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Remove *most* mentions of phantom fns and variance on traits. Leave some

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comments and also leave the entries in the variance tables for now.
This commit is contained in:
Niko Matsakis 2015-04-01 04:41:45 -04:00
parent 628d715ff4
commit 38fdd50e0b
12 changed files with 174 additions and 172 deletions
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28
src/libcore/marker.rs
View File

@ -276,12 +276,15 @@ macro_rules! impls{
#[unstable(feature = "core", reason = "deprecated")]
#[deprecated(since = "1.0.0", reason = "No longer needed")]
#[allow(deprecated)]
#[cfg(stage0)]
pub trait MarkerTrait : PhantomFn<Self,Self> { }
// ~~~~~ <-- FIXME(#22806)?
//
// Marker trait has been made invariant so as to avoid inf recursion,
// but we should ideally solve the underlying problem. That's a bit
// complicated.
/// `MarkerTrait` is deprecated and no longer needed.
#[unstable(feature = "core", reason = "deprecated")]
#[deprecated(since = "1.0.0", reason = "No longer needed")]
#[allow(deprecated)]
#[cfg(not(stage0))]
pub trait MarkerTrait { }
#[allow(deprecated)]
impl<T:?Sized> MarkerTrait for T { }
@ -290,7 +293,20 @@ impl<T:?Sized> MarkerTrait for T { }
#[lang="phantom_fn"]
#[unstable(feature = "core", reason = "deprecated")]
#[deprecated(since = "1.0.0", reason = "No longer needed")]
pub trait PhantomFn<A:?Sized,R:?Sized=()> { }
#[cfg(stage0)]
pub trait PhantomFn<A:?Sized,R:?Sized=()> {
}
/// `PhantomFn` is a deprecated marker trait that is no longer needed.
#[unstable(feature = "core", reason = "deprecated")]
#[deprecated(since = "1.0.0", reason = "No longer needed")]
#[cfg(not(stage0))]
pub trait PhantomFn<A:?Sized,R:?Sized=()> {
}
#[allow(deprecated)]
#[cfg(not(stage0))]
impl<A:?Sized,R:?Sized,T:?Sized> PhantomFn<A,R> for T { }
/// `PhantomData<T>` allows you to describe that a type acts as if it stores a value of type `T`,
/// even though it does not. This allows you to inform the compiler about certain safety properties

1
src/librustc/middle/lang_items.rs
View File

@ -321,7 +321,6 @@ lets_do_this! {
ExchangeHeapLangItem, "exchange_heap", exchange_heap;
OwnedBoxLangItem, "owned_box", owned_box;
PhantomFnItem, "phantom_fn", phantom_fn;
PhantomDataItem, "phantom_data", phantom_data;
// Deprecated:

9
src/librustc/middle/traits/object_safety.rs
View File

@ -138,11 +138,10 @@ fn supertraits_reference_self<'tcx>(tcx: &ty::ctxt<'tcx>,
match predicate {
ty::Predicate::Trait(ref data) => {
// In the case of a trait predicate, we can skip the "self" type.
Some(data.def_id()) != tcx.lang_items.phantom_fn() &&
data.0.trait_ref.substs.types.get_slice(TypeSpace)
.iter()
.cloned()
.any(is_self)
data.0.trait_ref.substs.types.get_slice(TypeSpace)
.iter()
.cloned()
.any(is_self)
}
ty::Predicate::Projection(..) |
ty::Predicate::TypeOutlives(..) |

8
src/librustc/middle/traits/select.rs
View File

@ -836,14 +836,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
ambiguous: false
};
// Check for the `PhantomFn` trait. This is really just a
// special annotation that is *always* considered to match, no
// matter what the type parameters are etc.
if self.tcx().lang_items.phantom_fn() == Some(obligation.predicate.def_id()) {
candidates.vec.push(PhantomFnCandidate);
return Ok(candidates);
}
// Other bounds. Consider both in-scope bounds from fn decl
// and applicable impls. There is a certain set of precedence rules here.

42
src/librustc_typeck/check/wf.rs
View File

@ -117,15 +117,10 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
self.check_variances_for_type_defn(item, ast_generics);
}
ast::ItemTrait(_, ref ast_generics, _, ref items) => {
ast::ItemTrait(_, _, _, ref items) => {
let trait_predicates =
ty::lookup_predicates(ccx.tcx, local_def(item.id));
reject_non_type_param_bounds(
ccx.tcx,
item.span,
&trait_predicates);
self.check_variances(item, ast_generics, &trait_predicates,
self.tcx().lang_items.phantom_fn());
reject_non_type_param_bounds(ccx.tcx, item.span, &trait_predicates);
if ty::trait_has_default_impl(ccx.tcx, local_def(item.id)) {
if !items.is_empty() {
ccx.tcx.sess.span_err(
@ -287,30 +282,7 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
ast_generics: &ast::Generics)
{
let item_def_id = local_def(item.id);
let predicates = ty::lookup_predicates(self.tcx(), item_def_id);
self.check_variances(item,
ast_generics,
&predicates,
self.tcx().lang_items.phantom_data());
}
fn check_variances(&self,
item: &ast::Item,
ast_generics: &ast::Generics,
ty_predicates: &ty::GenericPredicates<'tcx>,
suggested_marker_id: Option<ast::DefId>)
{
let variance_lang_items = &[
self.tcx().lang_items.phantom_fn(),
self.tcx().lang_items.phantom_data(),
];
let item_def_id = local_def(item.id);
let is_lang_item = variance_lang_items.iter().any(|n| *n == Some(item_def_id));
if is_lang_item {
return;
}
let ty_predicates = ty::lookup_predicates(self.tcx(), item_def_id);
let variances = ty::item_variances(self.tcx(), item_def_id);
let mut constrained_parameters: HashSet<_> =
@ -331,7 +303,7 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
continue;
}
let span = self.ty_param_span(ast_generics, item, space, index);
self.report_bivariance(span, param_ty.name, suggested_marker_id);
self.report_bivariance(span, param_ty.name);
}
for (space, index, &variance) in variances.regions.iter_enumerated() {
@ -342,7 +314,7 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
assert_eq!(space, TypeSpace);
let span = ast_generics.lifetimes[index].lifetime.span;
let name = ast_generics.lifetimes[index].lifetime.name;
self.report_bivariance(span, name, suggested_marker_id);
self.report_bivariance(span, name);
}
}
@ -377,14 +349,14 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
fn report_bivariance(&self,
span: Span,
param_name: ast::Name,
suggested_marker_id: Option<ast::DefId>)
param_name: ast::Name)
{
self.tcx().sess.span_err(
span,
&format!("parameter `{}` is never used",
param_name.user_string(self.tcx())));
let suggested_marker_id = self.tcx().lang_items.phantom_data();
match suggested_marker_id {
Some(def_id) => {
self.tcx().sess.fileline_help(

239
src/librustc_typeck/variance.rs
View File

@ -18,34 +18,121 @@
//! defined on type `X`, we only consider the definition of the type `X`
//! and the definitions of any types it references.
//!
//! We only infer variance for type parameters found on *types*: structs,
//! enums, and traits. We do not infer variance for type parameters found
//! on fns or impls. This is because those things are not type definitions
//! and variance doesn't really make sense in that context.
//!
//! It is worth covering what variance means in each case. For structs and
//! enums, I think it is fairly straightforward. The variance of the type
//! We only infer variance for type parameters found on *data types*
//! like structs and enums. In these cases, there is fairly straightforward
//! explanation for what variance means. The variance of the type
//! or lifetime parameters defines whether `T<A>` is a subtype of `T<B>`
//! (resp. `T<'a>` and `T<'b>`) based on the relationship of `A` and `B`
//! (resp. `'a` and `'b`). (FIXME #3598 -- we do not currently make use of
//! the variances we compute for type parameters.)
//! (resp. `'a` and `'b`).
//!
//! ### Variance on traits
//! We do not infer variance for type parameters found on traits, fns,
//! or impls. Variance on trait parameters can make indeed make sense
//! (and we used to compute it) but it is actually rather subtle in
//! meaning and not that useful in practice, so we removed it. See the
//! addendum for some details. Variances on fn/impl parameters, otoh,
//! doesn't make sense because these parameters are instantiated and
//! then forgotten, they don't persist in types or compiled
//! byproducts.
//!
//! The meaning of variance for trait parameters is more subtle and worth
//! expanding upon. There are in fact two uses of the variance values we
//! compute.
//! ### The algorithm
//!
//! #### Trait variance and object types
//! The basic idea is quite straightforward. We iterate over the types
//! defined and, for each use of a type parameter X, accumulate a
//! constraint indicating that the variance of X must be valid for the
//! variance of that use site. We then iteratively refine the variance of
//! X until all constraints are met. There is *always* a sol'n, because at
//! the limit we can declare all type parameters to be invariant and all
//! constraints will be satisfied.
//!
//! The first is for object types. Just as with structs and enums, we can
//! decide the subtyping relationship between two object types `&Trait<A>`
//! and `&Trait<B>` based on the relationship of `A` and `B`. Note that
//! for object types we ignore the `Self` type parameter -- it is unknown,
//! and the nature of dynamic dispatch ensures that we will always call a
//! As a simple example, consider:
//!
//! enum Option<A> { Some(A), None }
//! enum OptionalFn<B> { Some(|B|), None }
//! enum OptionalMap<C> { Some(|C| -> C), None }
//!
//! Here, we will generate the constraints:
//!
//! 1. V(A) <= +
//! 2. V(B) <= -
//! 3. V(C) <= +
//! 4. V(C) <= -
//!
//! These indicate that (1) the variance of A must be at most covariant;
//! (2) the variance of B must be at most contravariant; and (3, 4) the
//! variance of C must be at most covariant *and* contravariant. All of these
//! results are based on a variance lattice defined as follows:
//!
//! * Top (bivariant)
//! - +
//! o Bottom (invariant)
//!
//! Based on this lattice, the solution V(A)=+, V(B)=-, V(C)=o is the
//! optimal solution. Note that there is always a naive solution which
//! just declares all variables to be invariant.
//!
//! You may be wondering why fixed-point iteration is required. The reason
//! is that the variance of a use site may itself be a function of the
//! variance of other type parameters. In full generality, our constraints
//! take the form:
//!
//! V(X) <= Term
//! Term := + | - | * | o | V(X) | Term x Term
//!
//! Here the notation V(X) indicates the variance of a type/region
//! parameter `X` with respect to its defining class. `Term x Term`
//! represents the "variance transform" as defined in the paper:
//!
//! If the variance of a type variable `X` in type expression `E` is `V2`
//! and the definition-site variance of the [corresponding] type parameter
//! of a class `C` is `V1`, then the variance of `X` in the type expression
//! `C<E>` is `V3 = V1.xform(V2)`.
//!
//! ### Constraints
//!
//! If I have a struct or enum with where clauses:
//!
//! struct Foo<T:Bar> { ... }
//!
//! you might wonder whether the variance of `T` with respect to `Bar`
//! affects the variance `T` with respect to `Foo`. I claim no. The
//! reason: assume that `T` is invariant w/r/t `Bar` but covariant w/r/t
//! `Foo`. And then we have a `Foo<X>` that is upcast to `Foo<Y>`, where
//! `X <: Y`. However, while `X : Bar`, `Y : Bar` does not hold. In that
//! case, the upcast will be illegal, but not because of a variance
//! failure, but rather because the target type `Foo<Y>` is itself just
//! not well-formed. Basically we get to assume well-formedness of all
//! types involved before considering variance.
//!
//! ### Addendum: Variance on traits
//!
//! As mentioned above, we used to permit variance on traits. This was
//! computed based on the appearance of trait type parameters in
//! method signatures and was used to represent the compatibility of
//! vtables in trait objects (and also "virtual" vtables or dictionary
//! in trait bounds). One complication was that variance for
//! associated types is less obvious, since they can be projected out
//! and put to myriad uses, so it's not clear when it is safe to allow
//! `X<A>::Bar` to vary (or indeed just what that means). Moreover (as
//! covered below) all inputs on any trait with an associated type had
//! to be invariant, limiting the applicability. Finally, the
//! annotations (`MarkerTrait`, `PhantomFn`) needed to ensure that all
//! trait type parameters had a variance were confusing and annoying
//! for little benefit.
//!
//! Just for historical reference,I am going to preserve some text indicating
//! how one could interpret variance and trait matching.
//!
//! #### Variance and object types
//!
//! Just as with structs and enums, we can decide the subtyping
//! relationship between two object types `&Trait<A>` and `&Trait<B>`
//! based on the relationship of `A` and `B`. Note that for object
//! types we ignore the `Self` type parameter -- it is unknown, and
//! the nature of dynamic dispatch ensures that we will always call a
//! function that is expected the appropriate `Self` type. However, we
//! must be careful with the other type parameters, or else we could end
//! up calling a function that is expecting one type but provided another.
//! must be careful with the other type parameters, or else we could
//! end up calling a function that is expecting one type but provided
//! another.
//!
//! To see what I mean, consider a trait like so:
//!
@ -135,104 +222,24 @@
//!
//! These conditions are satisfied and so we are happy.
//!
//! ### The algorithm
//! #### Variance and associated types
//!
//! The basic idea is quite straightforward. We iterate over the types
//! defined and, for each use of a type parameter X, accumulate a
//! constraint indicating that the variance of X must be valid for the
//! variance of that use site. We then iteratively refine the variance of
//! X until all constraints are met. There is *always* a sol'n, because at
//! the limit we can declare all type parameters to be invariant and all
//! constraints will be satisfied.
//!
//! As a simple example, consider:
//!
//! enum Option<A> { Some(A), None }
//! enum OptionalFn<B> { Some(|B|), None }
//! enum OptionalMap<C> { Some(|C| -> C), None }
//!
//! Here, we will generate the constraints:
//!
//! 1. V(A) <= +
//! 2. V(B) <= -
//! 3. V(C) <= +
//! 4. V(C) <= -
//!
//! These indicate that (1) the variance of A must be at most covariant;
//! (2) the variance of B must be at most contravariant; and (3, 4) the
//! variance of C must be at most covariant *and* contravariant. All of these
//! results are based on a variance lattice defined as follows:
//!
//! * Top (bivariant)
//! - +
//! o Bottom (invariant)
//!
//! Based on this lattice, the solution V(A)=+, V(B)=-, V(C)=o is the
//! optimal solution. Note that there is always a naive solution which
//! just declares all variables to be invariant.
//!
//! You may be wondering why fixed-point iteration is required. The reason
//! is that the variance of a use site may itself be a function of the
//! variance of other type parameters. In full generality, our constraints
//! take the form:
//!
//! V(X) <= Term
//! Term := + | - | * | o | V(X) | Term x Term
//!
//! Here the notation V(X) indicates the variance of a type/region
//! parameter `X` with respect to its defining class. `Term x Term`
//! represents the "variance transform" as defined in the paper:
//!
//! If the variance of a type variable `X` in type expression `E` is `V2`
//! and the definition-site variance of the [corresponding] type parameter
//! of a class `C` is `V1`, then the variance of `X` in the type expression
//! `C<E>` is `V3 = V1.xform(V2)`.
//!
//! ### Constraints
//!
//! If I have a struct or enum with where clauses:
//!
//! struct Foo<T:Bar> { ... }
//!
//! you might wonder whether the variance of `T` with respect to `Bar`
//! affects the variance `T` with respect to `Foo`. I claim no. The
//! reason: assume that `T` is invariant w/r/t `Bar` but covariant w/r/t
//! `Foo`. And then we have a `Foo<X>` that is upcast to `Foo<Y>`, where
//! `X <: Y`. However, while `X : Bar`, `Y : Bar` does not hold. In that
//! case, the upcast will be illegal, but not because of a variance
//! failure, but rather because the target type `Foo<Y>` is itself just
//! not well-formed. Basically we get to assume well-formedness of all
//! types involved before considering variance.
//!
//! ### Associated types
//!
//! Any trait with an associated type is invariant with respect to all
//! of its inputs. To see why this makes sense, consider what
//! subtyping for a trait reference means:
//! Traits with associated types -- or at minimum projection
//! expressions -- must be invariant with respect to all of their
//! inputs. To see why this makes sense, consider what subtyping for a
//! trait reference means:
//!
//! <T as Trait> <: <U as Trait>
//!
//! means that if I know that `T as Trait`,
//! I also know that `U as
//! Trait`. Moreover, if you think of it as
//! dictionary passing style, it means that
//! a dictionary for `<T as Trait>` is safe
//! to use where a dictionary for `<U as
//! Trait>` is expected.
//! means that if I know that `T as Trait`, I also know that `U as
//! Trait`. Moreover, if you think of it as dictionary passing style,
//! it means that a dictionary for `<T as Trait>` is safe to use where
//! a dictionary for `<U as Trait>` is expected.
//!
//! The problem is that when you can
//! project types out from `<T as Trait>`,
//! the relationship to types projected out
//! of `<U as Trait>` is completely unknown
//! unless `T==U` (see #21726 for more
//! details). Making `Trait` invariant
//! ensures that this is true.
//!
//! *Historical note: we used to preserve this invariant another way,
//! by tweaking the subtyping rules and requiring that when a type `T`
//! appeared as part of a projection, that was considered an invariant
//! location, but this version does away with the need for those
//! somewhat "special-case-feeling" rules.*
//! The problem is that when you can project types out from `<T as
//! Trait>`, the relationship to types projected out of `<U as Trait>`
//! is completely unknown unless `T==U` (see #21726 for more
//! details). Making `Trait` invariant ensures that this is true.
//!
//! Another related reason is that if we didn't make traits with
//! associated types invariant, then projection is no longer a
@ -383,7 +390,6 @@ fn determine_parameters_to_be_inferred<'a, 'tcx>(tcx: &'a ty::ctxt<'tcx>,
fn lang_items(tcx: &ty::ctxt) -> Vec<(ast::NodeId,Vec<ty::Variance>)> {
let all = vec![
(tcx.lang_items.phantom_fn(), vec![ty::Contravariant, ty::Covariant]),
(tcx.lang_items.phantom_data(), vec![ty::Covariant]),
(tcx.lang_items.unsafe_cell_type(), vec![ty::Invariant]),
@ -520,6 +526,9 @@ impl<'a, 'tcx, 'v> Visitor<'v> for TermsContext<'a, 'tcx> {
self.add_inferreds_for_item(item.id, false, generics);
}
ast::ItemTrait(_, ref generics, _, _) => {
// Note: all inputs for traits are ultimately
// constrained to be invariant. See `visit_item` in
// the impl for `ConstraintContext` below.
self.add_inferreds_for_item(item.id, true, generics);
visit::walk_item(self, item);
}

4
src/test/compile-fail/variance-contravariant-arg-object.rs
View File

@ -10,8 +10,8 @@
#![allow(dead_code)]
// This test was previously testing variance on traits.
// But now that it is removed, both cases error.
// Test that even when `T` is only used in contravariant position, it
// is treated as invariant.
trait Get<T> : 'static {
fn get(&self, t: T);

3
src/test/compile-fail/variance-contravariant-arg-trait-match.rs
View File

@ -10,6 +10,9 @@
#![allow(dead_code)]
// Test that even when `T` is only used in contravariant position, it
// is treated as invariant.
trait Get<T> {
fn get(&self, t: T);
}

3
src/test/compile-fail/variance-contravariant-self-trait-match.rs
View File

@ -10,6 +10,9 @@
#![allow(dead_code)]
// Test that even when `Self` is only used in contravariant position, it
// is treated as invariant.
trait Get {
fn get(&self);
}

3
src/test/compile-fail/variance-covariant-arg-object.rs
View File

@ -10,6 +10,9 @@
#![allow(dead_code)]
// Test that even when `T` is only used in covariant position, it
// is treated as invariant.
trait Get<T> : 'static {
fn get(&self) -> T;
}

3
src/test/compile-fail/variance-covariant-arg-trait-match.rs
View File

@ -10,6 +10,9 @@
#![allow(dead_code)]
// Test that even when `T` is only used in covariant position, it
// is treated as invariant.
trait Get<T> {
fn get(&self) -> T;
}

3
src/test/compile-fail/variance-covariant-self-trait-match.rs
View File

@ -10,6 +10,9 @@
#![allow(dead_code)]
// Test that even when `Self` is only used in covariant position, it
// is treated as invariant.
trait Get {
fn get() -> Self;
}