Commits on Feb 22, 2022

1. Introduce ChunkedBitSet and use it for some dataflow analyses. …

   This reduces peak memory usage significantly for some programs with very
   large functions, such as:
   - `keccak`, `unicode_normalization`, and `match-stress-enum`, from
     the `rustc-perf` benchmark suite;
   - `http-0.2.6` from crates.io.
   
   The new type is used in the analyses where the bitsets can get huge
   (e.g. 10s of thousands of bits): `MaybeInitializedPlaces`,
   `MaybeUninitializedPlaces`, and `EverInitializedPlaces`.
   
   Some refactoring was required in `rustc_mir_dataflow`. All existing
   analysis domains are either `BitSet` or a trivial wrapper around
   `BitSet`, and access in a few places is done via `Borrow<BitSet>` or
   `BorrowMut<BitSet>`. Now that some of these domains are `ClusterBitSet`,
   that no longer works. So this commit replaces the `Borrow`/`BorrowMut`
   usage with a new trait `BitSetExt` containing the needed bitset
   operations. The impls just forward these to the underlying bitset type.
   This required fiddling with trait bounds in a few places.
   
   The commit also:
   - Moves `static_assert_size` from `rustc_data_structures` to
     `rustc_index` so it can be used in the latter; the former now
     re-exports it so existing users are unaffected.
   - Factors out some common "clear excess bits in the final word"
     functionality in `bit_set.rs`.
   - Uses `fill` in a few places instead of loops.

   

   nnethercote committed Feb 22, 2022
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