New Rule: Block not captured

[j8r]

Anonymous block annotations are present since a bit of time now. Adding & shows that the method yields a block, but using &block either means that or that the block is captured.

Here is a simple example:

# Valid
def m(&block)
  block.call 1
end

# Also valid
def m(&block)
  callback = block
end

# Invalid
def m(&block)
  yield
end

# Correct syntax
def m(&)
  yield
end

[veelenga]

Basically, we would like to report if there is a block argument and it is not used. Is this correct?

[Sija]

Hmm, this should be already covered by Lint/UnusedArgument rule 🤔

[straight-shoota]

And I hope eventually by crystal tool format (crystal-lang/crystal#8764)

[Sija]

@veelenga For some reason it doesn't seem to work though 🤔

[veelenga]

That's strange. It is covered by specs:

ameba/spec/ameba/rule/lint/unused_argument_spec.cr

Lines 118 to 133 in be65ba2

	it "reports if block arg is not used" do
	s = Source.new %(
	def method(&block)
	end
	)
	subject.catch(s).should_not be_valid
	end
	it "reports if unused and there is yield" do
	s = Source.new %(
	def method(&block)
	yield 1
	end
	)
	subject.catch(s).should_not be_valid
	end

But you are right, it doesn't work on the example in the description.

[Sija]

That's strange. It is covered by specs:

@veelenga I know, I was trying to understand wtf is goin' on there, but to no avail yet.

[veelenga]

@Sija it ignores defs by default. There is a configuration property:

ameba/src/ameba/rule/lint/unused_argument.cr

Line 31 in be65ba2

ignore_defs true

That was done because of this discussion (which makes sense) #52 (comment)

The rule still has to be tuned in order to properly support the example below:

def m(&)
  yield
end

currently it reports an issue:

[W] Lint/UnusedArgument: Unused argument ``. If it's necessary, use `_` as an argument name to indicate that it won't be used.
> def m(&)
        ^

[veelenga]

Maybe it makes more sense to correct Lint/UnusedArgument rule in order to ignore an example with (&) argument and create a new rule that will specifically report the unused block argument.