commit 2f80a99a164c7972cda2307bbeb1ef2d60407251
author Paul Berry <paulberry@google.com> Tue Aug 26 06:56:53 2025 -0700
committer Commit Queue <dart-scoped@luci-project-accounts.iam.gserviceaccount.com> Tue Aug 26 06:56:53 2025 -0700
tree d8497fe79a84df62c98e4fce7f74050870598cf9
parent 8fbf6030ef005b728944176a859d18bfb979e14e

[flow analysis] Clean up and document the condition variable feature.

The condition variable feature is a feature of flow analysis in which
writes to local variables cause flow analysis state to be saved, and
reads of local variables cause flow analysis state to be partially
restored. This is what allows type promotion in examples like the
following:

    int? x = ...;
    var xIsNonNull = x != null; // The following state is now saved: if
                                // `xIsNonNull` is `true`, `x` is known
                                // to be non-null
    ...Other statements...
    if (xIsNonNull) {           // The state is now restored
      print(x.isEven);          // Therefore this is ok, because `x` is
                                // known to be non-null.
    }

See https://github.com/dart-lang/language/issues/1274, the original
feature request for this feature.

This CL makes the following changes:

- It adds documentation of how the feature works.

- It renames `SsaNode.expressionInfo` to
  `SsaNode.conditionVariableState` and `_Reference.addPreviousInfo` to
  `_Reference.restoreConditionVariableState`, so that the connection
  to the "condition variable" feature is clearer.

- It removes an unnecessary call to `_Reference.addPreviousInfo` from
  `_FlowAnalysisImpl._handleEqualityCheckPattern`, and documents why
  it wasn't necessary.

- It changes the signature of the `SsaNode` constructor so that (a) at
  call sites that don't need to save flow analysis state, there's no
  need to pass in `null`, and (b) at call sites that do need to save
  flow analysis state, it's clear that the state that's being saved is
  `conditionVariableState`.

Change-Id: I6a6a6964f953b4e853efa95a32b1fbaa5f6c09ca
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446600
Reviewed-by: Erik Ernst <eernst@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>