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52 lines
2.1 KiB
Markdown
52 lines
2.1 KiB
Markdown
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# Provide more information to visitor methods
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- Status: accepted <!-- optional -->
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- Deciders: @kdy1, @kwonoj <!-- optional -->
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- Date: 2022-06-30 <!-- optional -->
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## Context and Problem Statement
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Currently one can't get any information about parent node.
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This is due to ownership restriction of rust.
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We can't pass a reference to the parent to a visitor method.
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Let's say we have a `CallExpr`, and we are now going to call `visit_mut_callee` from `visit_mut_call_expr`.
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We want `&mut callee`, because it's the signature of `visit_mut_callee`.
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So we **borrow it from CallExpr**.
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As we mutabily borrowed something from `CallExpr`, we cannot pass `&CallExpr` to `visit_mut_callee`.
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We can workaround this by
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- taking `CallExpr.callee`
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- call `visit_mut_callee(&mut self, callee: &mut Callee, parent: &CallExpr)`, with dummy callee in `parent.callee`.
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- restore the original callee from `visit_mut_call`.
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But doing this by hand is error-prone and doing this automatically by codegen is costly.
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This is explicit `memmove`, and `memmove` is quite costly.
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SWC moved from `Fold` to `VisitMut` because of `mmemove`.
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## Decision Drivers
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- Help plugin authors writing plugins.
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## Considered Options
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- [option 1] Stay with as-is.
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- [option 2] Provide full information using `unsafe`.
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- **[option 3] Provide small amount of information .**
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We will expose the spans and kinds of the parent ast nodes for `VisitMut` and `Fold`, while passing an enum with parent data for `Visit`.
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This difference is related to the restriction above.
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This is the exact problem rust is trying to solve, and we don't want to violate the rules of rust.
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## Decision Outcome
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Chosen option: **[option 3] Provide small amount of information .**
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This decision is taken because
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- It's too hard to **port** plugins, because a plugin author has to recreate logic instead of porting, if the original babel plugin uses parent node information.
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- Using `unsafe` in public API requires more discussion.
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- We don't have good debugging api for plugins at the moment, so using `unsafe` in plugins is strongly discourages.
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