Rebased refactorings for Chalk#47861

Merged

bors merged 22 commits intorust-lang:masterfrom

sgrif:sg-rebase-chalkify-universe-refactorings

Mar 2, 2018

Commits on Mar 1, 2018

introduce `Universe` struct

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add some comments to `Obligation`

nikomatsakis
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introduce `UniverseIndex` into `ParamEnv`

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use `{}` for `Known` variant just for more parity

nikomatsakis
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make `Default` Copy and Clone

nikomatsakis
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kill custom type inference defaults (these don't really work anyway)

nikomatsakis
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kill supporting code from type-variable defaults

nikomatsakis
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obtain `UnificationTable` and `snapshot_vec` from `ena` instead

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store type values in the unification table directly

nikomatsakis
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correct subtle bug in the type variable code

nikomatsakis
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have `probe()` return `TypeVariableValue`

nikomatsakis
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remove unnecessary clause propagating divergence

nikomatsakis
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add universes to type inference variables

nikomatsakis
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change skolemizations to use universe index

nikomatsakis
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fix tidy error

nikomatsakis
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fix tests in `librustc_driver`

nikomatsakis
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change skolemizations to use universe index
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Remove dead code
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Re-add some removed uses of `Kind`
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Fix bad rebase
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Fix breakage in rustdoc
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Bump ena
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