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Check only predicates with a single param with a concrete default.
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This is the most conservative possible and should be always correct.
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@leoyvens
leoyvens committed Feb 28, 2018
1 parent ac17948 commit c74f85f
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Showing 4 changed files with 49 additions and 101 deletions.
72 changes: 31 additions & 41 deletions src/librustc_typeck/check/wfcheck.rs
View file
Expand Up @@ -278,7 +278,7 @@ impl<'a, 'gcx> CheckTypeWellFormedVisitor<'a, 'gcx> {
fn check_trait(&mut self, item: &hir::Item) {
let trait_def_id = self.tcx.hir.local_def_id(item.id);
self.for_item(item).with_fcx(|fcx, _| {
self.check_trait_where_clauses(fcx, item.span, trait_def_id);
self.check_where_clauses(fcx, item.span, trait_def_id);
vec![]
});
}
Expand Down Expand Up @@ -354,23 +354,6 @@ impl<'a, 'gcx> CheckTypeWellFormedVisitor<'a, 'gcx> {
fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
span: Span,
def_id: DefId) {
self.inner_check_where_clauses(fcx, span, def_id, false)
}

fn check_trait_where_clauses<'fcx, 'tcx>(&mut self,
fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
span: Span,
def_id: DefId) {
self.inner_check_where_clauses(fcx, span, def_id, true)
}

/// Checks where clauses and inline bounds that are declared on def_id.
fn inner_check_where_clauses<'fcx, 'tcx>(&mut self,
fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
span: Span,
def_id: DefId,
is_trait: bool)
{
use ty::subst::Subst;
use rustc::ty::TypeFoldable;

Expand All @@ -390,43 +373,50 @@ impl<'a, 'gcx> CheckTypeWellFormedVisitor<'a, 'gcx> {
}

// Check that trait predicates are WF when params are substituted by their defaults.
// We don't want to overly constrain the predicates that may be written but we
// want to catch obviously wrong cases such as `struct Foo<T: Copy = String>`
// or cases where defaults don't work together such as:
// `struct Foo<T = i32, U = u8> where T: into<U>`
// Therefore we check if a predicate in which all type params are defaulted
// is WF with those defaults simultaneously substituted.
// We don't want to overly constrain the predicates that may be written but we want to
// catch cases where a default my never be applied such as `struct Foo<T: Copy = String>`.
// Therefore we check if a predicate which contains a single type param
// with a concrete default is WF with that default substituted.
// For more examples see tests `defaults-well-formedness.rs` and `type-check-defaults.rs`.
//
// First we build the defaulted substitution.
let substs = ty::subst::Substs::for_item(fcx.tcx, def_id, |def, _| {
// All regions are identity.
fcx.tcx.mk_region(ty::ReEarlyBound(def.to_early_bound_region_data()))
}, |def, _| {
if !is_our_default(def) {
// We don't want to use non-defaulted params in a substitution, mark as err.
fcx.tcx.types.err
} else {
// Substitute with default.
fcx.tcx.type_of(def.def_id)
// If the param has a default,
if is_our_default(def) {
let default_ty = fcx.tcx.type_of(def.def_id);
// and it's not a dependent default
if !default_ty.needs_subst() {
// then substitute with the default.
return default_ty;
}
}
// Mark unwanted params as err.
fcx.tcx.types.err
});
// Now we build the substituted predicates.
for &pred in predicates.predicates.iter() {
struct CountParams { params: FxHashSet<u32> }
impl<'tcx> ty::fold::TypeVisitor<'tcx> for CountParams {
fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
match t.sty {
ty::TyParam(p) => {
self.params.insert(p.idx);
t.super_visit_with(self)
}
_ => t.super_visit_with(self)
}
}
}
let mut param_count = CountParams { params: FxHashSet() };
pred.visit_with(&mut param_count);
let substituted_pred = pred.subst(fcx.tcx, substs);
// If there is a non-defaulted param in the predicate, don't check it.
if substituted_pred.references_error() {
// Don't check non-defaulted params, dependent defaults or preds with multiple params.
if substituted_pred.references_error() || param_count.params.len() > 1 {
continue;
}
// In trait defs, don't check `Self: Sized` when `Self` is the default.
if let ty::Predicate::Trait(trait_pred) = substituted_pred {
// `skip_binder()` is ok, we're only inspecting for `has_self_ty()`.
let lhs_is_self = trait_pred.skip_binder().self_ty().has_self_ty();
let pred_sized = Some(trait_pred.def_id()) == fcx.tcx.lang_items().sized_trait();
if is_trait && lhs_is_self && pred_sized {
continue;
}
}
// Avoid duplication of predicates that contain no parameters, for example.
if !predicates.predicates.contains(&substituted_pred) {
substituted_predicates.push(substituted_pred);
Expand Down
9 changes: 6 additions & 3 deletions src/test/run-pass/defaults-well-formedness.rs
View file
Expand Up @@ -14,12 +14,15 @@ struct Foo<U, V=i32>(U, V) where U: Trait<V>;
trait Marker {}
struct TwoParams<T, U>(T, U);
impl Marker for TwoParams<i32, i32> {}
// Check that defaults are substituted simultaneously.

// Clauses with more than 1 param are not checked.
struct IndividuallyBogus<T = i32, U = i32>(TwoParams<T, U>) where TwoParams<T, U>: Marker;
struct BogusTogether<T = u32, U = i32>(T, U) where TwoParams<T, U>: Marker;
// Clauses with non-defaulted params are not checked.
struct NonDefaultedInClause<T, U = i32>(TwoParams<T, U>) where TwoParams<T, U>: Marker;
struct DefaultedLhs<U, V=i32>(U, V) where V: Trait<U>;
// Dependent defaults.
struct Dependent<T: Copy, U = T>(T, U) where U: Copy;
// Dependent defaults are not checked.
struct Dependent<T, U = T>(T, U) where U: Copy;
trait SelfBound<T: Copy=Self> {}

fn main() {}
16 changes: 1 addition & 15 deletions src/test/ui/type-check-defaults.rs
View file
Expand Up @@ -30,25 +30,11 @@ struct WhereClause<T=String>(T) where T: Copy;
trait TraitBound<T:Copy=String> {}
//~^ error: the trait bound `std::string::String: std::marker::Copy` is not satisfied [E0277]

trait SelfBound<T:Copy=Self> {}
//~^ error: the trait bound `Self: std::marker::Copy` is not satisfied [E0277]

trait Super<T: Copy> { }
trait Base<T = String>: Super<T> { }
//~^ error: the trait bound `T: std::marker::Copy` is not satisfied [E0277]

trait ProjectionPred<T:Iterator = IntoIter<i32>> where T::Item : Add<u8> {}
//~^ error: the trait bound `i32: std::ops::Add<u8>` is not satisfied [E0277]

// Defaults must work together.
struct TwoParams<T = u32, U = i32>(T, U) where T: Bar<U>;
//~^ the trait bound `u32: Bar<i32>` is not satisfied [E0277]
trait Bar<V> {}
impl Bar<String> for u32 { }
impl Bar<i32> for String { }

// Dependent defaults.
struct Dependent<T, U = T>(T, U) where U: Copy;
//~^ the trait bound `T: std::marker::Copy` is not satisfied [E0277]
//~^ error: cannot add `u8` to `i32` [E0277]

fn main() { }
53 changes: 11 additions & 42 deletions src/test/ui/type-check-defaults.stderr
View file
Expand Up @@ -25,13 +25,14 @@ note: required by `Foo`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: the trait bound `A: std::iter::Iterator` is not satisfied
--> $DIR/type-check-defaults.rs:21:1
--> $DIR/type-check-defaults.rs:21:32
|
21 | struct WellFormedProjection<A, T=<A as Iterator>::Item>(A, T);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `A` is not an iterator; maybe try calling `.iter()` or a similar method
| ^ `A` is not an iterator; maybe try calling `.iter()` or a similar method
|
= help: the trait `std::iter::Iterator` is not implemented for `A`
= help: consider adding a `where A: std::iter::Iterator` bound
= note: required by `std::iter::Iterator`

error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not satisfied
--> $DIR/type-check-defaults.rs:24:1
Expand All @@ -57,59 +58,27 @@ error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not sa
|
= note: required by `std::marker::Copy`

error[E0277]: the trait bound `Self: std::marker::Copy` is not satisfied
--> $DIR/type-check-defaults.rs:33:1
|
33 | trait SelfBound<T:Copy=Self> {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `Self`
|
= help: consider adding a `where Self: std::marker::Copy` bound
= note: required by `std::marker::Copy`

error[E0277]: the trait bound `T: std::marker::Copy` is not satisfied
--> $DIR/type-check-defaults.rs:37:1
--> $DIR/type-check-defaults.rs:34:1
|
37 | trait Base<T = String>: Super<T> { }
34 | trait Base<T = String>: Super<T> { }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `T`
|
= help: consider adding a `where T: std::marker::Copy` bound
note: required by `Super`
--> $DIR/type-check-defaults.rs:36:1
--> $DIR/type-check-defaults.rs:33:1
|
36 | trait Super<T: Copy> { }
33 | trait Super<T: Copy> { }
| ^^^^^^^^^^^^^^^^^^^^

error[E0277]: the trait bound `i32: std::ops::Add<u8>` is not satisfied
--> $DIR/type-check-defaults.rs:40:1
error[E0277]: cannot add `u8` to `i32`
--> $DIR/type-check-defaults.rs:37:1
|
40 | trait ProjectionPred<T:Iterator = IntoIter<i32>> where T::Item : Add<u8> {}
37 | trait ProjectionPred<T:Iterator = IntoIter<i32>> where T::Item : Add<u8> {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `i32 + u8`
|
= help: the trait `std::ops::Add<u8>` is not implemented for `i32`
= note: required by `std::ops::Add`

error[E0277]: the trait bound `u32: Bar<i32>` is not satisfied
--> $DIR/type-check-defaults.rs:44:1
|
44 | struct TwoParams<T = u32, U = i32>(T, U) where T: Bar<U>;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar<i32>` is not implemented for `u32`
|
= help: the following implementations were found:
<u32 as Bar<std::string::String>>
note: required by `Bar`
--> $DIR/type-check-defaults.rs:46:1
|
46 | trait Bar<V> {}
| ^^^^^^^^^^^^

error[E0277]: the trait bound `T: std::marker::Copy` is not satisfied
--> $DIR/type-check-defaults.rs:51:1
|
51 | struct Dependent<T, U = T>(T, U) where U: Copy;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `T`
|
= help: consider adding a `where T: std::marker::Copy` bound
= note: required by `std::marker::Copy`

error: aborting due to 11 previous errors
error: aborting due to 8 previous errors

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