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transmutability: shift abstraction boundary #142040

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15 changes: 7 additions & 8 deletions compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2558,32 +2558,31 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
rustc_transmute::Reason::SrcIsNotYetSupported => {
format!("analyzing the transmutability of `{src}` is not yet supported")
}

rustc_transmute::Reason::DstIsNotYetSupported => {
format!("analyzing the transmutability of `{dst}` is not yet supported")
}

rustc_transmute::Reason::DstIsBitIncompatible => {
format!(
"at least one value of `{src}` isn't a bit-valid value of `{dst}`"
)
}

rustc_transmute::Reason::DstUninhabited => {
format!("`{dst}` is uninhabited")
}

rustc_transmute::Reason::DstMayHaveSafetyInvariants => {
format!("`{dst}` may carry safety invariants")
}
rustc_transmute::Reason::DstIsTooBig => {
format!("the size of `{src}` is smaller than the size of `{dst}`")
}
rustc_transmute::Reason::DstRefIsTooBig { src, dst } => {
let src_size = src.size;
let dst_size = dst.size;
rustc_transmute::Reason::DstRefIsTooBig {
src,
src_size,
dst,
dst_size,
} => {
format!(
"the referent size of `{src}` ({src_size} bytes) \
"the size of `{src}` ({src_size} bytes) \
is smaller than that of `{dst}` ({dst_size} bytes)"
)
}
Expand Down
143 changes: 46 additions & 97 deletions compiler/rustc_trait_selection/src/traits/select/confirmation.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,13 +9,12 @@

use std::ops::ControlFlow;

use rustc_ast::Mutability;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_hir::lang_items::LangItem;
use rustc_infer::infer::{DefineOpaqueTypes, HigherRankedType, InferOk};
use rustc_infer::traits::ObligationCauseCode;
use rustc_middle::traits::{BuiltinImplSource, SignatureMismatchData};
use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, Upcast, elaborate};
use rustc_middle::ty::{self, GenericArgsRef, Region, Ty, TyCtxt, Upcast, elaborate};
use rustc_middle::{bug, span_bug};
use rustc_span::def_id::DefId;
use thin_vec::thin_vec;
Expand Down Expand Up @@ -286,99 +285,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
) -> Result<PredicateObligations<'tcx>, SelectionError<'tcx>> {
use rustc_transmute::{Answer, Assume, Condition};

/// Generate sub-obligations for reference-to-reference transmutations.
fn reference_obligations<'tcx>(
tcx: TyCtxt<'tcx>,
obligation: &PolyTraitObligation<'tcx>,
(src_lifetime, src_ty, src_mut): (ty::Region<'tcx>, Ty<'tcx>, Mutability),
(dst_lifetime, dst_ty, dst_mut): (ty::Region<'tcx>, Ty<'tcx>, Mutability),
assume: Assume,
) -> PredicateObligations<'tcx> {
let make_transmute_obl = |src, dst| {
let transmute_trait = obligation.predicate.def_id();
let assume = obligation.predicate.skip_binder().trait_ref.args.const_at(2);
let trait_ref = ty::TraitRef::new(
tcx,
transmute_trait,
[
ty::GenericArg::from(dst),
ty::GenericArg::from(src),
ty::GenericArg::from(assume),
],
);
Obligation::with_depth(
tcx,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
trait_ref,
)
};

let make_freeze_obl = |ty| {
let trait_ref = ty::TraitRef::new(
tcx,
tcx.require_lang_item(LangItem::Freeze, obligation.cause.span),
[ty::GenericArg::from(ty)],
);
Obligation::with_depth(
tcx,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
trait_ref,
)
};

let make_outlives_obl = |target, region| {
let outlives = ty::OutlivesPredicate(target, region);
Obligation::with_depth(
tcx,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
outlives,
)
};

// Given a transmutation from `&'a (mut) Src` and `&'dst (mut) Dst`,
// it is always the case that `Src` must be transmutable into `Dst`,
// and that that `'src` must outlive `'dst`.
let mut obls = PredicateObligations::with_capacity(1);
obls.push(make_transmute_obl(src_ty, dst_ty));
if !assume.lifetimes {
obls.push(make_outlives_obl(src_lifetime, dst_lifetime));
}

// Given a transmutation from `&Src`, both `Src` and `Dst` must be
// `Freeze`, otherwise, using the transmuted value could lead to
// data races.
if src_mut == Mutability::Not {
obls.extend([make_freeze_obl(src_ty), make_freeze_obl(dst_ty)])
}

// Given a transmutation into `&'dst mut Dst`, it also must be the
// case that `Dst` is transmutable into `Src`. For example,
// transmuting bool -> u8 is OK as long as you can't update that u8
// to be > 1, because you could later transmute the u8 back to a
// bool and get undefined behavior. It also must be the case that
// `'dst` lives exactly as long as `'src`.
if dst_mut == Mutability::Mut {
obls.push(make_transmute_obl(dst_ty, src_ty));
if !assume.lifetimes {
obls.push(make_outlives_obl(dst_lifetime, src_lifetime));
}
}

obls
}

/// Flatten the `Condition` tree into a conjunction of obligations.
#[instrument(level = "debug", skip(tcx, obligation))]
fn flatten_answer_tree<'tcx>(
tcx: TyCtxt<'tcx>,
obligation: &PolyTraitObligation<'tcx>,
cond: Condition<rustc_transmute::layout::rustc::Ref<'tcx>>,
cond: Condition<Region<'tcx>, Ty<'tcx>>,
assume: Assume,
) -> PredicateObligations<'tcx> {
match cond {
Expand All @@ -388,13 +300,50 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
.into_iter()
.flat_map(|cond| flatten_answer_tree(tcx, obligation, cond, assume))
.collect(),
Condition::IfTransmutable { src, dst } => reference_obligations(
tcx,
obligation,
(src.lifetime, src.ty, src.mutability),
(dst.lifetime, dst.ty, dst.mutability),
assume,
),
Condition::Immutable { ty } => {
let trait_ref = ty::TraitRef::new(
tcx,
tcx.require_lang_item(LangItem::Freeze, obligation.cause.span),
[ty::GenericArg::from(ty)],
);
thin_vec![Obligation::with_depth(
tcx,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
trait_ref,
)]
}
Condition::Outlives { long, short } => {
let outlives = ty::OutlivesPredicate(long, short);
thin_vec![Obligation::with_depth(
tcx,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
outlives,
)]
}
Condition::Transmutable { src, dst } => {
let transmute_trait = obligation.predicate.def_id();
let assume = obligation.predicate.skip_binder().trait_ref.args.const_at(2);
let trait_ref = ty::TraitRef::new(
tcx,
transmute_trait,
[
ty::GenericArg::from(dst),
ty::GenericArg::from(src),
ty::GenericArg::from(assume),
],
);
thin_vec![Obligation::with_depth(
tcx,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
trait_ref,
)]
}
}
}

Expand Down
45 changes: 25 additions & 20 deletions compiler/rustc_transmute/src/layout/dfa.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,32 +2,35 @@ use std::fmt;
use std::iter::Peekable;
use std::sync::atomic::{AtomicU32, Ordering};

use super::{Byte, Ref, Tree, Uninhabited};
use super::{Byte, Reference, Region, Tree, Type, Uninhabited};
use crate::{Map, Set};

#[derive(PartialEq)]
#[cfg_attr(test, derive(Clone))]
pub(crate) struct Dfa<R>
pub(crate) struct Dfa<R, T>
where
R: Ref,
R: Region,
T: Type,
{
pub(crate) transitions: Map<State, Transitions<R>>,
pub(crate) transitions: Map<State, Transitions<R, T>>,
pub(crate) start: State,
pub(crate) accept: State,
}

#[derive(PartialEq, Clone, Debug)]
pub(crate) struct Transitions<R>
pub(crate) struct Transitions<R, T>
where
R: Ref,
R: Region,
T: Type,
{
byte_transitions: EdgeSet<State>,
ref_transitions: Map<R, State>,
ref_transitions: Map<Reference<R, T>, State>,
}

impl<R> Default for Transitions<R>
impl<R, T> Default for Transitions<R, T>
where
R: Ref,
R: Region,
T: Type,
{
fn default() -> Self {
Self { byte_transitions: EdgeSet::empty(), ref_transitions: Map::default() }
Expand All @@ -51,9 +54,10 @@ impl fmt::Debug for State {
}
}

impl<R> Dfa<R>
impl<R, T> Dfa<R, T>
where
R: Ref,
R: Region,
T: Type,
{
#[cfg(test)]
pub(crate) fn bool() -> Self {
Expand All @@ -64,7 +68,7 @@ where
}

pub(crate) fn unit() -> Self {
let transitions: Map<State, Transitions<R>> = Map::default();
let transitions: Map<State, Transitions<R, T>> = Map::default();
let start = State::new();
let accept = start;

Expand All @@ -78,21 +82,21 @@ where
})
}

pub(crate) fn from_ref(r: R) -> Self {
pub(crate) fn from_ref(r: Reference<R, T>) -> Self {
Self::from_transitions(|accept| Transitions {
byte_transitions: EdgeSet::empty(),
ref_transitions: [(r, accept)].into_iter().collect(),
})
}

fn from_transitions(f: impl FnOnce(State) -> Transitions<R>) -> Self {
fn from_transitions(f: impl FnOnce(State) -> Transitions<R, T>) -> Self {
let start = State::new();
let accept = State::new();

Self { transitions: [(start, f(accept))].into_iter().collect(), start, accept }
}

pub(crate) fn from_tree(tree: Tree<!, R>) -> Result<Self, Uninhabited> {
pub(crate) fn from_tree(tree: Tree<!, R, T>) -> Result<Self, Uninhabited> {
Ok(match tree {
Tree::Byte(b) => Self::from_byte(b),
Tree::Ref(r) => Self::from_ref(r),
Expand Down Expand Up @@ -125,7 +129,7 @@ where
let start = self.start;
let accept = other.accept;

let mut transitions: Map<State, Transitions<R>> = self.transitions;
let mut transitions: Map<State, Transitions<R, T>> = self.transitions;

for (source, transition) in other.transitions {
let fix_state = |state| if state == other.start { self.accept } else { state };
Expand Down Expand Up @@ -169,7 +173,7 @@ where
};

let start = mapped((Some(a.start), Some(b.start)));
let mut transitions: Map<State, Transitions<R>> = Map::default();
let mut transitions: Map<State, Transitions<R, T>> = Map::default();
let empty_transitions = Transitions::default();

struct WorkQueue {
Expand Down Expand Up @@ -257,7 +261,7 @@ where
.flat_map(|transitions| transitions.byte_transitions.iter())
}

pub(crate) fn refs_from(&self, start: State) -> impl Iterator<Item = (R, State)> {
pub(crate) fn refs_from(&self, start: State) -> impl Iterator<Item = (Reference<R, T>, State)> {
self.transitions
.get(&start)
.into_iter()
Expand Down Expand Up @@ -297,9 +301,10 @@ where
}

/// Serialize the DFA using the Graphviz DOT format.
impl<R> fmt::Debug for Dfa<R>
impl<R, T> fmt::Debug for Dfa<R, T>
where
R: Ref,
R: Region,
T: Type,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
writeln!(f, "digraph {{")?;
Expand Down
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