split file and replace with deep dive doc
This commit is contained in:
Boxy
Michael Goulet
parent
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commit
0effe03071
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@ -109,9 +109,10 @@
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- [`TypeFolder` and `TypeFoldable`](./ty-fold.md)
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- [Generic arguments](./generic_arguments.md)
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- [Constants in the type system](./constants.md)
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- [Bound vars and Parameters](./bound-vars-and-params.md)
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- [Type inference](./type-inference.md)
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- [Trait solving](./traits/resolution.md)
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- [Early and Late Bound Parameters](./early-late-bound.md)
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- [Early and Late Bound Parameter Definitions](./early-late-bound.md)
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- [Higher-ranked trait bounds](./traits/hrtb.md)
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- [Caching subtleties](./traits/caching.md)
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- [Specialization](./traits/specialization.md)
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@ -0,0 +1,61 @@
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# Bound vars and parameters
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## Early-bound parameters
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Early-bound parameters in rustc are identified by an index, stored in the
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[`ParamTy`] struct for types or the [`EarlyBoundRegion`] struct for lifetimes.
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The index counts from the outermost declaration in scope. This means that as you
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add more binders inside, the index doesn't change.
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For example,
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```rust,ignore
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trait Foo<T> {
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type Bar<U> = (Self, T, U);
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}
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```
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Here, the type `(Self, T, U)` would be `($0, $1, $2)`, where `$N` means a
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[`ParamTy`] with the index of `N`.
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In rustc, the [`Generics`] structure carries this information. So the
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[`Generics`] for `Bar` above would be just like for `U` and would indicate the
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'parent' generics of `Foo`, which declares `Self` and `T`. You can read more
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in [this chapter](./generics.md).
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[`ParamTy`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.ParamTy.html
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[`EarlyBoundRegion`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.EarlyBoundRegion.html
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[`Generics`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Generics.html
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## Late-bound parameters
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Late-bound parameters in `rustc` are handled quite differently (they are also
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specialized to lifetimes since, right now, only late-bound lifetimes are
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supported, though with GATs that has to change). We indicate their potential
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presence by a [`Binder`] type. The [`Binder`] doesn't know how many variables
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there are at that binding level. This can only be determined by walking the
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type itself and collecting them. So a type like `for<'a, 'b> ('a, 'b)` would be
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`for (^0.a, ^0.b)`. Here, we just write `for` because we don't know the names
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of the things bound within.
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Moreover, a reference to a late-bound lifetime is written `^0.a`:
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- The `0` is the index; it identifies that this lifetime is bound in the
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innermost binder (the `for`).
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- The `a` is the "name"; late-bound lifetimes in rustc are identified by a
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"name" -- the [`BoundRegionKind`] enum. This enum can contain a
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[`DefId`][defid] or it might have various "anonymous" numbered names. The
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latter arise from types like `fn(&u32, &u32)`, which are equivalent to
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something like `for<'a, 'b> fn(&'a u32, &'b u32)`, but the names of those
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lifetimes must be generated.
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This setup of not knowing the full set of variables at a binding level has some
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advantages and some disadvantages. The disadvantage is that you must walk the
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type to find out what is bound at the given level and so forth. The advantage
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is primarily that, when constructing types from Rust syntax, if we encounter
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anonymous regions like in `fn(&u32)`, we just create a fresh index and don't have
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to update the binder.
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[`Binder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Binder.html
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[`BoundRegionKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.BoundRegionKind.html
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[defid]: ./hir.html#identifiers-in-the-hir
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@ -1,107 +1,199 @@
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# Early and Late Bound Variables
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# Early and Late Bound Parameter Definitions
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In Rust, item definitions (like `fn`) can often have generic parameters, which
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are always [_universally_ quantified][quant]. That is, if you have a function
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like
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Understanding this page likely requires a rudimentary understanding of higher ranked
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trait bounds/`for<'a>`and also what types such as `dyn for<'a> Trait<'a>` and
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`for<'a> fn(&'a u32)` mean. Reading [the nomincon chapter](https://doc.rust-lang.org/nomicon/hrtb.html)
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on HRTB may be useful for understanding this syntax. The meaning of `for<'a> fn(&'a u32)`
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is incredibly similar to the meaning of `T: for<'a> Trait<'a>`.
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If you are looking for information on the `RegionKind` variants `ReLateBound` and `ReEarlyBound`
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you should look at the section on [bound vars and params](./bound-vars-and-params.md). This section
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discusses what makes generic parameters on functions and closures late/early bound. Not the general
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concept of bound vars and generic parameters which `RegionKind` has named somewhat confusingly
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with this topic.
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## What does it mean for parameters to be early or late bound
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All function definitions conceptually have a zst (this is represented by `TyKind::FnDef` in rustc).
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The only generics on this zst are the early bound parameters of the function definition. e.g.
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```rust
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fn foo<T>(x: T) { }
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```
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fn foo<'a>(_: &'a u32) {}
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this function is defined "for all T" (not "for some specific T", which would be
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[_existentially_ quantified][quant]).
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[quant]: ./appendix/background.md#quantified
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While Rust *items* can be quantified over types, lifetimes, and constants, the
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types of values in Rust are only ever quantified over lifetimes. So you can
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have a type like `for<'a> fn(&'a u32)`, which represents a function pointer
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that takes a reference with any lifetime, or `for<'a> dyn Trait<'a>`, which is
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a `dyn` trait for a trait implemented for any lifetime; but we have no type
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like `for<T> fn(T)`, which would be a function that takes a value of *any type*
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as a parameter. This is a consequence of monomorphization -- to support a value
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of type `for<T> fn(T)`, we would need a single function pointer that can be
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used for a parameter of any type, but in Rust we generate customized code for
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each parameter type.
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One consequence of this asymmetry is a weird split in how we represent some
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generic types: _early-_ and _late-_ bound parameters.
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Basically, if we cannot represent a type (e.g. a universally quantified type),
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we have to bind it _early_ so that the unrepresentable type is never around.
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Consider the following example:
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```rust,ignore
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fn foo<'a, 'b, T>(x: &'a u32, y: &'b T) where T: 'b { ... }
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```
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We cannot treat `'a`, `'b`, and `T` in the same way. Types in Rust can't have
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`for<T> { .. }`, only `for<'a> {...}`, so whenever you reference `foo` the type
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you get back can't be `for<'a, 'b, T> fn(&'a u32, y: &'b T)`. Instead, the `T`
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must be substituted early. In particular, you have:
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```rust,ignore
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let x = foo; // T, 'b have to be substituted here
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x(...); // 'a substituted here, at the point of call
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x(...); // 'a substituted here with a different value
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```
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## Early-bound parameters
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Early-bound parameters in rustc are identified by an index, stored in the
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[`ParamTy`] struct for types or the [`EarlyBoundRegion`] struct for lifetimes.
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The index counts from the outermost declaration in scope. This means that as you
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add more binders inside, the index doesn't change.
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For example,
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```rust,ignore
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trait Foo<T> {
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type Bar<U> = (Self, T, U);
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fn main() {
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let b = foo;
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// ^ `b` has type `FnDef(foo, [])` (no substs because `'a` is late bound)
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assert!(std::mem::size_of_val(&b) == 0);
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}
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```
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Here, the type `(Self, T, U)` would be `($0, $1, $2)`, where `$N` means a
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[`ParamTy`] with the index of `N`.
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In order to call `b` the late bound parameters do need to be provided, these are inferred at the
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call site instead of when we refer to `foo`.
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```rust
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fn main() {
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let b = foo;
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let a: &'static u32 = &10;
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foo(a);
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// the lifetime argument for `'a` on `foo` is inferred at the callsite
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// the generic parameter `'a` on `foo` is inferred to `'static` here
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}
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```
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In rustc, the [`Generics`] structure carries this information. So the
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[`Generics`] for `Bar` above would be just like for `U` and would indicate the
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'parent' generics of `Foo`, which declares `Self` and `T`. You can read more
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in [this chapter](./generics.md).
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Because late bound parameters are not part of the `FnDef`'s substs this allows us to prove trait
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bounds such as `F: for<'a> Fn(&'a u32)` where `F` is `foo`'s `FnDef`. e.g.
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```rust
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fn foo_early<'a, T: Trait<'a>>(_: &'a u32, _: T) {}
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fn foo_late<'a, T>(_: &'a u32, _: T) {}
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[`ParamTy`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.ParamTy.html
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[`EarlyBoundRegion`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.EarlyBoundRegion.html
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[`Generics`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Generics.html
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fn accepts_hr_func<F: for<'a> Fn(&'a u32, u32)>(_: F) {}
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## Late-bound parameters
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fn main() {
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// doesnt work, the substituted bound is `for<'a> FnDef<'?0>: Fn(&'a u32, u32)`
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// `foo_early` only implements `for<'a> FnDef<'a>: Fn(&'a u32, u32)`- the lifetime
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// of the borrow in the function argument must be the same as the lifetime
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// on the `FnDef`.
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accepts_hr_func(foo_early);
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Late-bound parameters in `rustc` are handled quite differently (they are also
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specialized to lifetimes since, right now, only late-bound lifetimes are
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supported, though with GATs that has to change). We indicate their potential
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presence by a [`Binder`] type. The [`Binder`] doesn't know how many variables
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there are at that binding level. This can only be determined by walking the
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type itself and collecting them. So a type like `for<'a, 'b> ('a, 'b)` would be
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`for (^0.a, ^0.b)`. Here, we just write `for` because we don't know the names
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of the things bound within.
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// works, the substituted bound is `for<'a> FnDef: Fn(&'a u32, u32)`
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accepts_hr_func(foo_late);
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}
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Moreover, a reference to a late-bound lifetime is written `^0.a`:
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// the builtin `Fn` impls for `foo_early` and `foo_late` look something like:
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// `foo_early`
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impl<'a, T: Trait<'a>> Fn(&'a u32, T) for FooEarlyFnDef<'a, T> { ... }
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// `foo_late`
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impl<'a, T> Fn(&'a u32, T) for FooLateFnDef<T> { ... }
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- The `0` is the index; it identifies that this lifetime is bound in the
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innermost binder (the `for`).
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- The `a` is the "name"; late-bound lifetimes in rustc are identified by a
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"name" -- the [`BoundRegionKind`] enum. This enum can contain a
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[`DefId`][defid] or it might have various "anonymous" numbered names. The
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latter arise from types like `fn(&u32, &u32)`, which are equivalent to
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something like `for<'a, 'b> fn(&'a u32, &'b u32)`, but the names of those
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lifetimes must be generated.
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```
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This setup of not knowing the full set of variables at a binding level has some
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advantages and some disadvantages. The disadvantage is that you must walk the
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type to find out what is bound at the given level and so forth. The advantage
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is primarily that, when constructing types from Rust syntax, if we encounter
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anonymous regions like in `fn(&u32)`, we just create a fresh index and don't have
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to update the binder.
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Early bound parameters are present on the `FnDef`. Late bound generic parameters are not present
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on the `FnDef` but are instead constrained by the builtin `Fn*` impl.
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The same distinction applies to closures. Instead of `FnDef` we are talking about the anonymous
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closure type. Closures are [currently unsound](https://github.com/rust-lang/rust/issues/84366) in
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ways that are closely related to the distinction between early/late bound
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parameters (more on this later)
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The early/late boundness of generic parameters is only relevent for the desugaring of
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functions/closures into types with builtin `Fn*` impls. It does not make sense to talk about
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in other contexts.
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The `generics_of` query in rustc only contains early bound parameters. In this way it acts more
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like `generics_of(my_func)` is the generics for the FnDef than the generics provided to the function
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body although it's not clear to the author of this section if this was the actual justification for
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making `generics_of` behave this way.
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## What parameters are currently late bound
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Below are the current requirements for determining if a generic parameter is late bound. It is worth
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keeping in mind that these are not necessarily set in stone and it is almost certainly possible to
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be more flexible.
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### Must be a lifetime parameter
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Rust can't support types such as `for<T> dyn Trait<T>` or `for<T> fn(T)`, this is a
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fundamental limitation of the language as we are required to monomorphize type/const
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parameters and cannot do so behind dynamic dispatch. (technically we could probably
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support `for<T> dyn MarkerTrait<T>` as there is nothing to monomorphize)
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Not being able to support `for<T> dyn Trait<T>` resulted in making all type and const
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parameters early bound. Only lifetime parameters can be late bound.
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### Must not appear in the where clauses
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In order for a generic parameter to be late bound it must not appear in any where clauses.
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This is currently an incredibly simplistic check that causes lifetimes to be early bound even
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if the where clause they appear in are always true, or implied by well formedness of function
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arguments. e.g.
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```rust
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fn foo1<'a: 'a>(_: &'a u32) {}
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// ^^ early bound parameter because it's in a `'a: 'a` clause
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// even though the bound obviously holds all the time
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fn foo2<'a, T: Trait<'a>(a: T, b: &'a u32) {}
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// ^^ early bound parameter because it's used in the `T: Trait<'a>` clause
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fn foo3<'a, T: 'a>(_: &'a T) {}
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// ^^ early bound parameter because it's used in the `T: 'a` clause
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// even though that bound is implied by wellformedness of `&'a T`
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fn foo4<'a, 'b: 'a>(_: Inv<&'a ()>, _: Inv<&'b ()>) {}
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// ^^ ^^ ^^^ note:
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// ^^ ^^ `Inv` stands for `Invariant` and is used to
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// ^^ ^^ make the the type parameter invariant. This
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// ^^ ^^ is necessary for demonstration purposes as
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// ^^ ^^ `for<'a, 'b> fn(&'a (), &'b ())` and
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// ^^ ^^ `for<'a> fn(&'a u32, &'a u32)` are subtypes-
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// ^^ ^^ of eachother which makes the bound trivially
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// ^^ ^^ satisfiable when making the fnptr. `Inv`
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// ^^ ^^ disables this subtyping.
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// ^^ ^^
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// ^^^^^^ both early bound parameters because they are present in the
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// `'b: 'a` clause
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```
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The reason for this requirement is that we cannot represent the `T: Trait<'a>` or `'a: 'b` clauses
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on a function pointer. `for<'a, 'b> fn(Inv<&'a ()>, Inv<&'b ()>)` is not a valid function pointer to
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represent`foo4` as it would allow calling the function without `'b: 'a` holding.
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### Must be constrained by where clauses or function argument types
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The builtin impls of the `Fn*` traits for closures and `FnDef`s cannot not have any unconstrained
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parameters. For example the following impl is illegal:
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```rust
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impl<'a> Trait for u32 { type Assoc = &'a u32; }
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```
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We must not end up with a similar impl for the `Fn*` traits e.g.
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```rust
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impl<'a> Fn<()> for FnDef { type Assoc = &'a u32 }
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```
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|
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Violating this rule can trivially lead to unsoundness as seen in [#84366](https://github.com/rust-lang/rust/issues/84366).
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Additionally if we ever support late bound type params then an impl like:
|
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```rust
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impl<T> Fn<()> for FnDef { type Assoc = T; }
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```
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would break the compiler in various ways.
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In order to ensure that everything functions correctly, we do not allow generic parameters to
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be late bound if it would result in a builtin impl that does not constrain all of the generic
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parameters on the builtin impl. Making a generic parameter be early bound trivially makes it be
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constrained by the builtin impl as it ends up on the self type.
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Because of the requirement that late bound parameters must not appear in where clauses, checking
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this is simpler than the rules for checking impl headers constrain all the parameters on the impl.
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We only have to ensure that all late bound parameters appear at least once in the function argument
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types outside of an alias (e.g. an associated type).
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The requirement that they not indirectly be in the substs of an alias for it to count is the
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same as why the follow code is forbidden:
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```rust
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impl<T: Trait> OtherTrait for <T as Trait>::Assoc { type Assoc = T }
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```
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There is no guarantee that `<T as Trait>::Assoc` will normalize to different types for every
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instantiation of `T`. If we were to allow this impl we could get overlapping impls and the
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same is true of the builtin `Fn*` impls.
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## Making more generic parameters late bound
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It is generally considered desirable for more parameters to be late bound as it makes
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the builtin `Fn*` impls more flexible. Right now many of the requirements for making
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a parameter late bound are overly restrictive as they are tied to what we can currently
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(or can ever) do with fn ptrs.
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|
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It would be theoretically possible to support late bound params in `where`-clauses in the
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language by introducing implication types which would allow us to express types such as:
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`for<'a, 'b: 'a> fn(Inv<&'a u32>, Inv<&'b u32>)` which would ensure `'b: 'a` is upheld when
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calling the function pointer.
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|
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It would also be theoretically possible to support it by making the coercion to a fn ptr
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instantiate the parameter with an infer var while still allowing the FnDef to not have the
|
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generic parameter present as trait impls are perfectly capable of representing the where clauses
|
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on the function on the impl itself. This would also allow us to support late bound type/const
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vars allowing bounds like `F: for<T> Fn(T)` to hold.
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|
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It is almost somewhat unclear if we can change the `Fn` traits to be structured differently
|
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so that we never have to make a parameter early bound just to make the builtin impl have all
|
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generics be constrained. Of all the possible causes of a generic parameter being early bound
|
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this seems the most difficult to remove.
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|
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Whether these would be good ideas to implement is a separate question- they are only brought
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up to illustrate that the current rules are not necessarily set in stone and a result of
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"its the only way of doing this".
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[`Binder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Binder.html
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[`BoundRegionKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.BoundRegionKind.html
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[defid]: ./hir.html#identifiers-in-the-hir
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Reference in New Issue