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33 lines
1.0 KiB
Markdown
33 lines
1.0 KiB
Markdown
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# Reserve `BytePos(0)` for dummy spans
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- Status: accepted
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- Deciders: @kdy1
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- Date: 2020-05-11
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## Context and Problem Statement
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`BytePos(0)` was causing lots of problems because a span generated for user input can have `BytePos(0)`.
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We tried various ways to distinguish the user input from synthesized spans, but all were not successful.
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It especially caused various issues on source maps.
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## Decision Drivers <!-- optional -->
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- Source map issues, cause by `BytePos(0)`.
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## Considered Options
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- Adding one empty file to `SourceMap` each time we create one.
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## Decision Outcome
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Reserving `0` of `BytePos` provides guarantee which can be used by various codegen crates.
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As `BytePos(0)` is reserved, `swc_ecma_codegen` can know if a `BytePos` is dummy without checking both of `lo` and `hi`.
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### Positive Consequences
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- Cleaner code, as detecting if a `BytePos` is dummy become easier.
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### Adding one empty file to `SourceMap` each time we create one.
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This is a code smell, and if we select this, we can't depend on the fact `BytePos(0)` is dummy.
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