Rollup merge of rust-lang#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 rust-lang#55437

Author: Mark-Simulacrum

src/doc/unstable-book/src/language-features/re-rebalance-coherence.md

@@ -0,0 +1,23 @@
+# `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.

src/librustc/traits/coherence.rs

@@ -344,43 +344,68 @@ 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)

src/libsyntax/feature_gate.rs

@@ -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! (

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

@@ -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,
+{}

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

@@ -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
 

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

@@ -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
 

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

@@ -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
 

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

@@ -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
 

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

@@ -1,4 +1,7 @@
 // run-pass
+// revisions: old re
+
+#![cfg_attr(re, feature(re_rebalance_coherence))]
 #![allow(dead_code)]
 #![allow(non_camel_case_types)]
 

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

@@ -1,4 +1,7 @@
 // run-pass
+// revisions: old re
+
+#![cfg_attr(re, feature(re_rebalance_coherence))]
 #![allow(dead_code)]
 // aux-build:coherence_lib.rs