Auto merge of rust-lang#126139 - compiler-errors:specializes, r=<try>

Only compute `specializes` query if (min)specialization is enabled in the crate of the specializing impl

Fixes (after backport) rust-lang#125197

### What

rust-lang#122791 makes it so that inductive cycles are no longer hard errors. That means that when we are testing, for example, whether these impls overlap:

```rust
impl PartialEq<Self> for AnyId {
    fn eq(&self, _: &Self) -> bool {
        todo!()
    }
}

impl<T: Identifier> PartialEq<T> for AnyId {
    fn eq(&self, _: &T) -> bool {
        todo!()
    }
}
```

...given...

```rust
pub trait Identifier: Display + 'static {}

impl<T> Identifier for T where T: PartialEq + Display + 'static {}
```

Then we try to see if the second impl holds given `T = AnyId`. That requires `AnyId: Identifier`, which requires that `AnyId: PartialEq`, which is satisfied by these two impl candidates... The `PartialEq<T>` impl is a cycle, and we used to winnow it when we used to treat inductive cycles as errors.

However, now that we don't winnow it, this means that we *now* try calling `candidate_should_be_dropped_in_favor_of`, which tries to check whether one of the impls specializes the other: the `specializes` query. In that query, we currently bail early if the impl is local:

However, in a foreign crate, we try to compute if the two impls specialize each other by doing trait solving. This may itself lead to the same situation where we call `specializes`, which will lead to a query cycle.

### How does this fix the problem

We now record whether specialization is enabled in foreign crates, and extend this early-return behavior to foreign impls too. This means that we can only encounter these cycles if we truly have a specializing impl from a crate with specialization enabled.

-----

r? `@oli-obk` or `@lcnr`

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -314,6 +314,7 @@ provide! { tcx, def_id, other, cdata,
     extern_crate => { cdata.extern_crate.map(|c| &*tcx.arena.alloc(c)) }
     is_no_builtins => { cdata.root.no_builtins }
     symbol_mangling_version => { cdata.root.symbol_mangling_version }
+    specialization_enabled_in => { cdata.root.specialization_enabled_in }
     reachable_non_generics => {
         let reachable_non_generics = tcx
             .exported_symbols(cdata.cnum)

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -741,6 +741,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 expn_data,
                 expn_hashes,
                 def_path_hash_map,
+                specialization_enabled_in: tcx.specialization_enabled_in(LOCAL_CRATE),
             })
         });
 

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -290,6 +290,8 @@ pub(crate) struct CrateRoot {
     panic_runtime: bool,
     profiler_runtime: bool,
     symbol_mangling_version: SymbolManglingVersion,
+
+    specialization_enabled_in: bool,
 }
 
 /// On-disk representation of `DefId`.

compiler/rustc_middle/src/query/mod.rs

@@ -1494,6 +1494,11 @@ rustc_queries! {
         separate_provide_extern
     }
 
+    query specialization_enabled_in(cnum: CrateNum) -> bool {
+        desc { "checking whether the crate enabled `specialization`/`min_specialization`" }
+        separate_provide_extern
+    }
+
     query specializes(_: (DefId, DefId)) -> bool {
         desc { "computing whether impls specialize one another" }
     }

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -623,6 +623,7 @@ pub fn provide(providers: &mut Providers) {
     *providers = Providers {
         specialization_graph_of: specialize::specialization_graph_provider,
         specializes: specialize::specializes,
+        specialization_enabled_in: specialize::specialization_enabled_in,
         instantiate_and_check_impossible_predicates,
         is_impossible_associated_item,
         ..*providers

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -24,6 +24,7 @@ use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{codes::*, Diag, EmissionGuarantee};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::bug;
+use rustc_middle::query::LocalCrate;
 use rustc_middle::ty::{self, ImplSubject, Ty, TyCtxt, TypeVisitableExt};
 use rustc_middle::ty::{GenericArgs, GenericArgsRef};
 use rustc_session::lint::builtin::COHERENCE_LEAK_CHECK;
@@ -136,38 +137,29 @@ pub fn translate_args_with_cause<'tcx>(
     source_args.rebase_onto(infcx.tcx, source_impl, target_args)
 }
 
+pub(super) fn specialization_enabled_in(tcx: TyCtxt<'_>, _: LocalCrate) -> bool {
+    tcx.features().specialization || tcx.features().min_specialization
+}
+
 /// Is `impl1` a specialization of `impl2`?
 ///
 /// Specialization is determined by the sets of types to which the impls apply;
 /// `impl1` specializes `impl2` if it applies to a subset of the types `impl2` applies
 /// to.
 #[instrument(skip(tcx), level = "debug")]
 pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId, DefId)) -> bool {
-    // The feature gate should prevent introducing new specializations, but not
-    // taking advantage of upstream ones.
-    // If specialization is enabled for this crate then no extra checks are needed.
-    // If it's not, and either of the `impl`s is local to this crate, then this definitely
-    // isn't specializing - unless specialization is enabled for the `impl` span,
-    // e.g. if it comes from an `allow_internal_unstable` macro
-    let features = tcx.features();
-    let specialization_enabled = features.specialization || features.min_specialization;
-    if !specialization_enabled {
-        if impl1_def_id.is_local() {
-            let span = tcx.def_span(impl1_def_id);
-            if !span.allows_unstable(sym::specialization)
-                && !span.allows_unstable(sym::min_specialization)
-            {
-                return false;
-            }
-        }
-
-        if impl2_def_id.is_local() {
-            let span = tcx.def_span(impl2_def_id);
-            if !span.allows_unstable(sym::specialization)
-                && !span.allows_unstable(sym::min_specialization)
-            {
-                return false;
-            }
+    // We check that the specializing impl comes from a crate that has specialization enabled,
+    // or if the specializing impl is marked with `allow_internal_unstable`.
+    //
+    // We don't really care if the specialized impl (the parent) is in a crate that has
+    // specialization enabled, since it's not being specialized, and it's already been checked
+    // for coherence.
+    if !tcx.specialization_enabled_in(impl1_def_id.krate) {
+        let span = tcx.def_span(impl1_def_id);
+        if !span.allows_unstable(sym::specialization)
+            && !span.allows_unstable(sym::min_specialization)
+        {
+            return false;
         }
     }
 

tests/ui/coherence/coherence-impls-copy.stderr

@@ -1,14 +1,3 @@
-error[E0117]: only traits defined in the current crate can be implemented for primitive types
-  --> $DIR/coherence-impls-copy.rs:5:1
-   |
-LL | impl Copy for i32 {}
-   | ^^^^^^^^^^^^^^---
-   | |             |
-   | |             `i32` is not defined in the current crate
-   | impl doesn't use only types from inside the current crate
-   |
-   = note: define and implement a trait or new type instead
-
 error[E0119]: conflicting implementations of trait `Copy` for type `&NotSync`
   --> $DIR/coherence-impls-copy.rs:28:1
    |
@@ -30,6 +19,17 @@ LL | impl Copy for &'static [NotSync] {}
    |
    = note: define and implement a trait or new type instead
 
+error[E0117]: only traits defined in the current crate can be implemented for primitive types
+  --> $DIR/coherence-impls-copy.rs:5:1
+   |
+LL | impl Copy for i32 {}
+   | ^^^^^^^^^^^^^^---
+   | |             |
+   | |             `i32` is not defined in the current crate
+   | impl doesn't use only types from inside the current crate
+   |
+   = note: define and implement a trait or new type instead
+
 error[E0206]: the trait `Copy` cannot be implemented for this type
   --> $DIR/coherence-impls-copy.rs:21:15
    |

tests/ui/specialization/anyid-repro-125197.rs

@@ -0,0 +1,17 @@
+//@ aux-build: anyid-repro-125197.rs
+//@ check-pass
+
+// Makes sure that we don't check `specializes(impl1, impl2)` for a pair of impls that don't
+// actually participate in specialization. Since <https://github.com/rust-lang/rust/pull/122791>,
+// we don't treat inductive cycles as errors -- so we may need to winnow more pairs of impls, and
+// we try to winnow impls in favor of other impls. However, if we're *inside* the `specializes`
+// query, then may have a query cycle if we call `specializes` again!
+
+extern crate anyid_repro_125197;
+use anyid_repro_125197::AnyId;
+
+fn main() {
+    let x = "hello, world";
+    let y: AnyId = x.into();
+    let _ = y == x;
+}

tests/ui/specialization/auxiliary/anyid-repro-125197.rs

@@ -0,0 +1,26 @@
+use std::fmt::Display;
+use std::sync::Arc;
+
+pub struct AnyId(());
+
+impl PartialEq<Self> for AnyId {
+    fn eq(&self, _: &Self) -> bool {
+        todo!()
+    }
+}
+
+impl<T: Identifier> PartialEq<T> for AnyId {
+    fn eq(&self, _: &T) -> bool {
+        todo!()
+    }
+}
+
+impl<T: Identifier> From<T> for AnyId {
+    fn from(_: T) -> Self {
+        todo!()
+    }
+}
+
+pub trait Identifier: Display + 'static {}
+
+impl<T> Identifier for T where T: PartialEq + Display + 'static {}