Annotation/diagnostic to enforce exact type definition

[firas-assaad]

Currently if you write the following:

  ---@class C
  ---@field one integer
  ---@field two string
  local c = {}
  
  print(c.one)
  print(c.three) -- undefined field
  
  c.four = 5
  print(c.four) -- works fine, c.four is 5

You can still add more inline fields to the table without warnings, as long as they're defined before they're used. This is useful because Lua tables are used in many ways and lend themselves to being extended with more fields.

However, sometimes you're working with a strict class definition (e.g. a vendor API) that are not meant to be extended. In these cases, you can end up adding fields to them by mistake. e.g. if you type c.twwo = 'x' when you meant c.two. Another scenario is trying to add a new index to a C/C++ usertype that doesn't support this operation, resulting in a crash that can only be caught at runtime.

My suggestion is to introduce an annotation for classes that triggers a diagnostic warning when trying to add a new index to objects of that class. Some name ideas: @exact, @closed, @final. It could also be part of @class annotation (e.g. @class exact C or @class{exact} C).

  ---@class C
  ---@exact
  ---@field one integer
  ---@field two string
  local c = {}
  
  print(c.one)
  print(c.three) -- undefined field
  
  c.four = 5 -- undefined field
  print(c.four) -- undefined field

The suggestion for exact types in TypeScript might be relevant.

[firas-assaad]

Similar suggestions/feature requests:

• Table interface definitions #1652
• Validation and enforcement of @class fields is incomplete/inconsistent #1823
• [Feature Request] Add a strong typed struct/class that cannot assign a new field that is not defined in the struct/class. #1990

In addition to my example (preventing undocumented properties from being defined), the following features would are also desired:

• Warn when non-nil field is missing from the initialization (incomplete type)
• Validate the exact type in method calls and return statements