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Always try to project predicates when finding auto traits in rustdoc 

Fixes #60726

Previous, AutoTraitFinder would only try to project predicates when the
predicate type contained an inference variable. When finding auto
traits, we only project to try to unify inference variables - we don't
otherwise learn any new information about the required bounds.

However, this lead to failing to properly generate a negative auto trait
impl (indicating that a type never implements a certain auto trait) in
the following unusual scenario:

In almost all cases, a type has an (implicit) negative impl of an auto
trait due some other type having an explicit *negative* impl of that
auto trait. For example:

struct MyType<T> {
    field: *const T
}

has an implicit 'impl<T> !Send for MyType<T>', due to the explicit
negative impl (in libcore) 'impl<T: ?Sized> !Send for *const T'.

However, as exposed by the 'abi_stable' crate, this isn't always the
case. This minimzed example shows how a type can never implement
'Send', due to a projection error:

```
pub struct True;
pub struct False;

pub trait MyTrait {
    type Project;
}

pub struct MyStruct<T> {
    field: T
}

impl MyTrait for u8 {
    type Project = False;
}

unsafe impl<T> Send for MyStruct<T>
    where T: MyTrait<Project=True> {}

pub struct Wrapper {
    inner: MyStruct<u8>
}
```

In this example, `<u8 as MyTrait>::Project == True'
must hold for 'MyStruct<u8>: Send' to hold.
However, '<u8 as MyTrait>::Project == False' holds instead

To properly account for this unusual case, we need to call
'poly_project_and_unify' on *all* predicates, not just those with
inference variables. This ensures that we catch the projection error
that occurs above, and don't incorrectly determine that 'Wrapper: Send'
holds.
This commit is contained in:
Aaron Hill 2019-05-13 01:37:04 -04:00
parent af39a1fd73
commit 476ea9ef1c
No known key found for this signature in database
GPG Key ID: B4087E510E98B164
2 changed files with 103 additions and 17 deletions
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85
src/librustc/traits/auto_trait.rs
View File

@ -700,22 +700,64 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
}
}
// We can only call poly_project_and_unify_type when our predicate's
// Ty contains an inference variable - otherwise, there won't be anything to
// unify
if p.ty().skip_binder().has_infer_types() {
debug!("Projecting and unifying projection predicate {:?}",
predicate);
match poly_project_and_unify_type(select, &obligation.with(p)) {
Err(e) => {
debug!(
"evaluate_nested_obligations: Unable to unify predicate \
'{:?}' '{:?}', bailing out",
ty, e
);
return false;
}
Ok(Some(v)) => {
// There are three possible cases when we project a predicate:
//
// 1. We encounter an error. This means that it's impossible for
// our current type to implement the auto trait - there's bound
// that we could add to our ParamEnv that would 'fix' this kind
// of error, as it's not caused by an unimplemented type.
//
// 2. We succesfully project the predicate (Ok(Some(_))), generating
// some subobligations. We then process these subobligations
// like any other generated sub-obligations.
//
// 3. We receieve an 'ambiguous' result (Ok(None))
// If we were actually trying to compile a crate,
// we would need to re-process this obligation later.
// However, all we care about is finding out what bounds
// are needed for our type to implement a particular auto trait.
// We've already added this obligation to our computed ParamEnv
// above (if it was necessary). Therefore, we don't need
// to do any further processing of the obligation.
//
// Note that we *must* try to project *all* projection predicates
// we encounter, even ones without inference variable.
// This ensures that we detect any projection errors,
// which indicate that our type can *never* implement the given
// auto trait. In that case, we will generate an explicit negative
// impl (e.g. 'impl !Send for MyType'). However, we don't
// try to process any of the generated subobligations -
// they contain no new information, since we already know
// that our type implements the projected-through trait,
// and can lead to weird region issues.
//
// Normally, we'll generate a negative impl as a result of encountering
// a type with an explicit negative impl of an auto trait
// (for example, raw pointers have !Send and !Sync impls)
// However, through some **interesting** manipulations of the type
// system, it's actually possible to write a type that never
// implements an auto trait due to a projection error, not a normal
// negative impl error. To properly handle this case, we need
// to ensure that we catch any potential projection errors,
// and turn them into an explicit negative impl for our type.
debug!("Projecting and unifying projection predicate {:?}",
predicate);
match poly_project_and_unify_type(select, &obligation.with(p)) {
Err(e) => {
debug!(
"evaluate_nested_obligations: Unable to unify predicate \
'{:?}' '{:?}', bailing out",
ty, e
);
return false;
}
Ok(Some(v)) => {
// We only care about sub-obligations
// when we started out trying to unify
// some inference variables. See the comment above
// for more infomration
if p.ty().skip_binder().has_infer_types() {
if !self.evaluate_nested_obligations(
ty,
v.clone().iter().cloned(),
@ -728,7 +770,16 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
return false;
}
}
Ok(None) => {
}
Ok(None) => {
// It's ok not to make progress when hvave no inference variables -
// in that case, we were only performing unifcation to check if an
// error occured (which would indicate that it's impossible for our
// type to implement the auto trait).
// However, we should always make progress (either by generating
// subobligations or getting an error) when we started off with
// inference variables
if p.ty().skip_binder().has_infer_types() {
panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
}
}

35
src/test/rustdoc/issue-60726.rs Normal file
View File

@ -0,0 +1,35 @@
use std::marker::PhantomData;
pub struct True;
pub struct False;
pub trait InterfaceType{
type Send;
}
pub struct FooInterface<T>(PhantomData<fn()->T>);
impl<T> InterfaceType for FooInterface<T> {
type Send=False;
}
pub struct DynTrait<I>{
_interface:PhantomData<fn()->I>,
_unsync_unsend:PhantomData<::std::rc::Rc<()>>,
}
unsafe impl<I> Send for DynTrait<I>
where
I:InterfaceType<Send=True>
{}
// @has issue_60726/struct.IntoIter.html
// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//code' "impl<T> !Send for \
// IntoIter<T>"
// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//code' "impl<T> !Sync for \
// IntoIter<T>"
pub struct IntoIter<T>{
hello:DynTrait<FooInterface<T>>,
}