Warn about expressions with no effect in void context

[lukehutch]

Re-filing from dart-lang/language#3334 at the recommendation of @lrhn .

I have run into this issue numerous times, so I thought I would suggest a linter check to catch this...

(1):

            onValueChanged: (_) => ProfileData.profileChanged,  // Wrong: parens are missing
            onCommit: ProfileData.commitProfile,                // Correct

(2):

            onValueChanged: (_) => ProfileData.profileChanged(),  // Correct
            onCommit: ProfileData.commitProfile,                  // Correct

The type of onValueChanged is void Function(String newValue), so there is no expected return value.

Returning a non-void value from a void lambda, especially if it is only a reference (not the return value of a function call), should show a linter warning, since void functions should have side effects.

(Dart 3.1.0 / Linux)

[lukehutch]

In general, I think all functions with void return type should report a warning or even an error if they are asked to return a non-void value.

[lrhn]

Hear, hear on the stronger void requirements.

However, it would mean that set foo(int x) => _x = x; would be disallowed, and people really do love their one-line setters.
(If the formatter could format set foo(int x) { _x = x; } on one line, I think we would have a much better chance of disallowing void..=> functions in general.)

[lukehutch]

I would assume an assignment would have the same type as the right-hand expression, so that x = y = z works, the same as in C?

[lrhn]

@lukehutch Exactly, so set foo(int x) => _x = x; has type int and an implicit return type of void, so that declaration would be an error with the rule against returning values from void functions.
Same for a void updateFoo(int x) => _x = x; function with an explicit void return type. (Setters are a bit special, since they ignore their return values.)

[lukehutch]

@lrhn ah, of course.

Maybe assignments should be assumed to be of void type if their value is ignored, or if they are returned by a void lambda? It's probably safe to assume a user's intent when the user has initiated an action that has side effects.

I am mostly concerned about the case where a lambda is created that was unintentionally written as a no-op.

[lukehutch]

These are also valid statements with no effect:

int;
5;

Any statement that clearly has no effect should be flagged with a warning.