Define promotion contexts and promotability #28
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This needs rebasing now that my PR landed. |
ecstatic-morse
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Rebased. There's now two sections about |
This is useful inside a const-context as well.
ecstatic-morse
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Oct 11, 2019
ecstatic-morse
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Oct 11, 2019
oli-obk
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Oct 19, 2019
promotion.md
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| ### Access to a `const` or `static` | ||
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| Accesses to `const`s are always promotable, regardless of the body of the |
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well, not quite "regardless". You still can't promote Cells
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lgtm other than the clarification about which constants get promoted |
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RalfJung
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| # Const promotion | |||
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| ["(Implicit) Promotion"][promotion-rfc] is a mechanism that affects code like `&3`: | |||
| "Promotion" is the act of guaranteeing that code not written in a const context | |||
| (e.g. initalizer of a `const` or `static`, or an array length expression) will | |||
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These are examples for code in const-context, right? Because to me this reads like examples for "code not written in a const context".
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This adds an introduction to
promotion.mdthat explains promotion in terms of "promotion contexts" (e.g.&expr,[expr; 32]) and "promotability" (the rules for which expressions can be promoted in a given context). This is a variation on the terminology described in this comment, which I was using to reason about the code inqualify_consts.rswhile working on dataflow-based const qualification.I'm not asserting that this mental model is the correct one. In particular, you can see some inconsistencies in the section called "Lifetime extension in
constandstatic". Hopefully it can serve as a starting point.This also adds some requirements that were missing from the "Rules" section (now renamed to "Promotability"). These requirements are mostly structural ones (single assignment, no named variables), so maybe they should go in a different section?
cc @RalfJung @oli-obk