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Auto merge of #56145 - weiznich:re_rebalance_coherence, r=nikomatsakis
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Implement the Re-rebalance coherence RFC

This is the first time I touch anything in the compiler so just tell me if I got something wrong.

Big thanks to @sgrif for the pointers where to look for those things.
cc #55437
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bors committed Jan 5, 2019
2 parents bf6bb14 + d758e4d commit 244b05d
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# `re_rebalance_coherence`

The tracking issue for this feature is: [#55437]

[#55437]: https://github.com/rust-lang/rust/issues/55437

------------------------

The `re_rebalance_coherence` feature tweaks the rules regarding which trait
impls are allowed in crates.
The following rule is used:

Given `impl<P1..=Pn> Trait<T1..=Tn> for T0`, an impl is valid only if at
least one of the following is true:
- `Trait` is a local trait
- All of
- At least one of the types `T0..=Tn` must be a local type. Let `Ti` be the
first such type.
- No uncovered type parameters `P1..=Pn` may appear in `T0..Ti` (excluding
`Ti`)


See the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/2451-re-rebalancing-coherence.md) for details.
89 changes: 57 additions & 32 deletions src/librustc/traits/coherence.rs
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Expand Up @@ -371,43 +371,68 @@ fn orphan_check_trait_ref<'tcx>(tcx: TyCtxt<'_, '_, '_>,
trait_ref);
}

// First, create an ordered iterator over all the type parameters to the trait, with the self
// type appearing first.
// Find the first input type that either references a type parameter OR
// some local type.
for input_ty in trait_ref.input_types() {
if ty_is_local(tcx, input_ty, in_crate) {
debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);

// First local input type. Check that there are no
// uncovered type parameters.
let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
for uncovered_ty in uncovered_tys {
if let Some(param) = uncovered_ty.walk()
.find(|t| is_possibly_remote_type(t, in_crate))
{
debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
return Err(OrphanCheckErr::UncoveredTy(param));
}
if tcx.features().re_rebalance_coherence {
// Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
// if at least one of the following is true:
//
// - Trait is a local trait
// (already checked in orphan_check prior to calling this function)
// - All of
// - At least one of the types T0..=Tn must be a local type.
// Let Ti be the first such type.
// - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
//
for input_ty in trait_ref.input_types() {
debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
if ty_is_local(tcx, input_ty, in_crate) {
debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
return Ok(());
} else if let ty::Param(_) = input_ty.sty {
debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
return Err(OrphanCheckErr::UncoveredTy(input_ty))
}

// OK, found local type, all prior types upheld invariant.
return Ok(());
}
// If we exit above loop, never found a local type.
debug!("orphan_check_trait_ref: no local type");
Err(OrphanCheckErr::NoLocalInputType)
} else {
// First, create an ordered iterator over all the type
// parameters to the trait, with the self type appearing
// first. Find the first input type that either references a
// type parameter OR some local type.
for input_ty in trait_ref.input_types() {
if ty_is_local(tcx, input_ty, in_crate) {
debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);

// First local input type. Check that there are no
// uncovered type parameters.
let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
for uncovered_ty in uncovered_tys {
if let Some(param) = uncovered_ty.walk()
.find(|t| is_possibly_remote_type(t, in_crate))
{
debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
return Err(OrphanCheckErr::UncoveredTy(param));
}
}

// Otherwise, enforce invariant that there are no type
// parameters reachable.
if let Some(param) = input_ty.walk()
.find(|t| is_possibly_remote_type(t, in_crate))
{
debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
return Err(OrphanCheckErr::UncoveredTy(param));
// OK, found local type, all prior types upheld invariant.
return Ok(());
}

// Otherwise, enforce invariant that there are no type
// parameters reachable.
if let Some(param) = input_ty.walk()
.find(|t| is_possibly_remote_type(t, in_crate))
{
debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
return Err(OrphanCheckErr::UncoveredTy(param));
}
}
// If we exit above loop, never found a local type.
debug!("orphan_check_trait_ref: no local type");
Err(OrphanCheckErr::NoLocalInputType)
}

// If we exit above loop, never found a local type.
debug!("orphan_check_trait_ref: no local type");
return Err(OrphanCheckErr::NoLocalInputType);
}

fn uncovered_tys<'tcx>(tcx: TyCtxt<'_, '_, '_>, ty: Ty<'tcx>, in_crate: InCrate)
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3 changes: 3 additions & 0 deletions src/libsyntax/feature_gate.rs
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Expand Up @@ -479,6 +479,9 @@ declare_features! (

// Allows paths to enum variants on type aliases.
(active, type_alias_enum_variants, "1.31.0", Some(49683), None),

// Re-Rebalance coherence
(active, re_rebalance_coherence, "1.32.0", Some(55437), None),
);

declare_features! (
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pub trait Backend{}
pub trait SupportsDefaultKeyword {}

impl SupportsDefaultKeyword for Postgres {}

pub struct Postgres;

impl Backend for Postgres {}

pub struct AstPass<DB>(::std::marker::PhantomData<DB>);

pub trait QueryFragment<DB: Backend> {}


#[derive(Debug, Clone, Copy)]
pub struct BatchInsert<'a, T: 'a, Tab> {
_marker: ::std::marker::PhantomData<(&'a T, Tab)>,
}

impl<'a, T:'a, Tab, DB> QueryFragment<DB> for BatchInsert<'a, T, Tab>
where DB: SupportsDefaultKeyword + Backend,
{}
3 changes: 3 additions & 0 deletions src/test/run-pass/coherence/coherence-bigint-int.rs
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@@ -1,5 +1,8 @@
// run-pass
// aux-build:coherence_lib.rs
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]

// pretty-expanded FIXME #23616

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3 changes: 3 additions & 0 deletions src/test/run-pass/coherence/coherence-bigint-vecint.rs
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@@ -1,5 +1,8 @@
// run-pass
// aux-build:coherence_lib.rs
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]

// pretty-expanded FIXME #23616

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3 changes: 3 additions & 0 deletions src/test/run-pass/coherence/coherence-blanket.rs
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@@ -1,6 +1,9 @@
// run-pass
#![allow(unused_imports)]
// aux-build:coherence_lib.rs
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]

// pretty-expanded FIXME #23616

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@@ -1,6 +1,9 @@
// run-pass
#![allow(dead_code)]
// aux-build:coherence_lib.rs
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]

// pretty-expanded FIXME #23616

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3 changes: 3 additions & 0 deletions src/test/run-pass/coherence/coherence-impl-in-fn.rs
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@@ -1,4 +1,7 @@
// run-pass
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]
#![allow(dead_code)]
#![allow(non_camel_case_types)]

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@@ -1,4 +1,7 @@
// run-pass
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]
#![allow(dead_code)]
// aux-build:coherence_lib.rs

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3 changes: 3 additions & 0 deletions src/test/run-pass/coherence/coherence-iterator-vec.rs
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@@ -1,4 +1,7 @@
// run-pass
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]
#![allow(dead_code)]
// aux-build:coherence_lib.rs

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3 changes: 3 additions & 0 deletions src/test/run-pass/coherence/coherence-multidispatch-tuple.rs
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@@ -1,4 +1,7 @@
// run-pass
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]
#![allow(unused_imports)]
// pretty-expanded FIXME #23616

Expand Down
3 changes: 3 additions & 0 deletions src/test/run-pass/coherence/coherence-negative-impls-safe.rs
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@@ -1,4 +1,7 @@
// run-pass
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]
#![allow(dead_code)]
// pretty-expanded FIXME #23616

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3 changes: 3 additions & 0 deletions src/test/run-pass/coherence/coherence-rfc447-constrained.rs
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@@ -1,4 +1,7 @@
// run-pass
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]
// check that trait matching can handle impls whose types are only
// constrained by a projection.

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4 changes: 4 additions & 0 deletions src/test/run-pass/coherence/coherence-where-clause.rs
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@@ -1,4 +1,8 @@
// run-pass
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]

use std::fmt::Debug;
use std::default::Default;

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3 changes: 3 additions & 0 deletions src/test/run-pass/coherence/coherence_copy_like.rs
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@@ -1,4 +1,7 @@
// run-pass
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]
#![allow(dead_code)]
// Test that we are able to introduce a negative constraint that
// `MyType: !MyTrait` along with other "fundamental" wrappers.
Expand Down
14 changes: 14 additions & 0 deletions src/test/run-pass/coherence/re-rebalance-coherence.rs
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@@ -0,0 +1,14 @@
#![allow(dead_code)]
#![feature(re_rebalance_coherence)]

// run-pass
// aux-build:re_rebalance_coherence_lib.rs

extern crate re_rebalance_coherence_lib as lib;
use lib::*;

struct Oracle;
impl Backend for Oracle {}
impl<'a, T:'a, Tab> QueryFragment<Oracle> for BatchInsert<'a, T, Tab> {}

fn main() {}
23 changes: 23 additions & 0 deletions src/test/ui/coherence/auxiliary/re_rebalance_coherence_lib.rs
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@@ -0,0 +1,23 @@

pub trait Backend{}
pub trait SupportsDefaultKeyword {}

impl SupportsDefaultKeyword for Postgres {}

pub struct Postgres;

impl Backend for Postgres {}

pub struct AstPass<DB>(::std::marker::PhantomData<DB>);

pub trait QueryFragment<DB: Backend> {}


#[derive(Debug, Clone, Copy)]
pub struct BatchInsert<'a, T: 'a, Tab> {
_marker: ::std::marker::PhantomData<(&'a T, Tab)>,
}

impl<'a, T:'a, Tab, DB> QueryFragment<DB> for BatchInsert<'a, T, Tab>
where DB: SupportsDefaultKeyword + Backend,
{}
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@@ -1,5 +1,5 @@
error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
--> $DIR/coherence-all-remote.rs:6:1
--> $DIR/coherence-all-remote.rs:9:1
|
LL | impl<T> Remote1<T> for isize { }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `T` must be used as the type parameter for some local type
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11 changes: 11 additions & 0 deletions src/test/ui/coherence/coherence-all-remote.re.stderr
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@@ -0,0 +1,11 @@
error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
--> $DIR/coherence-all-remote.rs:9:1
|
LL | impl<T> Remote1<T> for isize { }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `T` must be used as the type parameter for some local type
|
= note: only traits defined in the current crate can be implemented for a type parameter

error: aborting due to previous error

For more information about this error, try `rustc --explain E0210`.
6 changes: 5 additions & 1 deletion src/test/ui/coherence/coherence-all-remote.rs
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@@ -1,9 +1,13 @@
// aux-build:coherence_lib.rs
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]

extern crate coherence_lib as lib;
use lib::Remote1;

impl<T> Remote1<T> for isize { }
//~^ ERROR E0210
//[old]~^ ERROR E0210
//[re]~^^ ERROR E0210

fn main() { }
Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
--> $DIR/coherence-bigint-param.rs:8:1
--> $DIR/coherence-bigint-param.rs:11:1
|
LL | impl<T> Remote1<BigInt> for T { }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `T` must be used as the type parameter for some local type
Expand Down
11 changes: 11 additions & 0 deletions src/test/ui/coherence/coherence-bigint-param.re.stderr
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@@ -0,0 +1,11 @@
error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
--> $DIR/coherence-bigint-param.rs:11:1
|
LL | impl<T> Remote1<BigInt> for T { }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `T` must be used as the type parameter for some local type
|
= note: only traits defined in the current crate can be implemented for a type parameter

error: aborting due to previous error

For more information about this error, try `rustc --explain E0210`.
6 changes: 5 additions & 1 deletion src/test/ui/coherence/coherence-bigint-param.rs
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@@ -1,11 +1,15 @@
// aux-build:coherence_lib.rs
// revisions: old re

#![cfg_attr(re, feature(re_rebalance_coherence))]

extern crate coherence_lib as lib;
use lib::Remote1;

pub struct BigInt;

impl<T> Remote1<BigInt> for T { }
//~^ ERROR type parameter `T` must be used as the type parameter for some local type
//[old]~^ ERROR type parameter `T` must be used as the type parameter for some local type
//[re]~^^ ERROR E0210

fn main() { }
Original file line number Diff line number Diff line change
@@ -1,10 +1,10 @@
error[E0119]: conflicting implementations of trait `MyTrait`:
--> $DIR/coherence-blanket-conflicts-with-blanket-implemented.rs:24:1
--> $DIR/coherence-blanket-conflicts-with-blanket-implemented.rs:28:1
|
LL | impl<T:Even> MyTrait for T {
| -------------------------- first implementation here
...
LL | impl<T:Odd> MyTrait for T { //~ ERROR E0119
LL | impl<T:Odd> MyTrait for T {
| ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

error: aborting due to previous error
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@@ -0,0 +1,12 @@
error[E0119]: conflicting implementations of trait `MyTrait`:
--> $DIR/coherence-blanket-conflicts-with-blanket-implemented.rs:28:1
|
LL | impl<T:Even> MyTrait for T {
| -------------------------- first implementation here
...
LL | impl<T:Odd> MyTrait for T {
| ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

error: aborting due to previous error

For more information about this error, try `rustc --explain E0119`.
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