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rewrite the section about regions in the trait solver #1508

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Nov 10, 2022
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rewrite the section about regions in the trait solver #1508

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0ee9284
extract regions
lcnr Nov 7, 2022
0c26684
Update src/type-inference.md
lcnr Nov 9, 2022
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30 changes: 17 additions & 13 deletions src/type-inference.md
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Expand Up @@ -218,29 +218,33 @@ algorithms.
[`region_constraints`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/region_constraints/index.html
[`opportunistic_resolve_var`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/region_constraints/struct.RegionConstraintCollector.html#method.opportunistic_resolve_var

## Extracting region constraints
## Solving region constraints

Ultimately, region constraints are only solved at the very end of
type-checking, once all other constraints are known. There are two
Region constraints are only solved at the very end of
typechecking, once all other constraints are known and
all other obligations have been proven. There are two
ways to solve region constraints right now: lexical and
non-lexical. Eventually there will only be one.

An exception here is the leak-check which is used during trait solving
and relies on region constraints containing higher-ranked regions. Region
constraints in the root universe (i.e. not arising from a `for<'a>`) must
not influence the trait system, as these regions are all erased during
codegen.

To solve **lexical** region constraints, you invoke
[`resolve_regions_and_report_errors`]. This "closes" the region
constraint process and invokes the [`lexical_region_resolve`] code. Once
this is done, any further attempt to equate or create a subtyping
relationship will yield an ICE.

Non-lexical region constraints are not handled within the inference
context. Instead, the NLL solver (actually, the MIR type-checker)
invokes [`take_and_reset_region_constraints`] periodically. This
extracts all of the outlives constraints from the region solver, but
leaves the set of variables intact. This is used to get *just* the
region constraints that resulted from some particular point in the
program, since the NLL solver needs to know not just *what* regions
were subregions, but also *where*. Finally, the NLL solver invokes
[`take_region_var_origins`], which "closes" the region constraint
process in the same way as normal solving.
The NLL solver (actually, the MIR type-checker) does things slightly
differently. It uses canonical queries for trait solving which use
[`take_and_reset_region_constraints`] at the end. This extracts all of the
outlives constraints added during the canonical query. This is required
as the NLL solver must not only know *what* regions outlive each other,
but also *where*. Finally, the NLL solver invokes [`take_region_var_origins`],
providing all region variables to the solver.

[`resolve_regions_and_report_errors`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.resolve_regions_and_report_errors
[`lexical_region_resolve`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/lexical_region_resolve/index.html
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