Add type inference for complex assignments whose LHS is an index expression.

Add type inference for complex assignments whose LHS is an index expression.

In later CLs I will expand on this code to handle other kinds of
complex assignments.

R=scheglov@google.com

Committed: https://github.com/dart-lang/sdk/commit/029771d746fd77e75af2089201eafc5314bf5db5

[Paul Berry, 2017-06-07 22:06:02]

paulberry@google.com changed reviewers:
+ ahe@google.com, scheglov@google.com

[scheglov, 2017-06-08 00:14:06]

LGTM

[ahe, 2017-06-08 14:16:33]

Don't feel blocked by my comments. I'm just trying to understand this, and
failing :-(

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
File pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart (right):

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart:235: /// Concrete
shadow object representing a complex assignment in kernel form.
What is a complex assignment?

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart:1807: final bool
_isDiscarding;
It would be easier for for me to understand the meaning of this if it was called
"voidContext" or something like that.

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
File pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart (right):

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart:251: // to fail. 
TODO(paulberry): fix this.
As far as I understand, interface targets are for strong mode only. Obviously,
that can be changed if it's a problem.

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart:499: e =
variable.initializer;
I don't understand this. Isn't [e] supposed to be the main effect? Also, how do
you know that let.body is really a VariableGet?

[Paul Berry, 2017-06-08 16:17:04]

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
File pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart (right):

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart:235: /// Concrete
shadow object representing a complex assignment in kernel form.
On 2017/06/08 14:16:33, ahe wrote:
> What is a complex assignment?

I'm still figuring out exactly where to draw the line.  It will probably be
something like: an assignment that desugars to a let expression.

For now I've added a TODO comment.

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart:1807: final bool
_isDiscarding;
On 2017/06/08 14:16:33, ahe wrote:
> It would be easier for for me to understand the meaning of this if it was
called
> "voidContext" or something like that.

I'm open to changing this, but I think "voidContext" would be confusing, because
the only time it's true is when we're *not* in a void context.  For example,
when desugaring the assignment `var x = t1[t2] = t3;`, we generate:

var x =
 let t1 = a in
  let t2 = b in
   let t3 = c in
    let t4 = a.[]=(b, c)
     t3

The variable declaration for t4 is marked with _isDiscarding = true to indicate
that the value of t4 will not be used.  But the overall expression `a[b] = c` is
in a non-void context.

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
File pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart (right):

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart:251: // to fail. 
TODO(paulberry): fix this.
On 2017/06/08 14:16:33, ahe wrote:
> As far as I understand, interface targets are for strong mode only. Obviously,
> that can be changed if it's a problem.

Acknowledged.

https://codereview.chromium.org/2926763003/diff/1/pkg/front_end/lib/src/fasta...
pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart:499: e =
variable.initializer;
On 2017/06/08 14:16:33, ahe wrote:
> I don't understand this. Isn't [e] supposed to be the main effect? Also, how
do
> you know that let.body is really a VariableGet?

Again considering the example of `var x = a[b] = c;`, which desugars to:

var x =
 let t1 = a in
  let t2 = b in
   let t3 = c in
    let t4 = t1.[]=(t2, t3)
     t3

When we reach the "let" expression for t4, the main effect of the expression is
found in the initializer for t4, namely `t1.[]=(t2, t3)`.

As for how we know the body is a VariableGet, we know it because the only time
desugaring produces a "let" expression whose variable has the `isDiscarding`
flag set, it is doing so in order to allow the final value of the expression to
be some previously computed value (t3 in this instance).  So the body of the
"let" is always a VariableGet which reads the previously computed value.

I've added comments to this effect, and I've also added an assertion
`assert(let.body is VariableGet);` to make the assumption about the body
clearer.

[Paul Berry, 2017-06-08 16:25:29]

Description was changed from

==========
Add type inference for complex assignments whose LHS is an index expression.

In later CLs I will expand on this code to handle other kinds of
complex assignments.
==========

to

==========
Add type inference for complex assignments whose LHS is an index expression.

In later CLs I will expand on this code to handle other kinds of
complex assignments.

R=scheglov@google.com

Committed:
https://github.com/dart-lang/sdk/commit/029771d746fd77e75af2089201eafc5314bf5db5
==========

[Paul Berry, 2017-06-08 16:25:31]

Committed patchset #1 (id:1) manually as
029771d746fd77e75af2089201eafc5314bf5db5 (presubmit successful).