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Revise never type fallback algorithm #88804

Merged
merged 7 commits into from
Sep 24, 2021
Merged

Revise never type fallback algorithm #88804

merged 7 commits into from
Sep 24, 2021

Commits on Sep 17, 2021

  1. introduce new fallback algorithm 

    We now fallback type variables using the following rules:

* Construct a coercion graph `A -> B` where `A` and `B` are unresolved
  type variables or the `!` type.
* Let D be those variables that are reachable from `!`.
* Let N be those variables that are reachable from a variable not in
D.
* All variables in (D \ N) fallback to `!`.
* All variables in (D & N) fallback to `()`.
    @nikomatsakis @Mark-Simulacrum
    nikomatsakis authored and Mark-Simulacrum committed Sep 17, 2021
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  2. remove diverging type variables from fn check 

    The comment seems incorrect. Testing revealed that the examples in
question still work (as well as some variants) even without the
special casing here.
    @nikomatsakis @Mark-Simulacrum
    nikomatsakis authored and Mark-Simulacrum committed Sep 17, 2021
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  3. remove reliance on "diverging" type variables 

    Instead, we now record those type variables that are the target of a
`NeverToAny` adjustment and consider those to be the "diverging" type
variables. This allows us to remove the special case logic that
creates a type variable for `!` in coercion.
    @nikomatsakis @Mark-Simulacrum
    nikomatsakis authored and Mark-Simulacrum committed Sep 17, 2021
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  4. stop categorizing inference variables as diverging when created 

    Instead, we now rely on the code that looks for a NeverToAny adjustment.
    @nikomatsakis @Mark-Simulacrum
    nikomatsakis authored and Mark-Simulacrum committed Sep 17, 2021
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  5. optimization: use a single DepthFirstSearch instead of hashsets 

    Extend the `DepthFirstSearch` iterator so that it can be re-used and
extended with add'l start nodes. Then replace the FxHashSets of nodes
we were using in the fallback analysis with a single iterator. This
way we won't re-walk portions of the graph that are reached more than
once, and we also do less allocation etc.
    @nikomatsakis @Mark-Simulacrum
    nikomatsakis authored and Mark-Simulacrum committed Sep 17, 2021
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  6. Add another case of fallback to () avoid breakage 

    This adds src/test/ui/never_type/fallback-closure-ret.rs as a test case which
showcases the failure mode fixed by this commit.
    @Mark-Simulacrum
    Mark-Simulacrum committed Sep 17, 2021
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  7. Add nofallback tests

    @Mark-Simulacrum
    Mark-Simulacrum committed Sep 17, 2021
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