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[WIP] Remove placeholders completely #130227

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[WIP] Remove placeholders completely #130227

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+933 −769

Commits on Nov 6, 2024

  1. Preliminary attempt at removing placeholders out of the borrow checker. 

    This version is maximally naive, and currently unsound.

Handle more placeholder errors.

This update extends the SCC metadata tracking a lot
and uses the extra information to add p: 'static for
any placeholder that reaches another placeholder.

It also inlines the few measly bits of `init_free_and_bound_regions()`
that still remain as relevant. This increases the constructor for
`RegionInferenceContext`s somewhat, but I still think it's
readable.

The documentation for `init_free_and_bound_regions()` was out of
date, and the correct, up to date version is available in the various
places where the logic was moved.

Use annotions on the outlives-static constraints to
help the search for who is to blame.

Invalid placeholder relation constraints are redirects

This commit makes the search for blamable outlives constraints
treat an added `x: 'static` edge as a redirect to figure out
why it reached an invalid placeholder.

As a drive-by it also refactors the blame search somewhat, renames
a few methods, and allows iterating over outgoing constraints
without the implied edges from 'static.

This version mostly works!

I've switched to encoding our new outlives-static constraints
with two extra parameters: the source and drain of a path in the
original constraint graph that caused a placeholder issue.

This handles all of the soundness issues, leaving 16 failing
diagnostics.

Remove some stuff we don't need or can't use anymore

Figure out better names for opaque types!

Fix tidy error

Fix most of the cases of missing "implied 'static" warnings

Fix documentation that build-failed
    @amandasystems
    amandasystems committed Nov 6, 2024
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  2. Produce slightly fewer placeholder constraints 

    This seems to hit the sweet spot between enabling debugging
and being efficient.

Somewhat cleaner representation that separates concerns

Avoid handling placeholders for MIR bodies without them

Determine once and for all if there are higher-kinded concerns and nope out

This also changes how annotations for SCCs work.

This version finally does away with all kinds of universe handling

It does so in a preprocessing step that now also removes higher-ranked
concerns from type tests.

Fix handling of opaque regions
    @amandasystems
    amandasystems committed Nov 6, 2024
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