Auto merge of #116821 - Nadrieril:fix-opaque-ice, r=compiler-errors

Exhaustiveness: reveal opaque types properly

Previously, exhaustiveness had no clear policy around opaque types. In this PR I propose the following policy: within the body of an item that defines the hidden type of some opaque type, exhaustiveness checking on a value of that opaque type is performed using the concrete hidden type inferred in this body.

I'm not sure how consistent this is with other operations allowed on opaque types; I believe this will require FCP.

From what I can tell, this doesn't change anything for non-empty types.

The observable changes are:
- when the real type is uninhabited, matches within the defining scopes can now rely on that for exhaustiveness, e.g.:

```rust
#[derive(Copy, Clone)]
enum Void {}
fn return_never_rpit(x: Void) -> impl Copy {
    if false {
        match return_never_rpit(x) {}
    }
    x
}
```
- this properly fixes ICEs like https://github.com/rust-lang/rust/issues/117100 that occurred because a same match could have some patterns where the type is revealed and some where it is not.

Bonus subtle point: if `x` is opaque, a match like `match x { ("", "") => {} ... }` will constrain its type ([playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=901d715330eac40339b4016ac566d6c3)). This is not the case for `match x {}`: this will not constain the type, and will only compile if something else constrains the type to be empty.

Fixes https://github.com/rust-lang/rust/issues/117100

r? `@oli-obk`

Edited for precision of the wording

[Included](https://github.com/rust-lang/rust/pull/116821#issuecomment-1813171764) in the FCP on this PR is this rule:

> Within the body of an item that defines the hidden type of some opaque type, exhaustiveness checking on a value of that opaque type is performed using the concrete hidden type inferred in this body.
This commit is contained in:
bors 2023-12-22 12:12:12 +00:00
commit c1fc1d18cd
7 changed files with 284 additions and 33 deletions

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@ -28,6 +28,7 @@ use rustc_span::hygiene::DesugaringKind;
use rustc_span::Span;
pub(crate) fn check_match(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(), ErrorGuaranteed> {
let typeck_results = tcx.typeck(def_id);
let (thir, expr) = tcx.thir_body(def_id)?;
let thir = thir.borrow();
let pattern_arena = TypedArena::default();
@ -35,6 +36,7 @@ pub(crate) fn check_match(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(), Err
let mut visitor = MatchVisitor {
tcx,
thir: &*thir,
typeck_results,
param_env: tcx.param_env(def_id),
lint_level: tcx.local_def_id_to_hir_id(def_id),
let_source: LetSource::None,
@ -80,6 +82,7 @@ enum LetSource {
struct MatchVisitor<'thir, 'p, 'tcx> {
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
typeck_results: &'tcx ty::TypeckResults<'tcx>,
thir: &'thir Thir<'tcx>,
lint_level: HirId,
let_source: LetSource,
@ -382,6 +385,7 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
scrutinee.map(|scrut| self.is_known_valid_scrutinee(scrut)).unwrap_or(true);
MatchCheckCtxt {
tcx: self.tcx,
typeck_results: self.typeck_results,
param_env: self.param_env,
module: self.tcx.parent_module(self.lint_level).to_def_id(),
pattern_arena: self.pattern_arena,

View File

@ -62,7 +62,8 @@ pub trait TypeCx: Sized + Clone + fmt::Debug {
/// patterns during analysis.
type PatData: Clone + Default;
fn is_opaque_ty(ty: Self::Ty) -> bool;
/// FIXME(Nadrieril): `Cx` should only give us revealed types.
fn reveal_opaque_ty(&self, ty: Self::Ty) -> Self::Ty;
fn is_exhaustive_patterns_feature_on(&self) -> bool;
/// The number of fields for this constructor.

View File

@ -48,22 +48,14 @@ impl<'a, 'p, 'tcx> PatternColumn<'a, 'p, 'tcx> {
fn is_empty(&self) -> bool {
self.patterns.is_empty()
}
fn head_ty(&self) -> Option<Ty<'tcx>> {
fn head_ty(&self, cx: MatchCtxt<'a, 'p, 'tcx>) -> Option<Ty<'tcx>> {
if self.patterns.len() == 0 {
return None;
}
// If the type is opaque and it is revealed anywhere in the column, we take the revealed
// version. Otherwise we could encounter constructors for the revealed type and crash.
let first_ty = self.patterns[0].ty();
if RustcMatchCheckCtxt::is_opaque_ty(first_ty) {
for pat in &self.patterns {
let ty = pat.ty();
if !RustcMatchCheckCtxt::is_opaque_ty(ty) {
return Some(ty);
}
}
}
Some(first_ty)
let ty = self.patterns[0].ty();
// FIXME(Nadrieril): `Cx` should only give us revealed types.
Some(cx.tycx.reveal_opaque_ty(ty))
}
/// Do constructor splitting on the constructors of the column.
@ -125,7 +117,7 @@ fn collect_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
cx: MatchCtxt<'a, 'p, 'tcx>,
column: &PatternColumn<'a, 'p, 'tcx>,
) -> Vec<WitnessPat<'p, 'tcx>> {
let Some(ty) = column.head_ty() else {
let Some(ty) = column.head_ty(cx) else {
return Vec::new();
};
let pcx = &PlaceCtxt::new_dummy(cx, ty);
@ -226,7 +218,7 @@ pub(crate) fn lint_overlapping_range_endpoints<'a, 'p, 'tcx>(
cx: MatchCtxt<'a, 'p, 'tcx>,
column: &PatternColumn<'a, 'p, 'tcx>,
) {
let Some(ty) = column.head_ty() else {
let Some(ty) = column.head_ty(cx) else {
return;
};
let pcx = &PlaceCtxt::new_dummy(cx, ty);

View File

@ -44,6 +44,7 @@ pub type WitnessPat<'p, 'tcx> = crate::pat::WitnessPat<RustcMatchCheckCtxt<'p, '
#[derive(Clone)]
pub struct RustcMatchCheckCtxt<'p, 'tcx> {
pub tcx: TyCtxt<'tcx>,
pub typeck_results: &'tcx ty::TypeckResults<'tcx>,
/// The module in which the match occurs. This is necessary for
/// checking inhabited-ness of types because whether a type is (visibly)
/// inhabited can depend on whether it was defined in the current module or
@ -101,6 +102,21 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
}
}
/// Type inference occasionally gives us opaque types in places where corresponding patterns
/// have more specific types. To avoid inconsistencies as well as detect opaque uninhabited
/// types, we use the corresponding concrete type if possible.
fn reveal_opaque_ty(&self, ty: Ty<'tcx>) -> Ty<'tcx> {
if let ty::Alias(ty::Opaque, alias_ty) = ty.kind() {
if let Some(local_def_id) = alias_ty.def_id.as_local() {
let key = ty::OpaqueTypeKey { def_id: local_def_id, args: alias_ty.args };
if let Some(real_ty) = self.typeck_results.concrete_opaque_types.get(&key) {
return real_ty.ty;
}
}
}
ty
}
// In the cases of either a `#[non_exhaustive]` field list or a non-public field, we hide
// uninhabited fields in order not to reveal the uninhabitedness of the whole variant.
// This lists the fields we keep along with their types.
@ -873,8 +889,9 @@ impl<'p, 'tcx> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
fn is_exhaustive_patterns_feature_on(&self) -> bool {
self.tcx.features().exhaustive_patterns
}
fn is_opaque_ty(ty: Self::Ty) -> bool {
matches!(ty.kind(), ty::Alias(ty::Opaque, ..))
fn reveal_opaque_ty(&self, ty: Ty<'tcx>) -> Ty<'tcx> {
self.reveal_opaque_ty(ty)
}
fn ctor_arity(&self, ctor: &crate::constructor::Constructor<Self>, ty: Self::Ty) -> usize {

View File

@ -865,24 +865,14 @@ impl<'a, 'p, Cx: TypeCx> Matrix<'a, 'p, Cx> {
matrix
}
fn head_ty(&self) -> Option<Cx::Ty> {
fn head_ty(&self, mcx: MatchCtxt<'a, 'p, Cx>) -> Option<Cx::Ty> {
if self.column_count() == 0 {
return None;
}
let mut ty = self.wildcard_row.head().ty();
// If the type is opaque and it is revealed anywhere in the column, we take the revealed
// version. Otherwise we could encounter constructors for the revealed type and crash.
if Cx::is_opaque_ty(ty) {
for pat in self.heads() {
let pat_ty = pat.ty();
if !Cx::is_opaque_ty(pat_ty) {
ty = pat_ty;
break;
}
}
}
Some(ty)
let ty = self.wildcard_row.head().ty();
// FIXME(Nadrieril): `Cx` should only give us revealed types.
Some(mcx.tycx.reveal_opaque_ty(ty))
}
fn column_count(&self) -> usize {
self.wildcard_row.len()
@ -1181,7 +1171,7 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
) -> WitnessMatrix<Cx> {
debug_assert!(matrix.rows().all(|r| r.len() == matrix.column_count()));
let Some(ty) = matrix.head_ty() else {
let Some(ty) = matrix.head_ty(mcx) else {
// The base case: there are no columns in the matrix. We are morally pattern-matching on ().
// A row is useful iff it has no (unguarded) rows above it.
for row in matrix.rows_mut() {

View File

@ -0,0 +1,146 @@
#![feature(never_type)]
#![feature(exhaustive_patterns)]
#![feature(type_alias_impl_trait)]
#![feature(non_exhaustive_omitted_patterns_lint)]
#![deny(unreachable_patterns)]
// Test that the lint traversal handles opaques correctly
#![deny(non_exhaustive_omitted_patterns)]
fn main() {}
#[derive(Copy, Clone)]
enum Void {}
fn return_never_rpit(x: Void) -> impl Copy {
if false {
match return_never_rpit(x) {
_ => {} //~ ERROR unreachable
}
}
x
}
fn friend_of_return_never_rpit(x: Void) {
match return_never_rpit(x) {}
//~^ ERROR non-empty
}
type T = impl Copy;
fn return_never_tait(x: Void) -> T {
if false {
match return_never_tait(x) {
_ => {} //~ ERROR unreachable
}
}
x
}
fn friend_of_return_never_tait(x: Void) {
match return_never_tait(x) {}
//~^ ERROR non-empty
}
fn option_never(x: Void) -> Option<impl Copy> {
if false {
match option_never(x) {
None => {}
Some(_) => {} //~ ERROR unreachable
}
match option_never(x) {
None => {}
// FIXME: Unreachable not detected because `is_uninhabited` does not look into opaque
// types.
_ => {}
}
}
Some(x)
}
fn option_never2(x: Void) -> impl Copy {
if false {
match option_never2(x) {
None => {}
Some(_) => {} //~ ERROR unreachable
}
match option_never2(x) {
None => {}
_ => {} //~ ERROR unreachable
}
match option_never2(x) {
None => {}
}
}
Some(x)
}
fn inner_never(x: Void) {
type T = impl Copy;
let y: T = x;
match y {
_ => {} //~ ERROR unreachable
}
}
// This one caused ICE https://github.com/rust-lang/rust/issues/117100.
fn inner_tuple() {
type T = impl Copy;
let foo: T = Some((1u32, 2u32));
match foo {
_ => {}
Some((a, b)) => {} //~ ERROR unreachable
}
}
type U = impl Copy;
fn unify_never(x: Void, u: U) -> U {
if false {
match u {
_ => {} //~ ERROR unreachable
}
}
x
}
type V = impl Copy;
fn infer_in_match(x: Option<V>) {
match x {
None => {}
Some((a, b)) => {}
Some((mut x, mut y)) => {
//~^ ERROR unreachable
x = 42;
y = "foo";
}
}
}
type W = impl Copy;
#[derive(Copy, Clone)]
struct Rec<'a> {
n: u32,
w: Option<&'a W>,
}
fn recursive_opaque() -> W {
if false {
match recursive_opaque() {
// Check for the ol' ICE when the type is recursively opaque.
_ => {}
Rec { n: 0, w: Some(Rec { n: 0, w: _ }) } => {} //~ ERROR unreachable
}
}
let w: Option<&'static W> = None;
Rec { n: 0, w }
}
type X = impl Copy;
struct SecretelyVoid(X);
fn nested_empty_opaque(x: Void) -> X {
if false {
let opaque_void = nested_empty_opaque(x);
let secretely_void = SecretelyVoid(opaque_void);
match secretely_void {
// FIXME: Unreachable not detected because `is_uninhabited` does not look into opaque
// types.
_ => {}
}
}
x
}

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@ -0,0 +1,101 @@
error: unreachable pattern
--> $DIR/impl-trait.rs:17:13
|
LL | _ => {}
| ^
|
note: the lint level is defined here
--> $DIR/impl-trait.rs:5:9
|
LL | #![deny(unreachable_patterns)]
| ^^^^^^^^^^^^^^^^^^^^
error: unreachable pattern
--> $DIR/impl-trait.rs:31:13
|
LL | _ => {}
| ^
error: unreachable pattern
--> $DIR/impl-trait.rs:45:13
|
LL | Some(_) => {}
| ^^^^^^^
error: unreachable pattern
--> $DIR/impl-trait.rs:61:13
|
LL | Some(_) => {}
| ^^^^^^^
error: unreachable pattern
--> $DIR/impl-trait.rs:65:13
|
LL | _ => {}
| ^
error: unreachable pattern
--> $DIR/impl-trait.rs:78:9
|
LL | _ => {}
| ^
error: unreachable pattern
--> $DIR/impl-trait.rs:88:9
|
LL | _ => {}
| - matches any value
LL | Some((a, b)) => {}
| ^^^^^^^^^^^^ unreachable pattern
error: unreachable pattern
--> $DIR/impl-trait.rs:96:13
|
LL | _ => {}
| ^
error: unreachable pattern
--> $DIR/impl-trait.rs:107:9
|
LL | Some((mut x, mut y)) => {
| ^^^^^^^^^^^^^^^^^^^^
error: unreachable pattern
--> $DIR/impl-trait.rs:126:13
|
LL | _ => {}
| - matches any value
LL | Rec { n: 0, w: Some(Rec { n: 0, w: _ }) } => {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable pattern
error[E0004]: non-exhaustive patterns: type `impl Copy` is non-empty
--> $DIR/impl-trait.rs:23:11
|
LL | match return_never_rpit(x) {}
| ^^^^^^^^^^^^^^^^^^^^
|
= note: the matched value is of type `impl Copy`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown
|
LL ~ match return_never_rpit(x) {
LL + _ => todo!(),
LL + }
|
error[E0004]: non-exhaustive patterns: type `T` is non-empty
--> $DIR/impl-trait.rs:37:11
|
LL | match return_never_tait(x) {}
| ^^^^^^^^^^^^^^^^^^^^
|
= note: the matched value is of type `T`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown
|
LL ~ match return_never_tait(x) {
LL + _ => todo!(),
LL + }
|
error: aborting due to 12 previous errors
For more information about this error, try `rustc --explain E0004`.