Slice support #1086
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Slice support #1086
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bjorn3
reviewed
Apr 6, 2022
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Just as a heads up rust-lang/rust#94963 changed the way this kind of impls on builtin and foreign types are handled. Libcore now uses #![rustc_coherence_is_core] to allow impls on any builtin type without #[lang = "..."] and other sysroot crates use #[rustc_allow_incoherent_impl] on all impl items for non-local impls.
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CohenArthur
reviewed
Apr 11, 2022
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This is unfortunatly a mega commit, in testing gccrs against the slice code
which is highly generic stress tested our implementation of generics and
poked the hole in or lack of support of generic higher ranked trait bounds
and more specificily generic associated types. More refactoring is needed
to eventually remove the setup_associated_types and replace it entirely
with this new setup_associated_types2 which takes into account the trait
bound receiver and its predicate.
In order to support slices, the code in libcore defines an index lang item
```rust
impl<T, I> Index<I> for [T]
where
I: SliceIndex<[T]>,
{
type Output = I::Output;
fn index(&self, index: I) -> &I::Output {
index.index(self)
}
}
```
This is the entry point where by the self here is a generic slice. So in
our case we have:
```rust
let a = [1, 2, 3, 4, 5];
let b = &a[1..3];
```
'a' is an array and b is our desired slice, so we must remember that from
algebraic data type constructor. But our receiver is still an array, so in
order to be able to call this index lang item we must 'unsize' our array
(see #1045) this allows for method resolution to adjust an array into a
FatPtr which is simply a struct containing reference to the array and the
capacity (GCC MAX_DOMAIN) of the underlying array data type. So now we are
able to infer the substituions for this index fn call to:
```
fn index(&self : [<integer>], index: Range<integer>)
-> &I::Output->placeholder
```
The complex piece here is the Higher ranked trait bound:
```
where I: SliceIndex<[T]>
```
So in this method call no generic arguments are specified so we must try
and infer the types. So during monomorphization the inference variables
need to be recursively propogated into the higher ranked trait bound. So
that the higher ranked trait bound looks like:
```
SliceIndex<[<integer>]> // like we seen earlier for the Self type
```
The monomorphization stage also needs to take into account the higher
ranked trait bound's type which is 'I' and infered to be: Range<integer>.
This is where specialization needs to occur.
```rust
unsafe impl<T> SliceIndex<[T]> for Range<usize> {
type Output = [T];
unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
unsafe {
let a: *const T = slice.as_ptr();
let b: *const T = a.add(self.start);
slice_from_raw_parts(b, self.end - self.start)
}
}
fn index(self, slice: &[T]) -> &[T] {
unsafe { &*self.get_unchecked(slice) }
}
}
```
So now we need to compute the constrained type-parameters for this
specialized impl block. And in this case is fairly simple:
```
impl<T> SliceIndex<[T]> for Range<usize>
vs
I: SliceIndex<[<integer>]> and Range<<integer>>
```
Here we need to compute that T is <integer>, which is required since
associated type Output is used in our original method call and this
is generic which requires us to set it up but both the Self type or
the trait bound here in this impl block could be generic so special
care needs to be taken to compute this safely. Once the constrained
types are computer we can also unify the Self types which specializes
our original Range<integer> type into the correct Range<usize> that
this trait bound expects. We used a callback here when we reusively
pass down the SubstitutionArgumentMappings when any Parameter type
is substitued we get a callback to hold a set of mappings in a generic
way what generic types are being substituted.
From all of this work this stressed our generics implementation to
breaking point due to the use of the generic trait bound which was
not supported and it also exposed many bugs in our implementation.
This is why I feel it is best to keep this a large patch as so much
of this patch will cause regressions if we don't keep it together.
One of the main changes we have made is how we handle parameters
substitution for example we might have a generic such as '&Y' but
this gets substituted with Y=T which is a new type parameter. Before
we used to directly just change this from &Y to &T which is correct
but this looses context from the generic argument bindings. So now
we maintain the information that &Y changes to &(Y=T) so that we see
Y was substutued with T so that subsequent substitutions or inferences
can change Y=?T and correctly map &Y to &(Y=T) to &(Y=?T).
The other major piece which was changed during this patch was how
we perform the method resolution on higher ranked trait bound calls
where we compute the specified bound possible candidates once so that
in the case:
```
trait Bar {
fn baz(&self)
}
fn <T:Bar> foo(a: &T) {
a.baz()
}
```
Here the type parameter T gets derefed to find the specified bound of
Bar which contains the method baz. This means that we try calling baz
with T vs &T which fails then we try the reference type T again. This
results into two useless adjustments of indirection and referencing but
GCC optimizes this away. Before this patch we computed the specified bound
for each attempt which was wrong.
Fixes #849
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philberty
added a commit
that referenced
this pull request
Apr 28, 2022
From PR #1086 I introduced a new setup_associated_types2 interface which is used to ensure we handle the complex associated types in libcore slices but this interface was inconsistant as well as the get_projected_type. This path refactors the code base to get rid of the old setup_associated_types interface in favour of this new one. It also removes the get_projected_type interface which was not going to work either. Fixes #1105
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Please see the commit a8de089 for
a long explanation of what's going on in the patch. Unfortunately, I have not been
able to split this patch up anymore since supporting slices exposed many bugs
in the implementation of generics in general never main the missing support for
generic associated types.
Fixes #849