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Auto merge of #56145 - weiznich:re_rebalance_coherence, r=nikomatsakis
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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‎src/doc/unstable-book/src/language-features/re-rebalance-coherence.md

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# `re_rebalance_coherence`
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The tracking issue for this feature is: [#55437]
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[#55437]: https://github.com/rust-lang/rust/issues/55437
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------------------------
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The `re_rebalance_coherence` feature tweaks the rules regarding which trait
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impls are allowed in crates.
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The following rule is used:
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Given `impl<P1..=Pn> Trait<T1..=Tn> for T0`, an impl is valid only if at
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least one of the following is true:
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- `Trait` is a local trait
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- All of
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- At least one of the types `T0..=Tn` must be a local type. Let `Ti` be the
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first such type.
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- No uncovered type parameters `P1..=Pn` may appear in `T0..Ti` (excluding
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`Ti`)
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See the [RFC](https://github.com/rust-lang/rfcs/blob/master/text/2451-re-rebalancing-coherence.md) for details.

‎src/librustc/traits/coherence.rs

+57-32
Original file line numberDiff line numberDiff line change
@@ -371,43 +371,68 @@ fn orphan_check_trait_ref<'tcx>(tcx: TyCtxt<'_, '_, '_>,
371371
trait_ref);
372372
}
373373

374-
// First, create an ordered iterator over all the type parameters to the trait, with the self
375-
// type appearing first.
376-
// Find the first input type that either references a type parameter OR
377-
// some local type.
378-
for input_ty in trait_ref.input_types() {
379-
if ty_is_local(tcx, input_ty, in_crate) {
380-
debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
381-
382-
// First local input type. Check that there are no
383-
// uncovered type parameters.
384-
let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
385-
for uncovered_ty in uncovered_tys {
386-
if let Some(param) = uncovered_ty.walk()
387-
.find(|t| is_possibly_remote_type(t, in_crate))
388-
{
389-
debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
390-
return Err(OrphanCheckErr::UncoveredTy(param));
391-
}
374+
if tcx.features().re_rebalance_coherence {
375+
// Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
376+
// if at least one of the following is true:
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//
378+
// - Trait is a local trait
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// (already checked in orphan_check prior to calling this function)
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// - All of
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// - At least one of the types T0..=Tn must be a local type.
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// Let Ti be the first such type.
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// - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
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//
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for input_ty in trait_ref.input_types() {
386+
debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
387+
if ty_is_local(tcx, input_ty, in_crate) {
388+
debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
389+
return Ok(());
390+
} else if let ty::Param(_) = input_ty.sty {
391+
debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
392+
return Err(OrphanCheckErr::UncoveredTy(input_ty))
392393
}
393-
394-
// OK, found local type, all prior types upheld invariant.
395-
return Ok(());
396394
}
395+
// If we exit above loop, never found a local type.
396+
debug!("orphan_check_trait_ref: no local type");
397+
Err(OrphanCheckErr::NoLocalInputType)
398+
} else {
399+
// First, create an ordered iterator over all the type
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// parameters to the trait, with the self type appearing
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// first. Find the first input type that either references a
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// type parameter OR some local type.
403+
for input_ty in trait_ref.input_types() {
404+
if ty_is_local(tcx, input_ty, in_crate) {
405+
debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
406+
407+
// First local input type. Check that there are no
408+
// uncovered type parameters.
409+
let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
410+
for uncovered_ty in uncovered_tys {
411+
if let Some(param) = uncovered_ty.walk()
412+
.find(|t| is_possibly_remote_type(t, in_crate))
413+
{
414+
debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
415+
return Err(OrphanCheckErr::UncoveredTy(param));
416+
}
417+
}
397418

398-
// Otherwise, enforce invariant that there are no type
399-
// parameters reachable.
400-
if let Some(param) = input_ty.walk()
401-
.find(|t| is_possibly_remote_type(t, in_crate))
402-
{
403-
debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
404-
return Err(OrphanCheckErr::UncoveredTy(param));
419+
// OK, found local type, all prior types upheld invariant.
420+
return Ok(());
421+
}
422+
423+
// Otherwise, enforce invariant that there are no type
424+
// parameters reachable.
425+
if let Some(param) = input_ty.walk()
426+
.find(|t| is_possibly_remote_type(t, in_crate))
427+
{
428+
debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
429+
return Err(OrphanCheckErr::UncoveredTy(param));
430+
}
405431
}
432+
// If we exit above loop, never found a local type.
433+
debug!("orphan_check_trait_ref: no local type");
434+
Err(OrphanCheckErr::NoLocalInputType)
406435
}
407-
408-
// If we exit above loop, never found a local type.
409-
debug!("orphan_check_trait_ref: no local type");
410-
return Err(OrphanCheckErr::NoLocalInputType);
411436
}
412437

413438
fn uncovered_tys<'tcx>(tcx: TyCtxt<'_, '_, '_>, ty: Ty<'tcx>, in_crate: InCrate)

‎src/libsyntax/feature_gate.rs

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@@ -479,6 +479,9 @@ declare_features! (
479479

480480
// Allows paths to enum variants on type aliases.
481481
(active, type_alias_enum_variants, "1.31.0", Some(49683), None),
482+
483+
// Re-Rebalance coherence
484+
(active, re_rebalance_coherence, "1.32.0", Some(55437), None),
482485
);
483486

484487
declare_features! (

‎src/test/run-pass/coherence/auxiliary/re_rebalance_coherence_lib.rs

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pub trait Backend{}
3+
pub trait SupportsDefaultKeyword {}
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5+
impl SupportsDefaultKeyword for Postgres {}
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7+
pub struct Postgres;
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9+
impl Backend for Postgres {}
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11+
pub struct AstPass<DB>(::std::marker::PhantomData<DB>);
12+
13+
pub trait QueryFragment<DB: Backend> {}
14+
15+
16+
#[derive(Debug, Clone, Copy)]
17+
pub struct BatchInsert<'a, T: 'a, Tab> {
18+
_marker: ::std::marker::PhantomData<(&'a T, Tab)>,
19+
}
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21+
impl<'a, T:'a, Tab, DB> QueryFragment<DB> for BatchInsert<'a, T, Tab>
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where DB: SupportsDefaultKeyword + Backend,
23+
{}

‎src/test/run-pass/coherence/coherence-bigint-int.rs

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// run-pass
22
// aux-build:coherence_lib.rs
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// revisions: old re
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#![cfg_attr(re, feature(re_rebalance_coherence))]
36

47
// pretty-expanded FIXME #23616
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‎src/test/run-pass/coherence/coherence-bigint-vecint.rs

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// run-pass
22
// aux-build:coherence_lib.rs
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// revisions: old re
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#![cfg_attr(re, feature(re_rebalance_coherence))]
36

47
// pretty-expanded FIXME #23616
58

‎src/test/run-pass/coherence/coherence-blanket.rs

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// run-pass
22
#![allow(unused_imports)]
33
// aux-build:coherence_lib.rs
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// revisions: old re
5+
6+
#![cfg_attr(re, feature(re_rebalance_coherence))]
47

58
// pretty-expanded FIXME #23616
69

‎src/test/run-pass/coherence/coherence-covered-type-parameter.rs

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11
// run-pass
22
#![allow(dead_code)]
33
// aux-build:coherence_lib.rs
4+
// revisions: old re
5+
6+
#![cfg_attr(re, feature(re_rebalance_coherence))]
47

58
// pretty-expanded FIXME #23616
69

‎src/test/run-pass/coherence/coherence-impl-in-fn.rs

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11
// run-pass
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
25
#![allow(dead_code)]
36
#![allow(non_camel_case_types)]
47

‎src/test/run-pass/coherence/coherence-iterator-vec-any-elem.rs

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11
// run-pass
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
25
#![allow(dead_code)]
36
// aux-build:coherence_lib.rs
47

‎src/test/run-pass/coherence/coherence-iterator-vec.rs

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11
// run-pass
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
25
#![allow(dead_code)]
36
// aux-build:coherence_lib.rs
47

‎src/test/run-pass/coherence/coherence-multidispatch-tuple.rs

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@@ -1,4 +1,7 @@
11
// run-pass
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
25
#![allow(unused_imports)]
36
// pretty-expanded FIXME #23616
47

‎src/test/run-pass/coherence/coherence-negative-impls-safe.rs

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@@ -1,4 +1,7 @@
11
// run-pass
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
25
#![allow(dead_code)]
36
// pretty-expanded FIXME #23616
47

‎src/test/run-pass/coherence/coherence-rfc447-constrained.rs

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@@ -1,4 +1,7 @@
11
// run-pass
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
25
// check that trait matching can handle impls whose types are only
36
// constrained by a projection.
47

‎src/test/run-pass/coherence/coherence-where-clause.rs

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11
// run-pass
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
5+
26
use std::fmt::Debug;
37
use std::default::Default;
48

‎src/test/run-pass/coherence/coherence_copy_like.rs

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@@ -1,4 +1,7 @@
11
// run-pass
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
25
#![allow(dead_code)]
36
// Test that we are able to introduce a negative constraint that
47
// `MyType: !MyTrait` along with other "fundamental" wrappers.

‎src/test/run-pass/coherence/re-rebalance-coherence.rs

+14
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@@ -0,0 +1,14 @@
1+
#![allow(dead_code)]
2+
#![feature(re_rebalance_coherence)]
3+
4+
// run-pass
5+
// aux-build:re_rebalance_coherence_lib.rs
6+
7+
extern crate re_rebalance_coherence_lib as lib;
8+
use lib::*;
9+
10+
struct Oracle;
11+
impl Backend for Oracle {}
12+
impl<'a, T:'a, Tab> QueryFragment<Oracle> for BatchInsert<'a, T, Tab> {}
13+
14+
fn main() {}

‎src/test/ui/coherence/auxiliary/re_rebalance_coherence_lib.rs

+23
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,23 @@
1+
2+
pub trait Backend{}
3+
pub trait SupportsDefaultKeyword {}
4+
5+
impl SupportsDefaultKeyword for Postgres {}
6+
7+
pub struct Postgres;
8+
9+
impl Backend for Postgres {}
10+
11+
pub struct AstPass<DB>(::std::marker::PhantomData<DB>);
12+
13+
pub trait QueryFragment<DB: Backend> {}
14+
15+
16+
#[derive(Debug, Clone, Copy)]
17+
pub struct BatchInsert<'a, T: 'a, Tab> {
18+
_marker: ::std::marker::PhantomData<(&'a T, Tab)>,
19+
}
20+
21+
impl<'a, T:'a, Tab, DB> QueryFragment<DB> for BatchInsert<'a, T, Tab>
22+
where DB: SupportsDefaultKeyword + Backend,
23+
{}

‎src/test/ui/coherence/coherence-all-remote.stderr renamed to ‎src/test/ui/coherence/coherence-all-remote.old.stderr

+1-1
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,5 @@
11
error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
2-
--> $DIR/coherence-all-remote.rs:6:1
2+
--> $DIR/coherence-all-remote.rs:9:1
33
|
44
LL | impl<T> Remote1<T> for isize { }
55
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `T` must be used as the type parameter for some local type

‎src/test/ui/coherence/coherence-all-remote.re.stderr

+11
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,11 @@
1+
error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
2+
--> $DIR/coherence-all-remote.rs:9:1
3+
|
4+
LL | impl<T> Remote1<T> for isize { }
5+
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `T` must be used as the type parameter for some local type
6+
|
7+
= note: only traits defined in the current crate can be implemented for a type parameter
8+
9+
error: aborting due to previous error
10+
11+
For more information about this error, try `rustc --explain E0210`.

‎src/test/ui/coherence/coherence-all-remote.rs

+5-1
Original file line numberDiff line numberDiff line change
@@ -1,9 +1,13 @@
11
// aux-build:coherence_lib.rs
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
25

36
extern crate coherence_lib as lib;
47
use lib::Remote1;
58

69
impl<T> Remote1<T> for isize { }
7-
//~^ ERROR E0210
10+
//[old]~^ ERROR E0210
11+
//[re]~^^ ERROR E0210
812

913
fn main() { }

‎src/test/ui/coherence/coherence-bigint-param.stderr renamed to ‎src/test/ui/coherence/coherence-bigint-param.old.stderr

+1-1
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,5 @@
11
error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
2-
--> $DIR/coherence-bigint-param.rs:8:1
2+
--> $DIR/coherence-bigint-param.rs:11:1
33
|
44
LL | impl<T> Remote1<BigInt> for T { }
55
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `T` must be used as the type parameter for some local type

‎src/test/ui/coherence/coherence-bigint-param.re.stderr

+11
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,11 @@
1+
error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
2+
--> $DIR/coherence-bigint-param.rs:11:1
3+
|
4+
LL | impl<T> Remote1<BigInt> for T { }
5+
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type parameter `T` must be used as the type parameter for some local type
6+
|
7+
= note: only traits defined in the current crate can be implemented for a type parameter
8+
9+
error: aborting due to previous error
10+
11+
For more information about this error, try `rustc --explain E0210`.

‎src/test/ui/coherence/coherence-bigint-param.rs

+5-1
Original file line numberDiff line numberDiff line change
@@ -1,11 +1,15 @@
11
// aux-build:coherence_lib.rs
2+
// revisions: old re
3+
4+
#![cfg_attr(re, feature(re_rebalance_coherence))]
25

36
extern crate coherence_lib as lib;
47
use lib::Remote1;
58

69
pub struct BigInt;
710

811
impl<T> Remote1<BigInt> for T { }
9-
//~^ ERROR type parameter `T` must be used as the type parameter for some local type
12+
//[old]~^ ERROR type parameter `T` must be used as the type parameter for some local type
13+
//[re]~^^ ERROR E0210
1014

1115
fn main() { }

‎src/test/ui/coherence/coherence-blanket-conflicts-with-blanket-implemented.stderr renamed to ‎src/test/ui/coherence/coherence-blanket-conflicts-with-blanket-implemented.old.stderr

+2-2
Original file line numberDiff line numberDiff line change
@@ -1,10 +1,10 @@
11
error[E0119]: conflicting implementations of trait `MyTrait`:
2-
--> $DIR/coherence-blanket-conflicts-with-blanket-implemented.rs:24:1
2+
--> $DIR/coherence-blanket-conflicts-with-blanket-implemented.rs:28:1
33
|
44
LL | impl<T:Even> MyTrait for T {
55
| -------------------------- first implementation here
66
...
7-
LL | impl<T:Odd> MyTrait for T { //~ ERROR E0119
7+
LL | impl<T:Odd> MyTrait for T {
88
| ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
99

1010
error: aborting due to previous error

‎src/test/ui/coherence/coherence-blanket-conflicts-with-blanket-implemented.re.stderr

+12
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,12 @@
1+
error[E0119]: conflicting implementations of trait `MyTrait`:
2+
--> $DIR/coherence-blanket-conflicts-with-blanket-implemented.rs:28:1
3+
|
4+
LL | impl<T:Even> MyTrait for T {
5+
| -------------------------- first implementation here
6+
...
7+
LL | impl<T:Odd> MyTrait for T {
8+
| ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
9+
10+
error: aborting due to previous error
11+
12+
For more information about this error, try `rustc --explain E0119`.

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