stdlib: create an init function for records with complex default values

[frazze-jobb]

records that have field default values containing variables that are "free" was unsafe in functions that have variables with the same name. This commit creates init function for records to protect the variables in the default value.

e.g.
-record(r, {f = fun(X)->case X of {y, Y} -> Y; _ -> X end, g=..., h=abc}). foo(X)->#r{}. --> foo(X)->(r_init()){}.

r_init() will only initialize fields that will not be updated e.g.
foo(X)->#r{f=X} --> foo(X)->(r_init_f()){f=X}.
r_init_f will only initialize g and h with its default value, f will be initialized to undefined.

r_init() functions will not be generated if all fields of the record that contains "free variables" are initialized by the user.
e.g.
foo(X)->#r{f=X,g=X}. --> foo(X)->{r,X,X,abc}.

closes #9317

[github-actions]

CT Test Results

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// Erlang/OTP Github Action Bot

[jhogberg]

In general, prefer binaries over lists, and to sink expressions as far down as possible: the compiler has to maintain observability with regards to tracing, and cannot sink this expression down to the true branch below as that would result in a different trace.

[ilya-klyuchnikov]

Now, as "free vars" are allowed in record fields, these free vars can affect each other.

-module(z).

-export([mk1/0]).

-record(a, {
  a = X = id(1),
  b = X = id(2)
}).

id(X) -> X.

mk1() ->
  #a{}.

1> z:mk1().
** exception error: no match of right hand side value 2
     in function  z:'rec_init$^0'/0 (z.erl, line 13)

[frazze-jobb]

Now, as "free vars" are allowed in record fields, these free vars can affect each other.

Thanks, I'll get to it soon!

[frazze-jobb]

Now, as "free vars" are allowed in record fields, these free vars can affect each other.

Thanks, I'll get to it soon!

Actually, I spoke to soon. This is as intended. Just like how it works if you try to update the record like this: #r0{a=X=id(1),b=X=id(2)} that would not work either.

But I will update the linter so that it warns for this.

[elbrujohalcon]

Any chance of not including the new function in the error description?

So, from the other comment…

1> z:mk1().
** exception error: no match of right hand side value 2
     in function  z:'rec_init$^0'/0 (z.erl, line 13)

I would've preferred to see something like…

1> z:mk1().
** exception error: no match of right hand side value 2
     in function  z:mk1/0 (z.erl, line 13)

[frazze-jobb]

Any chance of not including the new function in the error description?

Tail calls, like in this case, makes it invisible in the stacktrace.
Instead I suggest:
in record default value (z.erl, line 13)

Would that work for you?

[michalmuskala]

Another option would be to explicitly prevent the compiler from tail calling into this helper function, and always emit a full call with building a stack frame

[vkatsuba]

Suggested change

	[ traverse_af(Ast, Fun, Fun(Ast,Acc)) || Ast <- AF];
	[traverse_af(Ast, Fun, Fun(Ast,Acc)) || Ast <- AF];