MIR-borrowck: Some minor fixes
- Remove parens when printing dereference (fix#45185)
- Change argument type of `autoderef` to `bool`
- Change argument type of `field_index` to `Field`
Fix typo in MIR "cannot move out of borrowed content"
I believe this all we need to change (#46018). Anyway, do let me know if there is anything else that needs to changed as well!
integrate MIR type-checker with NLL inference
This branch refactors NLL type inference so that it uses the MIR type-checker to gather constraints. Along the way, it also refactors how region constraints are gathered in the normal inference context mildly. The new setup is like this:
- What used to be `region_inference` is split into two parts:
- `region_constraints`, which just collects up sets of constraints
- `lexical_region_resolve`, which does the iterative, lexical region resolution
- When `resolve_regions_and_report_errors` is invoked, the inference engine converts the constraints into final values.
- In the MIR type checker, however, we do not invoke this method, but instead periodically take the region constraints and package them up for the NLL solver to use later.
- This allows us to track when and where those constraints were incurred.
- We also remove the central fulfillment context from the MIR type checker, instead instantiating new fulfillment contexts at each point. This allows us to capture the set of obligations that occurred at a particular point, and also to ensure that if the same obligation arises at two points, we will enforce the region constraints at both locations.
- The MIR type checker is also enhanced to instantiate late-bound-regions with fresh variables and handle a few other corner cases that arose.
- I also extracted some of the 'outlives' logic from the regionck, which will be needed later (see future work) to handle the type-outlives relationships.
One concern I have with this branch: since the MIR type checker is used even without the `-Znll` switch, I'm not sure if it will impact performance. One simple fix here would be to only enable the MIR type-checker if debug-assertions are enabled, since it just serves to validate the MIR. Longer term I hope to address this by improving the interface to the trait solver to be more query-based (ongoing work).
There is plenty of future work left. Here are two things that leap to mind:
- **Type-region outlives.** Currently, the NLL solver will ICE if it is required to handle a constraint like `T: 'a`. Fixing this will require a small amount of refactoring to extract the implied bounds code. I plan to follow a file-up bug on this (hopefully with mentoring instructions).
- **Testing.** It's a good idea to enumerate some of the tricky scenarios that need testing, but I think it'd be nice to try and parallelize some of the actual test writing (and resulting bug fixing):
- Same obligation occurring at two points.
- Well-formedness and trait obligations of various kinds (which are not all processed by the current MIR type-checker).
- More tests for how subtyping and region inferencing interact.
- More suggestions welcome!
r? @arielb1
MIR-borrowck: don't ICE for cannot move from array error
Closes#45694
compile-fail test E0508 now gives
```text
error[E0508]: cannot move out of type `[NonCopy; 1]`, a non-copy array (Ast)
--> .\src\test\compile-fail\E0508.rs:18:18
|
18 | let _value = array[0]; //[ast]~ ERROR E0508
| ^^^^^^^^
| |
| cannot move out of here
| help: consider using a reference instead: `&array[0]`
error[E0508]: cannot move out of type `[NonCopy; 1]`, a non-copy array (Mir)
--> .\src\test\compile-fail\E0508.rs:18:18
|
18 | let _value = array[0]; //[ast]~ ERROR E0508
| ^^^^^^^^ cannot move out of here
error: aborting due to 2 previous errors
```
Also, factor out `do_mir_borrowck`, which is the code that actually
performs the MIR borrowck from within the scope of an inference context.
This change should be a pure refactoring.
MIR-borrowck: moves of prefixes invalidate uses too
I overlooked the fact that when we check if a path is moved, we need to check for interference between the (shallow) prefixes and the use in question.
~~Long term, we may want to revise how this computation is done. For example, it might be better to represent the set of invalidated prefixes in the dataflow computation (the `maybe_uninitialized` dataflow), and thus avoid one of the loops in the code here.~~
* Update: I was wrong in my original recollection of the dataflow code, which actually does the right thing, in terms of precisely tracking substructure initialization and movement.
Fix#44833
----
Update: The initial version of this PR's description (and the code as well) erroneously focused on supporting prefixes. ~~But the two main cases of interest are: 1. the *shallow* prefixes, and 2. the deref-free prefix built off a local (if the lvalue is indeed built off a local)~~
Update 2: The main cases of interest are in fact: 1. the nearest prefix with a MovePath, and 2. the suffixes.
MIR borrowck: print lvalues in error messages in the same way that the AST borrowck
Fix#44974
- Print fields with `.name` rather than `.<num>`
- Autoderef values if followed by a field or an index
- Output `[..]` when borrowing inside a slice
Fix#44833 (a very specific instance of a very broad bug).
In `check_if_path_is_moved(L)`, check nearest prefix of L with
MovePath, and suffixes of L with MovePaths.
Over the course of review, ariel pointed out a number of issues that
led to this version of the commit:
1. Looking solely at supporting prefixes does not suffice: it
overlooks checking if the path was ever actually initialized in the
first place. So you need to be willing to consider non-supporting
prefixes. Once you are looking at all prefixes, you *could* just
look at the local that forms the base of the projection, but to
handle partial initialization (which still needs to be formally
specified), this code instead looks at the nearest prefix of L that
has an associated MovePath (which, in the limit, will end up being
a local).
2. You also need to consider the suffixes of the given Lvalue, due to
how dataflow is representing partial moves of individual fields out
of struct values.
3. (There was originally a third search, but ariel pointed out that
the first and third could be folded into one.)
Also includes some drive-by refactorings to simplify some method
signatures and prefer `for _ in _` over `loop { }` (at least when it
comes semi-naturally).
MIR-borrowck: gather and signal any move errors
When building up the `MoveData` structure for a given MIR, also accumulate any erroneous actions, and then report all of those errors when the construction is complete.
This PR adds a host of move-related error constructor methods to `trait BorrowckErrors`. I think I got the notes right; but we should plan to audit all of the notes before turning MIR-borrowck on by default.
Fix#44830
Previously the constant index was reported as `[x of y]` or `[-x of y]` where
`x` was the offset and `y` the minimum length of the slice. The minus sign
wasn't in the right case since for `&[_, x, .., _, _]`, the error reported was
`[-1 of 4]`, and for `&[_, _, .., x, _]`, the error reported was `[2 of 4]`.
This commit fixes the sign so that the indexes 1 and -2 are reported, and
remove the ` of y` part of the message to make it more succinct.
Overlapping borrows can point to different lvalues.
Overlapping borrows can point to different lvalues.
There's always a basis for the overlap, so instead of removing the assert entirely, I instead pass in the prefix that we found and check that it actually is a prefix of both lvalues.
Fix#44829
