Migrate borrowck dataflow impls to new framework
This uses #65672 to implement the dataflow analyses needed by borrowck. These include all the `InitializedPlaces` analyses as well as `Borrows`. Additionally, this PR contains several independent changes around the dataflow API which improve performance and make it more ergonomic.
* An optimization that inhibits the caching of block transfer functions for acyclic MIR (~0.3% I-CNT savings).
* A `ResultsVisitor` for dataflow results that is more efficient than `ResultsCursor` when we have to visit every statement unconditionally (~0.3% I-CNT savings).
* An `into_engine` method on `Analysis` that selects the appropriate `Engine` constructor.
* A `contains` method for `ResultsCursor` as a shorthand for `.get().contains()`.
* A `find_descendants` helper on `MovePath` that replaces `has_any_child_of` on the old `FlowsAtLocation`
These changes made porting the dataflow analyses much easier. Finally, this PR removes some now-unused code in `dataflow/at_location.rs` and elsewhere.
You can view the perf results for the final version of this PR [here](https://perf.rust-lang.org/compare.html?start=29b854fb741809c29764e33fc17c32ba9c6523ba&end=6e516c1410c18cfe4eb6d030a39fdb73c8d8a4fe). Here's an example of the graphviz diagrams that are generated for the `MaybeInitializedPlaces` analysis.
Preparation for allocator aware `Box`
This cleans up the `Box` code a bit, and uses `Box::from_raw(ptr)` instead of `Box(ptr)`.
Additionally, `box_free` and `exchange_malloc` now uses the `AllocRef` trait and a comment was added on how `box_free` is tied to `Box`.
This a preparation for an upcoming PR, which makes `Box` aware of an allocator.
r? @Amanieu
Add missing `_zeroed` varants to `AllocRef`
The majority of the allocator wg has decided to add the missing `_zeroed` variants to `AllocRef`:
> these should be added since they can be efficiently implemented with the `mremap` system call on Linux. `mremap` allows you to move/grow/shrink a memory mapping, and any new pages added for growth are guaranteed to be zeroed.
>
> If `AllocRef` does not have these methods then the user will have to manually write zeroes to the added memory since the API makes no guarantees on their contents.
For the full discussion please see https://github.com/rust-lang/wg-allocators/issues/14.
This PR provides default implementations for `realloc_zeroed`, `alloc_excess_zeroed`, `realloc_excess_zeroed`, and `grow_in_place_zeroed`.
r? @Amanieu
Remove common usage pattern from `AllocRef`
This removes the common usage patterns from `AllocRef`:
- `alloc_one`
- `dealloc_one`
- `alloc_array`
- `realloc_array`
- `dealloc_array`
Actually, they add nothing to `AllocRef` except a [convenience wrapper around `Layout` and other methods in this trait](https://doc.rust-lang.org/1.41.0/src/core/alloc.rs.html#1076-1240) but have a major flaw: The documentation of `AllocRefs` notes, that
> some higher-level allocation methods (`alloc_one`, `alloc_array`) are well-defined on zero-sized types and can optionally support them: it is left up to the implementor whether to return `Err`, or to return `Ok` with some pointer.
With the current API, `GlobalAlloc` does not have those methods, so they cannot be overridden for `liballoc::Global`, which means that even if the global allocator would support zero-sized allocations, `alloc_one`, `alloc_array`, and `realloc_array` for `liballoc::Global` will error, while calling `alloc` with a zeroed-size `Layout` could succeed. Even worse: allocating with `alloc` and deallocating with `dealloc_{one,array}` could end up with not calling `dealloc` at all!
For the full discussion please see https://github.com/rust-lang/wg-allocators/issues/18
r? @Amanieu
Python script PEP8 style guide space formatting and minor Python source cleanup
This PR includes the following changes in the Python sources based on a flake8 3.7.9 (mccabe: 0.6.1, pycodestyle: 2.5.0, pyflakes: 2.1.1) CPython 3.7.6 on Darwin lint:
- PEP8 style guide spacing updates *without* line length changes
- removal of unused local variable assignments in context managers and exception handling
- removal of unused Python import statements
- removal of unnecessary semicolons
Test failure of unchecked arithmetic intrinsics in const eval
Test that the unchecked arithmetic intrinsics that were made unstably const in #68809 emit an error during const-eval if given invalid input.
Addresses [this comment](https://github.com/rust-lang/rust/pull/68809#discussion_r375753066).
r? @RalfJung
[experiment] Support linking from a .rlink file
Flag `-Z no-link` was previously introduced, which allows creating an `.rlink` file to perform compilation without linking. This change enables linking from an `.rlink` file.
Part of Issue #64191
