commit a42244f73b27256eca44def4daa552e91c2dbc55
author Paul Berry <paulberry@google.com> Tue Feb 02 16:15:34 2021 +0000
committer commit-bot@chromium.org <commit-bot@chromium.org> Tue Feb 02 16:15:34 2021 +0000
tree d7c8ae6455299daf0ff95fb8e3639b29a7f54d8d
parent 91be638a56a811b46da88093b16408cea045bbe8

Flow analysis: begin tracking non-promotion reasons.

This CL implements the core flow analysis infrastructure for tracking
reasons why an expression was not promoted.  It supports the following
reasons:

- Expression was a property access
- Expression has been written to since it was promoted

I expect to add support for other non-promotion reasons in the future,
for example:

- `this` cannot be promoted
- Expression has been write captured
- Expression was a reference to a static field or top level variable

These non-promotion reasons are plumbed through to the CFE and
analyzer for the purpose of making errors easier for the user to
understand.  For example, given the following code:

  class C {
    int? i;
    f() {
      if (i == null) return;
      print(i.isEven);
    }
  }

The front end now prints:

  ../../tmp/test.dart:5:13: Error: Property 'isEven' cannot be accessed on 'int?' because it is potentially null.
  Try accessing using ?. instead.
      print(i.isEven);
              ^^^^^^
  Context: 'i' refers to a property so it could not be promoted.

Much work still needs to be done to round out this feature, for example:

- Currently the analyzer only shows the new "why not promoted"
  messages when the "--verbose" flag is specified; this means the
  feature is unlikely to be noticed by users.

- Currently the analyzer doesn't show a "why not promoted" message
  when the non-promotion reason is that the expression is a property
  access.

- We need one or more web pages explaining non-promotion reasons in
  more detail so that the error messages can contain pointers to them.

- The analyzer and front end currently only show non-promotion reasons
  for expressions of the form `x.y` where `x` fails to be promoted to
  non-nullable.  There are many other scenarios that should be
  handled.

Change-Id: I0a12df74d0fc6274dfb3cb555abea81a75884231
Bug: https://github.com/dart-lang/sdk/issues/38773
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/181741
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>