` shows no
+statistically significant difference on existing benchmark suite. This does complicate the `#` computation a little more which may affect JIT as well, but even if the
+table doesn't have a metatable the process of computing `#` involves a series of condition checks and as such will likely require slow paths anyway.
+
+This is technically changing semantics of `#` when called on tables with an existing `__len` metamethod, and as such has a potential to change behavior of an existing valid program.
+That said, it's unlikely that any table would have a metatable with `__len` metamethod as outside of userdata it would not anything, and this drawback is not feasible to resolve with any alternate version of the proposal.
+
+## Alternatives
+
+Do not implement `__len`.