Dumb NRVO
This is a very simple version of an NRVO pass, which scans backwards from the `return` terminator to see if there is an an assignment like `_0 = _1`. If a basic block with two or more predecessors is encountered during this scan without first seeing an assignment to the return place, we bail out. This avoids running a full "reaching definitions" dataflow analysis.
I wanted to see how much `rustc` would benefit from even a very limited version of this optimization. We should be able to use this as a point of comparison for more advanced versions that are based on live ranges.
r? @ghost
Stabilize saturating_abs and saturating_neg
Stabilizes the following signed integer functions with saturation mechanics:
* saturating_abs()
* saturating_neg()
Closes#59983
Miri interning: replace ICEs by proper errors
Fixes https://github.com/rust-lang/rust/issues/71316
I also did some refactoring, as I kept being confused by all the parameters to `intern_shallow`, some of which have invalid combinations (such as a mutable const). So instead `InternMode` now contains all the information that is needed and invalid combinations are ruled out by the type system.
Also I removed interpreter errors from interning. We already ignored almost all errors, and the `ValidationFailure` errors that we handled separately actually cannot ever happen here. The only interpreter failure that was actually reachable was the UB on dangling pointers -- and arguably, a dangling raw pointer is not UB, so the error was not even correct. It's just that the rest of the compiler does not like "dangling" `AllocId`.
It should be possible to review the 3 commits separately.
r? @oli-obk
Cc @rust-lang/wg-const-eval
Miri: unleash all feature gates
IMO it is silly to unleash features that do not even have a feature gate yet, but not unleash features that do. The only thing this achieves is making unleashed mode annoying to use as we have to figure out the feature flags to enable (and not always do the error messages say what that flag is).
Given that the point of `-Z unleash-the-miri-inside-of-you` is to debug the Miri internals, I see no good reason for this extra hurdle. I cannot imagine a situation where we'd use that flag, realize the program also requires some feature gate, and then be like "oh I guess if this feature is unstable I will do something else". Instead, we'll always just add that flag to the code as well, so requiring the flag achieves nothing.
r? @oli-obk @ecstatic-morse
Fixes https://github.com/rust-lang/rust/issues/71630
Allow `Downcast` projections unconditionally in const-checking
`ProjectionElem::Downcast` sounds scary, but it's really just the projection we use to access a particular enum variant. They usually appear in the lowering of a `match` statement, so they have been associated with control flow in const-checking, but they don't do any control flow by themselves. We already have a HIR pass that looks for `if` and `match` (even ones that have 1 or fewer reachable branches). That pass is double-checked by a MIR pass that looks for `SwitchInt`s and `FakeRead`s for match scrutinees. In my opinion, there's no need to look for `Downcast` as well.
r? @oli-obk
MIR dump: print pointers consistently with Miri output
This makes MIR allocation dump pointer printing consistent with Miri output: both use hexadecimal offsets with a `0x` prefix. To save some space, MIR dump replaces the `alloc` prefix by `a` when necessary.
I also made AllocId/Pointer printing more consistent in their Debug/Display handling, and adjusted Display printing for Scalar a bit to avoid using decimal printing when we do not know the sign with which to interpret the value (IMO using decimal then is misleading).
Rename Unique::empty() -> Unique::dangling()
A `FIXME` comment in `src/libcore/ptr/unique.rs` suggested refactoring `Unique::empty()` to `Unique::dangling()` which this PR does.
Miri: better document and fix dynamic const pattern soundness checks
https://github.com/rust-lang/const-eval/issues/42 got me thinking about soundness for consts being used in patterns, and I found a hole in our existing dynamic checks: a const referring to a mutable static *in a different crate* was not caught. This PR fixes that. It also adds some comments that explain which invariants are crucial for soundness of const-patterns.
Curiously, trying to weaponize this soundness hole failed: pattern matching compilation ICEd when encountering the cross-crate static, saying "expected allocation ID alloc0 to point to memory". I don't know why that would happen, statics *should* be entirely normal memory for pattern matching to access.
r? @oli-obk
Cc @rust-lang/wg-const-eval
Const qualification for `StructuralEq`
Furthers #62411. Resolves#62614.
The goal of this PR is to implement the logic in #67088 on the MIR instead of the HIR. It uses the `Qualif` trait to track `StructuralPartialEq`/`StructuralEq` in the final value of a `const`. Then, if we encounter a constant during HAIR lowering whose value may not be structurally matchable, we emit the `indirect_structural_match` lint.
This PR contains all the tests present in #67088 and emits the proper warnings for the corner cases. This PR does not handle #65466, which would require that we be [more aggressive](42abbd8878/src/librustc_mir_build/hair/pattern/const_to_pat.rs (L126-L130)) when checking matched types for `PartialEq`. I think that should be done separately.
Because this works on MIR and uses dataflow, this PR should accept more cases than #67088. Notably, the qualifs in the final value of a const are encoded cross-crate, so matching on a constant whose value is defined in another crate to be `Option::<TyWithCustomEqImpl>::None` should work. Additionally, if a `const` has branching/looping, we will only emit the warning if any possible control flow path could result in a type with a custom `PartialEq` impl ending up as the final value of a `const`. I'm not sure how #67088 handled this.
AFAIK, it's not settled that these are the semantics we actually want: it's just how the `Qualif` framework happens to work. If the cross-crate part is undesirable, it would be quite easy to change the result of `mir_const_qualif().custom_eq` to `true` before encoding it in the crate metadata. This way, other crates would have to assume that all publicly exported constants may not be safe for matching.
r? @pnkfelix
cc @eddyb
