mandelbrot: fix overlapping buffers
lint: add method to get level of a specific lint.

This allows lint traversals to emit more information (when a lint is
non-allow), or avoid doing expensive computations (when a lint is
allow).
Removed `index_to_bitset` from the dataflow context.

Part of addressing 15019.
Removed the `_frozen` methods from dataflow API.

Calls to methods like `each_bit_on_entry_frozen` and
`each_gen_bit_frozen` now go to the `each_bit_on_entry` and
`each_gen_bit` methods.
auto merge of #15737 : huonw/rust/lint-level-here, r=pnkfelix

This allows lint traversals to emit more information (when a lint is
non-allow), or avoid doing expensive computations (when a lint is
allow).
librustc: Implement unboxed closures with mutable receivers
auto merge of #14539 : pcwalton/rust/unboxed-closures, r=pnkfelix

@nikomatsakis r?
auto merge of #15742 : pnkfelix/rust/fsk-fix-15019, r=pcwalton

Removed `index_to_bitset` field and `_frozen` methods.

Drive-by: Added some missing docs on the `each_bit` method.

Drive-by: Put in a regular pattern: when calling `compute_id_range`, ensure `words_per_id > 0` by either asserting it or checking and returning early.  (The prior code did the latter in a few cases where necessary, but debugging is much aided by the asserts.)

Fix #15019.
auto merge of #15743 : Ryman/rust/mandelbrot_fix, r=alexcrichton

Matches the official sample output (N=200) again.

cc #15408
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