[code-reflection] RFR: Simplify code generation for quotable lambdas not to require duplicate capture argument lists

[Maurizio Cimadamore]

On Fri, 4 Oct 2024 16:56:48 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> When translating quotable lambdas/method references, we generate two captured argument lists:
> 
> * the captured arguments to be used by the functional interface implementation;
> * the captured arguments to be used by the quotable implementation
> 
> In most cases these two captured lists are identical, except for cosmetic differences.
> This is even more true after the backend of javac underwent significant rearchitecture, to better support the Flexible Constructor Bodies feature.
> 
> This PR simplifies `ReflectMethods` so that a visitor now computes all the required captured variables upfront.
> The captured variables are then inserted in the quoted block, so that the rest of `BodyScanner` will be able to resolve references to such variables w/o the need for ad-hoc logic.
> 
> One complication is that, under this new regime, we can no longer afford to treat captured variables whose value is constant (e.g. `final int k = 100`) as if they were true captures.
> This is because neither `Lower` nor `LambdaToMethod` capture constant variables. Since we're reusing the same dynamic arguments as `LambdaToMethod` we need to make sure that `ReflectMethods` also does not capture constant arguments.
> Special logic to avoid capture of constant arguments is baked into the new visitor.
> 
> I have also added a check in the visitor, so that if, inside a quotable lambda, we see a creation of a local class that is declared outside the lambda, an unsupported exception is thrown.
> 
> I will followup with a separate fix to deal with these cases.

test/langtools/tools/javac/reflect/quoted/TestCaptureQuotable.java line 69:

> 67:         Iterator<Object> it = quoted.capturedValues().values().iterator();
> 68:         assertEquals(((Var)it.next()).value(), hello);
> 69:         assertEquals(it.next(), x);

Note - the captured value for `x` is not longer a `Var`, but a true constant.

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PR Review Comment: https://git.openjdk.org/babylon/pull/249#discussion_r1787997816