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Use correct substs in enum discriminant cast #98429

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merged 5 commits into from
Jun 25, 2022
Merged

Use correct substs in enum discriminant cast #98429

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4c4fb71
add test
b-naber Jun 23, 2022
2e3221a
use correct substs in enum discriminant hack
b-naber Jun 23, 2022
38814fc
small refactor
b-naber Jun 24, 2022
f39c0d6
address review
b-naber Jun 24, 2022
bf48b62
fmt
b-naber Jun 24, 2022
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186 changes: 94 additions & 92 deletions compiler/rustc_mir_build/src/thir/cx/expr.rs
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
use crate::thir::cx::region::Scope;
use crate::thir::cx::Cx;
use crate::thir::util::UserAnnotatedTyHelpers;
use rustc_data_structures::stack::ensure_sufficient_stack;
Expand Down Expand Up @@ -158,6 +159,98 @@ impl<'tcx> Cx<'tcx> {
Expr { temp_lifetime, ty: adjustment.target, span, kind }
}

/// Lowers a cast expression.
///
/// Dealing with user type annotations is left to the caller.
fn mirror_expr_cast(
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&mut self,
source: &'tcx hir::Expr<'tcx>,
temp_lifetime: Option<Scope>,
span: Span,
) -> ExprKind<'tcx> {
let tcx = self.tcx;

// Check to see if this cast is a "coercion cast", where the cast is actually done
// using a coercion (or is a no-op).
if self.typeck_results().is_coercion_cast(source.hir_id) {
// Convert the lexpr to a vexpr.
ExprKind::Use { source: self.mirror_expr(source) }
} else if self.typeck_results().expr_ty(source).is_region_ptr() {
// Special cased so that we can type check that the element
// type of the source matches the pointed to type of the
// destination.
ExprKind::Pointer {
source: self.mirror_expr(source),
cast: PointerCast::ArrayToPointer,
}
} else {
// check whether this is casting an enum variant discriminant
// to prevent cycles, we refer to the discriminant initializer
// which is always an integer and thus doesn't need to know the
// enum's layout (or its tag type) to compute it during const eval
// Example:
// enum Foo {
// A,
// B = A as isize + 4,
// }
// The correct solution would be to add symbolic computations to miri,
// so we wouldn't have to compute and store the actual value

let hir::ExprKind::Path(ref qpath) = source.kind else {
return ExprKind::Cast { source: self.mirror_expr(source)};
};

let res = self.typeck_results().qpath_res(qpath, source.hir_id);
let ty = self.typeck_results().node_type(source.hir_id);
let ty::Adt(adt_def, substs) = ty.kind() else {
return ExprKind::Cast { source: self.mirror_expr(source)};
};

let Res::Def(DefKind::Ctor(CtorOf::Variant, CtorKind::Const), variant_ctor_id) = res else {
return ExprKind::Cast { source: self.mirror_expr(source)};
};

let idx = adt_def.variant_index_with_ctor_id(variant_ctor_id);
let (discr_did, discr_offset) = adt_def.discriminant_def_for_variant(idx);

use rustc_middle::ty::util::IntTypeExt;
let ty = adt_def.repr().discr_type();
let discr_ty = ty.to_ty(tcx);

let param_env_ty = self.param_env.and(discr_ty);
let size = tcx
.layout_of(param_env_ty)
.unwrap_or_else(|e| {
panic!("could not compute layout for {:?}: {:?}", param_env_ty, e)
})
.size;

let lit = ScalarInt::try_from_uint(discr_offset as u128, size).unwrap();
let kind = ExprKind::NonHirLiteral { lit, user_ty: None };
let offset = self.thir.exprs.push(Expr { temp_lifetime, ty: discr_ty, span, kind });

let source = match discr_did {
// in case we are offsetting from a computed discriminant
// and not the beginning of discriminants (which is always `0`)
Some(did) => {
let kind = ExprKind::NamedConst { def_id: did, substs, user_ty: None };
let lhs =
self.thir.exprs.push(Expr { temp_lifetime, ty: discr_ty, span, kind });
let bin = ExprKind::Binary { op: BinOp::Add, lhs, rhs: offset };
self.thir.exprs.push(Expr {
temp_lifetime,
ty: discr_ty,
span: span,
kind: bin,
})
}
None => offset,
};

ExprKind::Cast { source }
}
}

fn make_mirror_unadjusted(&mut self, expr: &'tcx hir::Expr<'tcx>) -> Expr<'tcx> {
let tcx = self.tcx;
let expr_ty = self.typeck_results().expr_ty(expr);
Expand Down Expand Up @@ -604,98 +697,7 @@ impl<'tcx> Cx<'tcx> {
expr, cast_ty.hir_id, user_ty,
);

// Check to see if this cast is a "coercion cast", where the cast is actually done
// using a coercion (or is a no-op).
let cast = if self.typeck_results().is_coercion_cast(source.hir_id) {
// Convert the lexpr to a vexpr.
ExprKind::Use { source: self.mirror_expr(source) }
} else if self.typeck_results().expr_ty(source).is_region_ptr() {
// Special cased so that we can type check that the element
// type of the source matches the pointed to type of the
// destination.
ExprKind::Pointer {
source: self.mirror_expr(source),
cast: PointerCast::ArrayToPointer,
}
} else {
// check whether this is casting an enum variant discriminant
// to prevent cycles, we refer to the discriminant initializer
// which is always an integer and thus doesn't need to know the
// enum's layout (or its tag type) to compute it during const eval
// Example:
// enum Foo {
// A,
// B = A as isize + 4,
// }
// The correct solution would be to add symbolic computations to miri,
// so we wouldn't have to compute and store the actual value
let var = if let hir::ExprKind::Path(ref qpath) = source.kind {
let res = self.typeck_results().qpath_res(qpath, source.hir_id);
self.typeck_results().node_type(source.hir_id).ty_adt_def().and_then(
|adt_def| match res {
Res::Def(
DefKind::Ctor(CtorOf::Variant, CtorKind::Const),
variant_ctor_id,
) => {
let idx = adt_def.variant_index_with_ctor_id(variant_ctor_id);
let (d, o) = adt_def.discriminant_def_for_variant(idx);
use rustc_middle::ty::util::IntTypeExt;
let ty = adt_def.repr().discr_type();
let ty = ty.to_ty(tcx);
Some((d, o, ty))
}
_ => None,
},
)
} else {
None
};

let source = if let Some((did, offset, var_ty)) = var {
let param_env_ty = self.param_env.and(var_ty);
let size = tcx
.layout_of(param_env_ty)
.unwrap_or_else(|e| {
panic!("could not compute layout for {:?}: {:?}", param_env_ty, e)
})
.size;
let lit = ScalarInt::try_from_uint(offset as u128, size).unwrap();
let kind = ExprKind::NonHirLiteral { lit, user_ty: None };
let offset = self.thir.exprs.push(Expr {
temp_lifetime,
ty: var_ty,
span: expr.span,
kind,
});
match did {
Some(did) => {
// in case we are offsetting from a computed discriminant
// and not the beginning of discriminants (which is always `0`)
let substs = InternalSubsts::identity_for_item(tcx, did);
let kind =
ExprKind::NamedConst { def_id: did, substs, user_ty: None };
let lhs = self.thir.exprs.push(Expr {
temp_lifetime,
ty: var_ty,
span: expr.span,
kind,
});
let bin = ExprKind::Binary { op: BinOp::Add, lhs, rhs: offset };
self.thir.exprs.push(Expr {
temp_lifetime,
ty: var_ty,
span: expr.span,
kind: bin,
})
}
None => offset,
}
} else {
self.mirror_expr(source)
};

ExprKind::Cast { source: source }
};
let cast = self.mirror_expr_cast(*source, temp_lifetime, expr.span);

if let Some(user_ty) = user_ty {
// NOTE: Creating a new Expr and wrapping a Cast inside of it may be
Expand Down
10 changes: 10 additions & 0 deletions src/test/ui/const-generics/issues/issue-97634.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
// build-pass

pub enum Register<const N: u16> {
Field0 = 40,
Field1,
}

fn main() {
let _b = Register::<0>::Field1 as u16;
}