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Erase late bound regions for dropped instances #107936

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Erase late bound regions for dropped instances #107936

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b00a3c0
Erase late bound regions while resolving drop_in_place
Rattenkrieg Feb 11, 2023
2826efc
fix imports after merge
Rattenkrieg Feb 14, 2023
6c7bcb5
move test from run-make to ui/coercion
Rattenkrieg Feb 17, 2023
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13 changes: 11 additions & 2 deletions compiler/rustc_middle/src/ty/instance.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
use crate::ty::print::{FmtPrinter, Printer};
use crate::ty::{self, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable};
use crate::ty::{self, Binder, Ty, TyCtxt, TyKind, TypeFoldable, TypeSuperFoldable};
use crate::ty::{EarlyBinder, InternalSubsts, SubstsRef};
use rustc_errors::ErrorGuaranteed;
use rustc_hir::def::Namespace;
Expand Down Expand Up @@ -539,8 +539,17 @@ impl<'tcx> Instance<'tcx> {
}

pub fn resolve_drop_in_place(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> ty::Instance<'tcx> {
let ty_erased =
if let (true, TyKind::FnPtr(poly_fn_sig)) = (ty.has_late_bound_regions(), ty.kind()) {
let re_erased = tcx.erase_late_bound_regions(*poly_fn_sig);
let rebound = Binder::dummy(re_erased);
let reassembled = tcx.mk_fn_ptr(rebound);
reassembled
} else {
ty
};
Comment on lines +542 to +550
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@cjgillot cjgillot Feb 17, 2023 •
edited
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Suggested change
let ty_erased =
if let (true, TyKind::FnPtr(poly_fn_sig)) = (ty.has_late_bound_regions(), ty.kind()) {
let re_erased = tcx.erase_late_bound_regions(*poly_fn_sig);
let rebound = Binder::dummy(re_erased);
let reassembled = tcx.mk_fn_ptr(rebound);
reassembled
} else {
ty
};
// Erase regions before calling resolve, as they must not change the resolution.
let ty_erased = tcx.erase_regions(ty);

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I think I tried that but normalizing late bound regions is not enough in this case. There's still a mismatch because one symbol looks like core::ptr::drop_in_place::<for<'a> fn(&'a ())> and the other like core::ptr::drop_in_place::<fn(&())>.

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Changed my suggestion to use erase_regions that should erase everything.

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Well, not sure actually. It's probably the instance resolution code that needs to be fortified against leaking regions.

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Changed my suggestion to use erase_regions that should erase everything.

I believe this already happening without desired effect wrt to this bug: Instance::expect_resolve -> Instance::resolve -> Instance::resolve_opt_const_arg -> tcx.erase_regions(substs). Anyway, I'm going to give it a try.

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@michaelwoerister michaelwoerister Feb 17, 2023 •
edited
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Changed my suggestion to use erase_regions that should erase everything.

That's what I thought too, but it doesn't seem to be the case:

/// Returns an equivalent value with all free regions removed (note
/// that late-bound regions remain, because they are important for
/// subtyping, but they are anonymized and normalized as well)..
pub fn erase_regions(self, value: T) -> T

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Just checked, it doesn't erase late bound regions.

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@Rattenkrieg Rattenkrieg Feb 17, 2023 •
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rust/compiler/rustc_middle/src/ty/erase_regions.rs

Lines 54 to 64 in b5c8c32

fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
// because late-bound regions affect subtyping, we can't
// erase the bound/free distinction, but we can replace
// all free regions with 'erased.
//
// Note that we *CAN* replace early-bound regions -- the
// type system never "sees" those, they get substituted
// away. In codegen, they will always be erased to 'erased
// whenever a substitution occurs.
match *r {
ty::ReLateBound(..) => r,

this is the TypeFolder invoked during erase_regions

let def_id = tcx.require_lang_item(LangItem::DropInPlace, None);
let substs = tcx.intern_substs(&[ty.into()]);
let substs = tcx.intern_substs(&[ty_erased.into()]);
Instance::expect_resolve(tcx, ty::ParamEnv::reveal_all(), def_id, substs)
}

Expand Down
18 changes: 18 additions & 0 deletions tests/ui/coercion/issue-107205.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,18 @@
// build-pass

use std::fmt;

pub struct Wrapper(fn(val: &()));

impl fmt::Debug for Wrapper {
fn fmt<'a>(&'a self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_tuple("Wrapper").field(&self.0 as &fn(&'a ())).finish()
}
}

fn useful(_: &()) {
}

fn main() {
println!("{:?}", Wrapper(useful));
}