Fix unused lambda captures in Blink

Fix unused lambda captures in Blink.

Clang just got a new warning about unused lambda captures,
and that requires us to clean all places with this issue
across all the Chromium code base. This CL fixes all such
cases in WebKit/Blink.

An example of a code that is no longer considered normal is

auto g = [i, j]() { printf("%d\n", i); };

in which j is never used in the lambda and therefore its capture is unnecessary.

BUG=681136
Review-Url: https://codereview.chromium.org/2645613003
Cr-Commit-Position: refs/heads/master@{#444477}
Committed: https://chromium.googlesource.com/chromium/src/+/5cbffe1aa36f518681414e7bed5627a57449b58a

[krasin1, 2017-01-18 18:45:29]

The CQ bit was checked by krasin@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-01-18 18:46:07]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[krasin1, 2017-01-18 18:46:32]

krasin@chromium.org changed reviewers:
+ esprehn@chromium.org

[krasin1, 2017-01-18 18:46:33]

Hi Elliott!

please, review this small pack of fixes to the unused lambda captures. We need
to do this cleanup in order to adapt to the new (more strict) version of Clang.

Generally, not having unused <imports/includes/arguments/lamda captures> is good
anyway.

[krasin1, 2017-01-18 19:26:54]

Description was changed from

==========
Fix unused lambda captures.

Clang just got more strict about unused lambda captures,
and that requires us to clean all places with this issue
across all the Chromium code base. This CL fixes all such
cases in WebKit/Blink.

BUG=681912
==========

to

==========
Fix unused lambda captures in Blink.

Clang just got more strict about unused lambda captures,
and that requires us to clean all places with this issue
across all the Chromium code base. This CL fixes all such
cases in WebKit/Blink.

BUG=681912
==========

[krasin1, 2017-01-18 19:36:32]

Description was changed from

==========
Fix unused lambda captures in Blink.

Clang just got more strict about unused lambda captures,
and that requires us to clean all places with this issue
across all the Chromium code base. This CL fixes all such
cases in WebKit/Blink.

BUG=681912
==========

to

==========
Fix unused lambda captures in Blink.

Clang just got a new warning about unused lambda captures,
and that requires us to clean all places with this issue
across all the Chromium code base. This CL fixes all such
cases in WebKit/Blink.

An example of a code that is no longer considered normal is

auto g = [i, j]() { printf("%d\n", i); };

in which j is never used in the lambda and therefore its capture is unnecessary.

BUG=681136
==========

[esprehn, 2017-01-18 19:57:52]

Does capturing |this| have any side effects? Ex. You need to do it to call any
instance methods?

Lgtm

[krasin1, 2017-01-18 20:40:45]

On 2017/01/18 19:57:52, esprehn wrote:
> Does capturing |this| have any side effects? Ex. You need to do it to call any
> instance methods?
> 
> Lgtm

Correct. If an instance method is accessed, the compiler will force to capture
this.
Consider the following program:

#include <stdio.h>

class Lala {
 public:
  void Do() {
    auto g = []() { return Print(); } ;
    g();
  }

  void Print() {
    printf("hello!\n");
  }

};

int main(void) {
  Lala lala;
  lala.Do();
}

In this case, Do creates a lambda that accesses an instance method Print, but it
does not capture |this|. Clang throws the following error:

$ clang++ lala.cc -o lala -std=c++11 && ./lala
lala.cc:6:28: error: 'this' cannot be implicitly captured in this context
    auto g = []() { return Print(); } ;
                           ^
1 error generated.

After changing the line to

auto g = [this]() { return Print(); } ;

the program compiles and runs:

$ clang++ lala.cc -o lala -std=c++11 && ./lala
hello!

Now, if we make Print static, it will complain about the capture of |this| being
unused:

$ clang++ lala.cc -o lala -std=c++11 -Wall -Werror && ./lala
lala.cc:6:15: error: lambda capture 'this' is not used
[-Werror,-Wunused-lambda-capture]
    auto g = [this]() { return Print(); } ;
              ^
1 error generated.

The cases fixed in the current CL fall in the latter case: they don't use this
by any means.

[krasin1, 2017-01-18 20:41:19]

The CQ bit was unchecked by krasin@chromium.org

[krasin1, 2017-01-18 20:41:24]

The CQ bit was checked by krasin@chromium.org

[commit-bot: I haz the power, 2017-01-18 20:41:33]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-01-18 21:06:23]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1484772084371900, "parent_rev":
"608ff43a3cbf5527c5197741409df6b1cb58a4fb", "commit_rev":
"5cbffe1aa36f518681414e7bed5627a57449b58a"}

[commit-bot: I haz the power, 2017-01-18 21:06:51]

Description was changed from

==========
Fix unused lambda captures in Blink.

Clang just got a new warning about unused lambda captures,
and that requires us to clean all places with this issue
across all the Chromium code base. This CL fixes all such
cases in WebKit/Blink.

An example of a code that is no longer considered normal is

auto g = [i, j]() { printf("%d\n", i); };

in which j is never used in the lambda and therefore its capture is unnecessary.

BUG=681136
==========

to

==========
Fix unused lambda captures in Blink.

Clang just got a new warning about unused lambda captures,
and that requires us to clean all places with this issue
across all the Chromium code base. This CL fixes all such
cases in WebKit/Blink.

An example of a code that is no longer considered normal is

auto g = [i, j]() { printf("%d\n", i); };

in which j is never used in the lambda and therefore its capture is unnecessary.

BUG=681136

Review-Url: https://codereview.chromium.org/2645613003
Cr-Commit-Position: refs/heads/master@{#444477}
Committed:
https://chromium.googlesource.com/chromium/src/+/5cbffe1aa36f518681414e7bed56...
==========

[commit-bot: I haz the power, 2017-01-18 21:06:52]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/5cbffe1aa36f518681414e7bed56...