This adds support for rewriting field projections like &(*p).x when p is a nullable pointer. The result looks like Some(&(*p.unwrap()).x).
I initially tried to avoid a panic when p is null by implementing a rewrite using Option::map: p.map(|ptr| &ptr.x). However, implementing this correctly wound up being quite complex. It's undefined behavior in C to do &p->x when p == NULL, so it seems reasonable to introduce a panic in that case.
The mir_op changes for this are relatively straightforward, but unlower, distribute, and convert needed some work. In particular, unlower now has a new variant MirOriginDesc::LoadFromTempForAdjustment(i), which disambiguates cases like this:
// Rust:
f(&(*p).x)
// MIR:
// Evaluate the main expression:
_tmp1 = &(*_p).x;
// unlower_map entries for &(*p).x:
// * Expr
// * StoreIntoLocal
// Adjustments inserted to coerce `&T` to `*const T`:
_tmp2 = addr_of!(*_tmp1);
// * LoadFromTempForAdjustment(0) (load of _tmp1)
// * Adjustment(0) (deref)
// * Adjustment(1) (addr-of)
// * StoreIntoLocal
// The actual function call:
_result = f(_tmp2);
// * LoadFromTemp (load final result of &(*p).x from _tmp2)
Previously, the LoadFromTempForAdjustment would be recorded as LoadFromTemp, meaning there would be two LoadFromTemp entries in the unlower_map for the expression &(*p).x. Rewrites attached to the first LoadFromTemp (in this case, the use of _tmp1 in the second statement) would be wrongly applied at the site of the last LoadFromTemp. This caused unwrap() and Some(_) rewrites to be applied in the wrong order, causing type errors in the rewritten code.
This adds support for rewriting field projections like
&(*p).xwhenpis a nullable pointer. The result looks likeSome(&(*p.unwrap()).x).I initially tried to avoid a panic when
pis null by implementing a rewrite usingOption::map:p.map(|ptr| &ptr.x). However, implementing this correctly wound up being quite complex. It's undefined behavior in C to do&p->xwhenp == NULL, so it seems reasonable to introduce a panic in that case.The
mir_opchanges for this are relatively straightforward, butunlower,distribute, andconvertneeded some work. In particular,unlowernow has a new variantMirOriginDesc::LoadFromTempForAdjustment(i), which disambiguates cases like this:Previously, the
LoadFromTempForAdjustmentwould be recorded asLoadFromTemp, meaning there would be twoLoadFromTempentries in the unlower_map for the expression&(*p).x. Rewrites attached to the firstLoadFromTemp(in this case, the use of_tmp1in the second statement) would be wrongly applied at the site of the lastLoadFromTemp. This causedunwrap()andSome(_)rewrites to be applied in the wrong order, causing type errors in the rewritten code.