rustc_next_trait_solver/coherence.rs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468
use std::fmt::Debug;
use std::ops::ControlFlow;
use derive_where::derive_where;
use rustc_type_ir::inherent::*;
use rustc_type_ir::visit::{TypeVisitable, TypeVisitableExt, TypeVisitor};
use rustc_type_ir::{self as ty, InferCtxtLike, Interner};
use tracing::instrument;
/// Whether we do the orphan check relative to this crate or to some remote crate.
#[derive(Copy, Clone, Debug)]
pub enum InCrate {
Local { mode: OrphanCheckMode },
Remote,
}
#[derive(Copy, Clone, Debug)]
pub enum OrphanCheckMode {
/// Proper orphan check.
Proper,
/// Improper orphan check for backward compatibility.
///
/// In this mode, type params inside projections are considered to be covered
/// even if the projection may normalize to a type that doesn't actually cover
/// them. This is unsound. See also [#124559] and [#99554].
///
/// [#124559]: https://github.com/rust-lang/rust/issues/124559
/// [#99554]: https://github.com/rust-lang/rust/issues/99554
Compat,
}
#[derive(Debug, Copy, Clone)]
pub enum Conflict {
Upstream,
Downstream,
}
/// Returns whether all impls which would apply to the `trait_ref`
/// e.g. `Ty: Trait<Arg>` are already known in the local crate.
///
/// This both checks whether any downstream or sibling crates could
/// implement it and whether an upstream crate can add this impl
/// without breaking backwards compatibility.
#[instrument(level = "debug", skip(infcx, lazily_normalize_ty), ret)]
pub fn trait_ref_is_knowable<Infcx, I, E>(
infcx: &Infcx,
trait_ref: ty::TraitRef<I>,
mut lazily_normalize_ty: impl FnMut(I::Ty) -> Result<I::Ty, E>,
) -> Result<Result<(), Conflict>, E>
where
Infcx: InferCtxtLike<Interner = I>,
I: Interner,
E: Debug,
{
if orphan_check_trait_ref(infcx, trait_ref, InCrate::Remote, &mut lazily_normalize_ty)?.is_ok()
{
// A downstream or cousin crate is allowed to implement some
// generic parameters of this trait-ref.
return Ok(Err(Conflict::Downstream));
}
if trait_ref_is_local_or_fundamental(infcx.cx(), trait_ref) {
// This is a local or fundamental trait, so future-compatibility
// is no concern. We know that downstream/cousin crates are not
// allowed to implement a generic parameter of this trait ref,
// which means impls could only come from dependencies of this
// crate, which we already know about.
return Ok(Ok(()));
}
// This is a remote non-fundamental trait, so if another crate
// can be the "final owner" of the generic parameters of this trait-ref,
// they are allowed to implement it future-compatibly.
//
// However, if we are a final owner, then nobody else can be,
// and if we are an intermediate owner, then we don't care
// about future-compatibility, which means that we're OK if
// we are an owner.
if orphan_check_trait_ref(
infcx,
trait_ref,
InCrate::Local { mode: OrphanCheckMode::Proper },
&mut lazily_normalize_ty,
)?
.is_ok()
{
Ok(Ok(()))
} else {
Ok(Err(Conflict::Upstream))
}
}
pub fn trait_ref_is_local_or_fundamental<I: Interner>(tcx: I, trait_ref: ty::TraitRef<I>) -> bool {
trait_ref.def_id.is_local() || tcx.trait_is_fundamental(trait_ref.def_id)
}
#[derive(Debug, Copy, Clone)]
pub enum IsFirstInputType {
No,
Yes,
}
impl From<bool> for IsFirstInputType {
fn from(b: bool) -> IsFirstInputType {
match b {
false => IsFirstInputType::No,
true => IsFirstInputType::Yes,
}
}
}
#[derive_where(Debug; I: Interner, T: Debug)]
pub enum OrphanCheckErr<I: Interner, T> {
NonLocalInputType(Vec<(I::Ty, IsFirstInputType)>),
UncoveredTyParams(UncoveredTyParams<I, T>),
}
#[derive_where(Debug; I: Interner, T: Debug)]
pub struct UncoveredTyParams<I: Interner, T> {
pub uncovered: T,
pub local_ty: Option<I::Ty>,
}
/// Checks whether a trait-ref is potentially implementable by a crate.
///
/// The current rule is that a trait-ref orphan checks in a crate C:
///
/// 1. Order the parameters in the trait-ref in generic parameters order
/// - Self first, others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
/// 2. Of these type parameters, there is at least one type parameter
/// in which, walking the type as a tree, you can reach a type local
/// to C where all types in-between are fundamental types. Call the
/// first such parameter the "local key parameter".
/// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
/// going through `Box`, which is fundamental.
/// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
/// the same reason.
/// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
/// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
/// the local type and the type parameter.
/// 3. Before this local type, no generic type parameter of the impl must
/// be reachable through fundamental types.
/// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
/// - while `impl<T> Trait<LocalType> for Box<T>` results in an error, as `T` is
/// reachable through the fundamental type `Box`.
/// 4. Every type in the local key parameter not known in C, going
/// through the parameter's type tree, must appear only as a subtree of
/// a type local to C, with only fundamental types between the type
/// local to C and the local key parameter.
/// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
/// is bad, because the only local type with `T` as a subtree is
/// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
/// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
/// the second occurrence of `T` is not a subtree of *any* local type.
/// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
/// `LocalType<Vec<T>>`, which is local and has no types between it and
/// the type parameter.
///
/// The orphan rules actually serve several different purposes:
///
/// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
/// every type local to one crate is unknown in the other) can't implement
/// the same trait-ref. This follows because it can be seen that no such
/// type can orphan-check in 2 such crates.
///
/// To check that a local impl follows the orphan rules, we check it in
/// InCrate::Local mode, using type parameters for the "generic" types.
///
/// In InCrate::Local mode the orphan check succeeds if the current crate
/// is definitely allowed to implement the given trait (no false positives).
///
/// 2. They ground negative reasoning for coherence. If a user wants to
/// write both a conditional blanket impl and a specific impl, we need to
/// make sure they do not overlap. For example, if we write
/// ```ignore (illustrative)
/// impl<T> IntoIterator for Vec<T>
/// impl<T: Iterator> IntoIterator for T
/// ```
/// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
/// We can observe that this holds in the current crate, but we need to make
/// sure this will also hold in all unknown crates (both "independent" crates,
/// which we need for link-safety, and also child crates, because we don't want
/// child crates to get error for impl conflicts in a *dependency*).
///
/// For that, we only allow negative reasoning if, for every assignment to the
/// inference variables, every unknown crate would get an orphan error if they
/// try to implement this trait-ref. To check for this, we use InCrate::Remote
/// mode. That is sound because we already know all the impls from known crates.
///
/// In InCrate::Remote mode the orphan check succeeds if a foreign crate
/// *could* implement the given trait (no false negatives).
///
/// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
/// add "non-blanket" impls without breaking negative reasoning in dependent
/// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
///
/// For that, we only allow a crate to perform negative reasoning on
/// non-local-non-`#[fundamental]` if there's a local key parameter as per (2).
///
/// Because we never perform negative reasoning generically (coherence does
/// not involve type parameters), this can be interpreted as doing the full
/// orphan check (using InCrate::Local mode), instantiating non-local known
/// types for all inference variables.
///
/// This allows for crates to future-compatibly add impls as long as they
/// can't apply to types with a key parameter in a child crate - applying
/// the rules, this basically means that every type parameter in the impl
/// must appear behind a non-fundamental type (because this is not a
/// type-system requirement, crate owners might also go for "semantic
/// future-compatibility" involving things such as sealed traits, but
/// the above requirement is sufficient, and is necessary in "open world"
/// cases).
///
/// Note that this function is never called for types that have both type
/// parameters and inference variables.
#[instrument(level = "trace", skip(infcx, lazily_normalize_ty), ret)]
pub fn orphan_check_trait_ref<Infcx, I, E: Debug>(
infcx: &Infcx,
trait_ref: ty::TraitRef<I>,
in_crate: InCrate,
lazily_normalize_ty: impl FnMut(I::Ty) -> Result<I::Ty, E>,
) -> Result<Result<(), OrphanCheckErr<I, I::Ty>>, E>
where
Infcx: InferCtxtLike<Interner = I>,
I: Interner,
E: Debug,
{
if trait_ref.has_param() {
panic!("orphan check only expects inference variables: {trait_ref:?}");
}
let mut checker = OrphanChecker::new(infcx, in_crate, lazily_normalize_ty);
Ok(match trait_ref.visit_with(&mut checker) {
ControlFlow::Continue(()) => Err(OrphanCheckErr::NonLocalInputType(checker.non_local_tys)),
ControlFlow::Break(residual) => match residual {
OrphanCheckEarlyExit::NormalizationFailure(err) => return Err(err),
OrphanCheckEarlyExit::UncoveredTyParam(ty) => {
// Does there exist some local type after the `ParamTy`.
checker.search_first_local_ty = true;
let local_ty = match trait_ref.visit_with(&mut checker) {
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(local_ty)) => Some(local_ty),
_ => None,
};
Err(OrphanCheckErr::UncoveredTyParams(UncoveredTyParams {
uncovered: ty,
local_ty,
}))
}
OrphanCheckEarlyExit::LocalTy(_) => Ok(()),
},
})
}
struct OrphanChecker<'a, Infcx, I: Interner, F> {
infcx: &'a Infcx,
in_crate: InCrate,
in_self_ty: bool,
lazily_normalize_ty: F,
/// Ignore orphan check failures and exclusively search for the first local type.
search_first_local_ty: bool,
non_local_tys: Vec<(I::Ty, IsFirstInputType)>,
}
impl<'a, Infcx, I, F, E> OrphanChecker<'a, Infcx, I, F>
where
Infcx: InferCtxtLike<Interner = I>,
I: Interner,
F: FnOnce(I::Ty) -> Result<I::Ty, E>,
{
fn new(infcx: &'a Infcx, in_crate: InCrate, lazily_normalize_ty: F) -> Self {
OrphanChecker {
infcx,
in_crate,
in_self_ty: true,
lazily_normalize_ty,
search_first_local_ty: false,
non_local_tys: Vec::new(),
}
}
fn found_non_local_ty(&mut self, t: I::Ty) -> ControlFlow<OrphanCheckEarlyExit<I, E>> {
self.non_local_tys.push((t, self.in_self_ty.into()));
ControlFlow::Continue(())
}
fn found_uncovered_ty_param(&mut self, ty: I::Ty) -> ControlFlow<OrphanCheckEarlyExit<I, E>> {
if self.search_first_local_ty {
return ControlFlow::Continue(());
}
ControlFlow::Break(OrphanCheckEarlyExit::UncoveredTyParam(ty))
}
fn def_id_is_local(&mut self, def_id: I::DefId) -> bool {
match self.in_crate {
InCrate::Local { .. } => def_id.is_local(),
InCrate::Remote => false,
}
}
}
enum OrphanCheckEarlyExit<I: Interner, E> {
NormalizationFailure(E),
UncoveredTyParam(I::Ty),
LocalTy(I::Ty),
}
impl<'a, Infcx, I, F, E> TypeVisitor<I> for OrphanChecker<'a, Infcx, I, F>
where
Infcx: InferCtxtLike<Interner = I>,
I: Interner,
F: FnMut(I::Ty) -> Result<I::Ty, E>,
{
type Result = ControlFlow<OrphanCheckEarlyExit<I, E>>;
fn visit_region(&mut self, _r: I::Region) -> Self::Result {
ControlFlow::Continue(())
}
fn visit_ty(&mut self, ty: I::Ty) -> Self::Result {
let ty = self.infcx.shallow_resolve(ty);
let ty = match (self.lazily_normalize_ty)(ty) {
Ok(norm_ty) if norm_ty.is_ty_var() => ty,
Ok(norm_ty) => norm_ty,
Err(err) => return ControlFlow::Break(OrphanCheckEarlyExit::NormalizationFailure(err)),
};
let result = match ty.kind() {
ty::Bool
| ty::Char
| ty::Int(..)
| ty::Uint(..)
| ty::Float(..)
| ty::Str
| ty::FnDef(..)
| ty::Pat(..)
| ty::FnPtr(..)
| ty::Array(..)
| ty::Slice(..)
| ty::RawPtr(..)
| ty::Never
| ty::Tuple(..) => self.found_non_local_ty(ty),
ty::Param(..) => panic!("unexpected ty param"),
ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => {
match self.in_crate {
InCrate::Local { .. } => self.found_uncovered_ty_param(ty),
// The inference variable might be unified with a local
// type in that remote crate.
InCrate::Remote => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
}
}
// A rigid alias may normalize to anything.
// * If it references an infer var, placeholder or bound ty, it may
// normalize to that, so we have to treat it as an uncovered ty param.
// * Otherwise it may normalize to any non-type-generic type
// be it local or non-local.
ty::Alias(kind, _) => {
if ty.has_type_flags(
ty::TypeFlags::HAS_TY_PLACEHOLDER
| ty::TypeFlags::HAS_TY_BOUND
| ty::TypeFlags::HAS_TY_INFER,
) {
match self.in_crate {
InCrate::Local { mode } => match kind {
ty::Projection => {
if let OrphanCheckMode::Compat = mode {
ControlFlow::Continue(())
} else {
self.found_uncovered_ty_param(ty)
}
}
_ => self.found_uncovered_ty_param(ty),
},
InCrate::Remote => {
// The inference variable might be unified with a local
// type in that remote crate.
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
}
}
} else {
// Regarding *opaque types* specifically, we choose to treat them as non-local,
// even those that appear within the same crate. This seems somewhat surprising
// at first, but makes sense when you consider that opaque types are supposed
// to hide the underlying type *within the same crate*. When an opaque type is
// used from outside the module where it is declared, it should be impossible to
// observe anything about it other than the traits that it implements.
//
// The alternative would be to look at the underlying type to determine whether
// or not the opaque type itself should be considered local.
//
// However, this could make it a breaking change to switch the underlying hidden
// type from a local type to a remote type. This would violate the rule that
// opaque types should be completely opaque apart from the traits that they
// implement, so we don't use this behavior.
// Addendum: Moreover, revealing the underlying type is likely to cause cycle
// errors as we rely on coherence / the specialization graph during typeck.
self.found_non_local_ty(ty)
}
}
// For fundamental types, we just look inside of them.
ty::Ref(_, ty, _) => ty.visit_with(self),
ty::Adt(def, args) => {
if self.def_id_is_local(def.def_id()) {
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
} else if def.is_fundamental() {
args.visit_with(self)
} else {
self.found_non_local_ty(ty)
}
}
ty::Foreign(def_id) => {
if self.def_id_is_local(def_id) {
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
} else {
self.found_non_local_ty(ty)
}
}
ty::Dynamic(tt, ..) => {
let principal = tt.principal().map(|p| p.def_id());
if principal.is_some_and(|p| self.def_id_is_local(p)) {
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
} else {
self.found_non_local_ty(ty)
}
}
ty::Error(_) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
ty::Closure(did, ..) | ty::CoroutineClosure(did, ..) | ty::Coroutine(did, ..) => {
if self.def_id_is_local(did) {
ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
} else {
self.found_non_local_ty(ty)
}
}
// This should only be created when checking whether we have to check whether some
// auto trait impl applies. There will never be multiple impls, so we can just
// act as if it were a local type here.
ty::CoroutineWitness(..) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
};
// A bit of a hack, the `OrphanChecker` is only used to visit a `TraitRef`, so
// the first type we visit is always the self type.
self.in_self_ty = false;
result
}
/// All possible values for a constant parameter already exist
/// in the crate defining the trait, so they are always non-local[^1].
///
/// Because there's no way to have an impl where the first local
/// generic argument is a constant, we also don't have to fail
/// the orphan check when encountering a parameter or a generic constant.
///
/// This means that we can completely ignore constants during the orphan check.
///
/// See `tests/ui/coherence/const-generics-orphan-check-ok.rs` for examples.
///
/// [^1]: This might not hold for function pointers or trait objects in the future.
/// As these should be quite rare as const arguments and especially rare as impl
/// parameters, allowing uncovered const parameters in impls seems more useful
/// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile.
fn visit_const(&mut self, _c: I::Const) -> Self::Result {
ControlFlow::Continue(())
}
}