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//! and definition contexts*. J. Funct. Program. 22, 2 (March 2012), 181-216.
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//! DOI=10.1017/S0956796812000093 <https://doi.org/10.1017/S0956796812000093>
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// Hygiene data is stored in a global variable and accessed via TLS, which
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// means that accesses are somewhat expensive. (`HygieneData::with`
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// encapsulates a single access.) Therefore, on hot code paths it is worth
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// ensuring that multiple HygieneData accesses are combined into a single
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// `HygieneData::with`.
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//
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// This explains why `HygieneData`, `SyntaxContext` and `Mark` have interfaces
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// with a certain amount of redundancy in them. For example,
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// `SyntaxContext::outer_expn_info` combines `SyntaxContext::outer` and
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// `Mark::expn_info` so that two `HygieneData` accesses can be performed within
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// a single `HygieneData::with` call.
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//
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// It also explains why many functions appear in `HygieneData` and again in
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// `SyntaxContext` or `Mark`. For example, `HygieneData::outer` and
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// `SyntaxContext::outer` do the same thing, but the former is for use within a
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// `HygieneData::with` call while the latter is for use outside such a call.
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// When modifying this file it is important to understand this distinction,
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// because getting it wrong can lead to nested `HygieneData::with` calls that
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// trigger runtime aborts. (Fortunately these are obvious and easy to fix.)
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use crate::GLOBALS;
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use crate::Span;
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use crate::edition::Edition;
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