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Document coroutines codegen: rest of suspend lambda

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Ilmir Usmanov
2020-08-17 20:12:14 +02:00
committed by Ilmir Usmanov
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compiler/backend/src/org/jetbrains/kotlin/codegen/coroutines/coroutines-codegen.md
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@@ -1335,3 +1335,39 @@ In experimental coroutines, the diagram is much simpler.
+-------------+
```
Every suspending function's continuation or suspend lambda extends `CoroutineImpl`.
### Suspend Lambda Layout
The ideal suspend lambda layout is the following:
1. supertypes: `kotlin/coroutines/jvm/internal/SuspendLambda` and `kotlin/jvm/functions/Function{N}`
2. package-private captured variables
3. private label field of int. Private, since it is used only in the lambda itself.
4. private fields for spilled variables. Same.
5. public final method `invokeSuspend` of type `(Ljava/lang/Object;)Ljava/lang/Object;`.
6. overrides `BaseContinuationImpl`'s `invokeSuspend`.
7. public final `create` of type `(<params>,Lkotlin/coroutines/Continuation)Lkotlin/coroutines/Continuation`.
`<params>` types are erased. In other words, their types are `Ljava/lang/Object;` as long as the number of parameters
is either 0 or 1. That is because the method overrides the base class's `create`.
8. public final `invoke` of type `(<params>,Ljava/lang/Object;)Ljava/lang/Object;`. Since it overrides `Function{N}`'s
`invoke`, types of `<params>` are erased.
9. public or package-private constructor: `<init>` of type `(<captured-variables>,Lkotlin/coroutines/Continuation;)V`,
where we call `SuspendLambda`'s constructor with arity and completion and initialize captured variables.
The compiler knows the arity, but the completion is provided as an argument to the constructor.
### kotlin.suspend
`suspend` soft keyword cannot be used with lambdas or function expression yet. It is not supported in the parser.
However, writing the type of the variable is quite annoying, so, since 1.3, there is a function `kotlin.suspend`, which
can precede lambda without parameters and turn it into suspend one. Since `suspend` is a soft keyword, it is possible to
name a function `suspend`. A user can define `suspend` function, which accepts lambdas, but token sequence `suspend {`
can only be used with `kotlin.suspend`. That is just for the transition period and while suspend lambdas and function
expressions are not supported in the parser.
FIXME: Support it in the parser and stop using the hack.
### Tail-Call Suspend Lambdas
FIXME: This feature is not implemented yet. Ideally, they should behave like callable references to suspend functions.
Meaning, they should
1. not have `create`, `invokeSuspend`, and all the fields, except for captured variables. Only constructor and `invoke`.
2. not inherit `BaseContinuationImpl` or any of its children.