Allow method call on string literals

Note: this RFC was adapted from an internal proposal that predates RFC process

Summary

Allow string literals to be indexed on without parentheses or from an identifier. That is, the following snippet will become legal under this proposal:

print("Hello, %s!":format("world"))
print("0123456789ABCDEF":sub(i, i))

Motivation

Experienced Lua developers occasionally run into this paper-cut even after years of working with the language. Programmers in Lua frequently wants to format a user-facing message using a constant string, but the parser will not accept it as legible syntax.

Design

Formally, the proposal is to move the String parser from exp to prefixexp:

  var ::= Name | prefixexp `[´ exp `]´ | prefixexp `.´ Name 
- exp ::= nil | false | true | Number | String | `...´ | function |
+ exp ::= nil | false | true | Number | `...´ | function
        | prefixexp | tableconstructor | exp binop exp | unop exp
- prefixexp ::= var | functioncall | `(´ exp `)´
+ prefixexp ::= String | var | functioncall | `(´ exp `)´
  functioncall ::= prefixexp args | prefixexp `:´ Name args

The recommendation is that we should keep statements starting with string tokens as illegal syntax, as it is too niche to support use cases with side-effecting functions.

Drawbacks

Statements starting by parsing prefixexp will now allow string tokens to be parsed despite that the return values of the function calls are always discarded. This is a non-issue if we obey the recommendation to ban statements starting with string tokens.

Alternatives

The infallible parser could be mended in this exact scenario to report a more friendly error message. We decided not to do this because there is more value to gain by simply supporting the main proposal.

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