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[RFC] Clarify (and improve) rules for projections and well-formedness #1214

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merged 10 commits into from
Aug 7, 2015
Merged

[RFC] Clarify (and improve) rules for projections and well-formedness #1214

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7902a6d
Initial version.
nikomatsakis Jul 17, 2015
bcc727f
Correct issue #.
nikomatsakis Jul 17, 2015
6b07c20
add clause that says that an object type is WF only if the
nikomatsakis Jul 17, 2015
ffeccfd
discuss type aliases a bit more
nikomatsakis Jul 17, 2015
fb3ef33
Apply various clarifications based on suggestions
nikomatsakis Jul 20, 2015
5614b94
Add text documenting the impact on crates.io
nikomatsakis Aug 6, 2015
b17fdb8
pnkfelix nits
nikomatsakis Aug 6, 2015
af21352
correct some out of date text that Ralf identified
nikomatsakis Aug 6, 2015
abcd4be
rewrite the inference section to be less reliant on the compiler
nikomatsakis Aug 7, 2015
a3d22d8
clarify object type fragments
nikomatsakis Aug 7, 2015
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Correct issue #.
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@nikomatsakis
nikomatsakis committed Jul 17, 2015
commit bcc727f9efb85a09a818e96f2d113bf22104f0ae
8 changes: 4 additions & 4 deletions text/0000-projections-lifetimes-and-wf.md
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Expand Up @@ -8,7 +8,7 @@
Type system changes to address the outlives relation with respect to
projections, and to better enforce that all types are well-formed
(meaning that they respect their declared bounds). The current
implementation can be both unsound ([#24662]), inconvenient
implementation can be both unsound ([#24622]), inconvenient
([#23442]), and surprising ([#21748], [#25692]). The changes are as follows:

- Simplify the outlives relation to be syntactically based.
Expand Down Expand Up @@ -147,12 +147,12 @@ being [adapted] to cover associated type projections like
still lead to [annoying errors in some situations][#23442]. Finding a
better solution has been on the agenda for some time.

Simultaneously, we realized in [#24662] that the compiler had a bug
Simultaneously, we realized in [#24622] that the compiler had a bug
that caused it erroneously assume that every projection like `I::Item`
outlived the current function body, just as it assumes that type
parameters like `I` outlive the current function body. **This bug can
lead to unsound behavior.** Unfortunately, simply implementing the
naive fix for #24662 exacerbates the shortcomings of the current rules
naive fix for #24622 exacerbates the shortcomings of the current rules
for projections, causing widespread compilation failures in all sorts
of reasonable and obviously correct code.

Expand Down Expand Up @@ -855,7 +855,7 @@ regions, so probably yes.
[RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md
[#21748]: https://github.com/rust-lang/rust/issues/21748
[#23442]: https://github.com/rust-lang/rust/issues/23442
[#24662]: https://github.com/rust-lang/rust/issues/24622
[#24622]: https://github.com/rust-lang/rust/issues/24622
[#22436]: https://github.com/rust-lang/rust/pull/22436
[#22246]: https://github.com/rust-lang/rust/issues/22246
[#25860]: https://github.com/rust-lang/rust/issues/25860
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