[ruby-core:117590] [Ruby master Feature#15554] warn/error passing a block to a method which never use a block

["jeremyevans0 (Jeremy Evans) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #15554 has been updated by jeremyevans0 (Jeremy Evans).


One approach that reduces false positives (invalid warnings) and potentially increases false negatives (missing warnings) would be to only warn for literal blocks and not passed blocks:

```ruby
def foo
end

def bar(&) = foo(&) # no warning

bar{}
foo{} # warning
```

Currently, this generates two warnings:

```
-:4: warning: the block passed to 'Object#foo' defined at -:1 may be ignored
-:7: warning: the block passed to 'Object#foo' defined at -:1 may be ignored
```

I propose the first warning be eliminated.  In the above example, it is always an error, but in real world code, such as when `foo` is called indirectly using `send`, or when the `foo` method is defined differently for different objects, it may not be an error.

Passing a literal block to a method that doesn't accept a block is almost always an error.  Passing an existing block to a method that doesn't accept it may or may not be an error.  I think it's better to only warn in cases where we are fairly sure it is an error.

----------------------------------------
Feature #15554: warn/error passing a block to a method which never use a block
https://bugs.ruby-lang.org/issues/15554#change-107991

* Author: ko1 (Koichi Sasada)
* Status: Closed
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
# Abstract

Warn or raise an ArgumentError if block is passed to a method which does not use a block.
In other words, detect "block user methods" implicitly and only "block user methods" can accept a block.

# Background

Sometimes, we pass a block to a method which ignores the passed block accidentally.

```
def my_open(name)
  open(name)
end

# user hopes it works as Kernel#open which invokes a block with opened file.
my_open(name){|f| important_work_with f }
# but simply ignored...
```

To solve this issue, this feature request propose showing warnings or raising an exception on such case.

Last developer's meeting, matz proposed `&nil` which declares this method never receive a block. It is explicit, but it is tough to add this `&nil` parameter declaration to all of methods (do you want to add it to `def []=(i, e, &nil)`?).
(I agree `&nil` is valuable on some situations)

# Spec

## Define "use a block" methods

We need to define which method accepts a block and which method does not.

* (1) method has a block parameter (`&b`)
* (2) method body has `yield'
* (3) method body has `super` (ZSUPER in internal terminology) or `super(...)`
* (4) method body has singleton method (optional)

(1) and (2) is very clear. I need to explain about (3) and (4).

(3). `super` (ZSUPER) passes all parameters as arguments. So there is no surprise that which can accept `block`.
However `super(...)` also passes a block if no explicit block passing (like `super(){}` or `super(&b)`) are written.
I'm not sure we need to continue this strange specification, but to keep compatibility depending this spec, I add this rule.

(4). surprisingly, the following code invoke a block:

```
def foo
  class << Object.new
    yield
  end
end

foo{ p :ok } #=> :ok

```

I'm also not sure we need to keep this spec, but to allow this spec, I added (4) rule.
Strictly speaking, it is not required, but we don't keep the link from singleton class ISeq to lexical parent iseq now, so I added it.

## Exceptional cases

A method called by `super` doesn`t warn warning even if this method doesn't use a block.
The rule (3) can pass blocks easily and there are many methods don`t use a block.

So my patch ignores callings by `super`.

## corner cases

There are several cases to use block without (1)-(4) rules.

### `Proc.new/proc/lambda` without a block

Now it was deprecated in r66772 (commit:9f1fb0a17febc59356d58cef5e98db61a3c03550).
Related discussion: [Bug #15539]

### `block_given?`

`block_given?` expects block, but I believe we use it with `yield` or a block parameter.
If you know the usecase without them, please tell us.

### `yield` in `eval`

We can't know `yield` (or (3), (4) rule) in an `eval` evaluating string at calling time.

```
def foo
  eval('yield`)
end

foo{} # at calling time,
      # we can't know the method foo can accept a block or not.
```

So I added a warning to use `yield` in `eval` like that: `test.rb:4: warning: use yield in eval will not be supported in Ruby 3.`

Workaround is use a block parameter explicitly.

```
def foo &b
  eval('b.call')
end

foo{ p :ok }
```

# Implementation

Strategy is:

* [compile time] introduce `iseq::has_yield` field and check it if the iseq (or child iseq) contains `yield` (or something)
* [calling time] if block is given, check `iseq::has_yield` flag and show warning (or raise an exception)

https://gist.github.com/ko1/c9148ad0224bf5befa3cc76ed2220c0b

On this patch, now it raises an error to make it easy to detect.
It is easy to switch to show the warning.

# Evaluation and discussion

I tried to avoid ruby's tests.

https://gist.github.com/ko1/37483e7940cdc4390bf8eb0001883786

Here is a patch.

There are several patterns to avoid warnings.

## tests for `block_given?`, `Proc.new` (and similar) without block

Add a dummy block parameter.
It is test-specific issue.

## empty `each`

Some tests add `each` methods do not `yield`, like: `def each; end`.
Maybe test-specific issue, and adding a dummy block parameter.

## Subtyping / duck typing

https://github.com/ruby/ruby/blob/c01a5ee85e2d6a7128cccafb143bfa694284ca87/lib/optparse.rb#L698

This `parse` method doesn't use `yield`, but other sub-type's `parse` methods use.

## `super` with `new` method

https://gist.github.com/ko1/37483e7940cdc4390bf8eb0001883786#file-tests-patch-L61

This method override `Class#new` method and introduce a hook with block (yield a block in this hook code).

https://github.com/ruby/ruby/blob/trunk/lib/rubygems/package/tar_writer.rb#L81

In this method, call `super` and it also passing a block. However, called `initialize` doesn't use a block.

## Change robustness

This change reduce robustness for API change.

`Delegator` requires to support `__getobj__` for client classes.
Now `__getobj__` should accept block but most of `__getobj__` clients do not call given block.

https://github.com/ruby/ruby/blob/trunk/lib/delegate.rb#L80

This is because of delegator.rb's API change.

https://gist.github.com/ko1/37483e7940cdc4390bf8eb0001883786#file-tests-patch-L86

Nobu says calling block is not required (ignoring a block is no problem) so it is not a bug for delegator client classes.

## Found issues.

```
[ 2945/20449] Rinda::TestRingServer#test_do_reply = 0.00 s
  1) Error:
Rinda::TestRingServer#test_do_reply:
ArgumentError: passing block to the method "with_timeout" (defined at /home/ko1/src/ruby/trunk/test/rinda/test_rinda.rb:787) is never used.
    /home/ko1/src/ruby/trunk/test/rinda/test_rinda.rb:635:in `test_do_reply'

[ 2946/20449] Rinda::TestRingServer#test_do_reply_local = 0.00 s
  2) Error:
Rinda::TestRingServer#test_do_reply_local:
ArgumentError: passing block to the method "with_timeout" (defined at /home/ko1/src/ruby/trunk/test/rinda/test_rinda.rb:787) is never used.
    /home/ko1/src/ruby/trunk/test/rinda/test_rinda.rb:657:in `test_do_reply_local'

[10024/20449] TestGemRequestSetGemDependencyAPI#test_platform_mswin = 0.01 s
  3) Error:
TestGemRequestSetGemDependencyAPI#test_platform_mswin:
ArgumentError: passing block to the method "util_set_arch" (defined at /home/ko1/src/ruby/trunk/lib/rubygems/test_case.rb:1053) is never used.
    /home/ko1/src/ruby/trunk/test/rubygems/test_gem_request_set_gem_dependency_api.rb:655:in `test_platform_mswin'

[10025/20449] TestGemRequestSetGemDependencyAPI#test_platforms = 0.01 s
  4) Error:
TestGemRequestSetGemDependencyAPI#test_platforms:
ArgumentError: passing block to the method "util_set_arch" (defined at /home/ko1/src/ruby/trunk/lib/rubygems/test_case.rb:1053) is never used.
    /home/ko1/src/ruby/trunk/test/rubygems/test_gem_request_set_gem_dependency_api.rb:711:in `test_platforms'
```

These 4 detection show the problem. `with_timeout` method (used in Rinda test) and `util_set_arch` method (used in Rubygems test) simply ignore the given block.
So these tests are simply ignored.

I reported them. (https://github.com/rubygems/rubygems/issues/2601)

## raise an error or show a warning?

At least, Ruby 2.7 should show warning for this kind of violation with `-w`.
How about for Ruby3?




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