Add a cycle detector for generic `Graph`s and `mir::Body`s
Cycle detection is one way to differentiate the upcoming `const_loop` feature flag (#52000) from the `const_if_match` one (#49146). It would be possible to use the existing implementation of strongly-connected components for this but less efficient.
The ["tri-color" terminology](http://www.cs.cornell.edu/courses/cs2112/2012sp/lectures/lec24/lec24-12sp.html) is common in introductory data structures and algorithms courses: black nodes are settled, grey nodes are visited, and white nodes have no state. This particular implementation is iterative and uses a well-known technique where "node settled" events are kept on the stack alongside nodes to visit. When a settled event is popped, we know that all successors of that node have been visited and themselves settled. If we encounter a successor node that has been visited (is on the stack) but not yet settled, we have found a cycle.
r? @eddyb
Currently, after a CALL terminator is created in MIR, we insert DROP
statements for all of its operands -- even though they were just moved
shortly before! These spurious drops are later removed, but not before
causing borrow check errors.
This PR series modifies the drop code to track operands that were
moved and avoid creating drops for them.
Right now, I'm only using this mechanism for calls, but it seems
likely it could be used in more places.
Add a `Place::is_indirect` method to determine whether a `Place` contains a `Deref` projection
Working on #63860 requires tracking some property about each local. This requires differentiating `Place`s like `x` and `x.field[index]` from ones like `*x` and `*x.field`, since the first two will always access the same region of memory as `x` while the latter two may access any region of memory. This functionality is duplicated in various places across the compiler. This PR adds a helper method to `Place` which determines whether that `Place` has a `Deref` projection at any point and changes some existing code to use the new method.
I've not converted `qualify_consts.rs` to use the new method, since it's not a trivial conversion and it will get replaced anyway by #63860. There may be other potential uses besides the two I change in this PR.
r? @oli-obk
Value was renamed to Operand in ad2de8b4ee
ScalarPair to Slice in fe50b4eb1d
Not familiar enough with rustc's source to know if the comment is even still applicable.
Make Allocation::bytes private
Fixes#62931.
Direct immutable access to the bytes is still possible but redirected through the new method `raw_bytes_with_undef_and_ptr`, similar to `get_bytes_with_undef_and_ptr` but without requiring an interpretation context and not doing *any* relocation or bounds checks. The `size` of the allocation is stored separately which makes access as `Size` and `usize` more ergonomic.
cc: @RalfJung
This returns whether a `Place` references the same region of memory
as its base, or equivalently whether it contains a `Deref` projection.
This is helpful for analyses that must track state for locals, since an
assignment to `x` or `x.field` is fundamentally different than one to
`*x`, which may mutate any memory region.
Resolve some small issues related to #63580
This resolves some feedback left on #63580 after it was merged:
- Adds documentation to `mir::Static` and `mir::StaticKind`
- Simplifies `maybe_get_optimized_mir()` and `maybe_get_promoted_mir()`
cc @bjorn3 @RalfJung
There is now a dedicate `len` method which avoids the need to access the
bytes. Access the length as `Size` can also be done by a direct member.
The constructors guarantee that these representations are convertable.
Access which relies on the bytes, such as snapshot, can use direct raw
access by reference as it does not care about undef and relocations or
properly checks them seperately.
Requires a manual implementation for Relocations since dereferencing to
SortedMap is not always implemented but that one is far more trivial.
Added fields could otherwise be silently forgotten since private fields
make destructing outside the module possible only with `..` pattern
which would then also be applicable to newly added public fields.
Direct access to the bytes was previously a problem (#62931) where
components would read their contents without properly checking
relocations and/or definedness.
Making bytes private instead of purely renaming them also helps in
allowing amendments to their allocation scheme (such as eliding
allocation for undef of constant regions).
