stabilize -Znext-solver=coherence again

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -666,7 +666,7 @@ fn check_incompatible_features(sess: &Session, features: &Features) {
 }
 
 fn check_new_solver_banned_features(sess: &Session, features: &Features) {
-    if !sess.opts.unstable_opts.next_solver.is_some_and(|n| n.globally) {
+    if !sess.opts.unstable_opts.next_solver.globally {
         return;
     }
 

compiler/rustc_interface/src/tests.rs

@@ -810,7 +810,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(mir_opt_level, Some(4));
     tracked!(move_size_limit, Some(4096));
     tracked!(mutable_noalias, false);
-    tracked!(next_solver, Some(NextSolverConfig { coherence: true, globally: false }));
+    tracked!(next_solver, NextSolverConfig { coherence: true, globally: true });
     tracked!(no_generate_arange_section, true);
     tracked!(no_jump_tables, true);
     tracked!(no_link, true);

compiler/rustc_middle/src/ty/context.rs

@@ -3128,11 +3128,11 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     pub fn next_trait_solver_globally(self) -> bool {
-        self.sess.opts.unstable_opts.next_solver.map_or(false, |c| c.globally)
+        self.sess.opts.unstable_opts.next_solver.globally
     }
 
     pub fn next_trait_solver_in_coherence(self) -> bool {
-        self.sess.opts.unstable_opts.next_solver.map_or(false, |c| c.coherence)
+        self.sess.opts.unstable_opts.next_solver.coherence
     }
 
     pub fn is_impl_trait_in_trait(self, def_id: DefId) -> bool {

compiler/rustc_session/src/config.rs

@@ -842,6 +842,11 @@ pub struct NextSolverConfig {
     /// This is only `true` if `coherence` is also enabled.
     pub globally: bool,
 }
+impl Default for NextSolverConfig {
+    fn default() -> Self {
+        NextSolverConfig { coherence: true, globally: false }
+    }
+}
 
 #[derive(Clone)]
 pub enum Input {

compiler/rustc_session/src/options.rs

@@ -403,7 +403,7 @@ mod desc {
     pub(crate) const parse_unpretty: &str = "`string` or `string=string`";
     pub(crate) const parse_treat_err_as_bug: &str = "either no value or a non-negative number";
     pub(crate) const parse_next_solver_config: &str =
-        "a comma separated list of solver configurations: `globally` (default), and `coherence`";
+        "either `globally` (when used without an argument), `coherence` (default) or `no`";
     pub(crate) const parse_lto: &str =
         "either a boolean (`yes`, `no`, `on`, `off`, etc), `thin`, `fat`, or omitted";
     pub(crate) const parse_linker_plugin_lto: &str =
@@ -1123,27 +1123,16 @@ mod parse {
         }
     }
 
-    pub(crate) fn parse_next_solver_config(
-        slot: &mut Option<NextSolverConfig>,
-        v: Option<&str>,
-    ) -> bool {
+    pub(crate) fn parse_next_solver_config(slot: &mut NextSolverConfig, v: Option<&str>) -> bool {
         if let Some(config) = v {
-            let mut coherence = false;
-            let mut globally = true;
-            for c in config.split(',') {
-                match c {
-                    "globally" => globally = true,
-                    "coherence" => {
-                        globally = false;
-                        coherence = true;
-                    }
-                    _ => return false,
-                }
-            }
-
-            *slot = Some(NextSolverConfig { coherence: coherence || globally, globally });
+            *slot = match config {
+                "no" => NextSolverConfig { coherence: false, globally: false },
+                "coherence" => NextSolverConfig { coherence: true, globally: false },
+                "globally" => NextSolverConfig { coherence: true, globally: true },
+                _ => return false,
+            };
         } else {
-            *slot = Some(NextSolverConfig { coherence: true, globally: true });
+            *slot = NextSolverConfig { coherence: true, globally: true };
         }
 
         true
@@ -1914,7 +1903,7 @@ options! {
         "the size at which the `large_assignments` lint starts to be emitted"),
     mutable_noalias: bool = (true, parse_bool, [TRACKED],
         "emit noalias metadata for mutable references (default: yes)"),
-    next_solver: Option<NextSolverConfig> = (None, parse_next_solver_config, [TRACKED],
+    next_solver: NextSolverConfig = (NextSolverConfig::default(), parse_next_solver_config, [TRACKED],
         "enable and configure the next generation trait solver used by rustc"),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],
         "dump facts from NLL analysis into side files (default: no)"),

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -36,10 +36,8 @@ where
         if infcx.next_trait_solver() {
             Box::new(NextFulfillmentCtxt::new(infcx))
         } else {
-            let new_solver_globally =
-                infcx.tcx.sess.opts.unstable_opts.next_solver.map_or(false, |c| c.globally);
             assert!(
-                !new_solver_globally,
+                !infcx.tcx.next_trait_solver_globally(),
                 "using old solver even though new solver is enabled globally"
             );
             Box::new(FulfillmentContext::new(infcx))

tests/ui/associated-types/associated-types-coherence-failure.stderr

@@ -1,20 +1,20 @@
-error[E0119]: conflicting implementations of trait `IntoCow<'_, _>` for type `Cow<'_, _>`
+error[E0119]: conflicting implementations of trait `IntoCow<'_, _>` for type `<_ as ToOwned>::Owned`
   --> $DIR/associated-types-coherence-failure.rs:21:1
    |
 LL | impl<'a, B: ?Sized> IntoCow<'a, B> for <B as ToOwned>::Owned where B: ToOwned {
    | ----------------------------------------------------------------------------- first implementation here
 ...
 LL | impl<'a, B: ?Sized> IntoCow<'a, B> for Cow<'a, B> where B: ToOwned {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Cow<'_, _>`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `<_ as ToOwned>::Owned`
 
-error[E0119]: conflicting implementations of trait `IntoCow<'_, _>` for type `&_`
+error[E0119]: conflicting implementations of trait `IntoCow<'_, _>` for type `<_ as ToOwned>::Owned`
   --> $DIR/associated-types-coherence-failure.rs:28:1
    |
 LL | impl<'a, B: ?Sized> IntoCow<'a, B> for <B as ToOwned>::Owned where B: ToOwned {
    | ----------------------------------------------------------------------------- first implementation here
 ...
 LL | impl<'a, B: ?Sized> IntoCow<'a, B> for &'a B where B: ToOwned {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&_`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `<_ as ToOwned>::Owned`
 
 error: aborting due to 2 previous errors
 

tests/ui/coherence/coherence-negative-outlives-lifetimes.stock.stderr

@@ -5,6 +5,8 @@ LL | impl<'a, T: MyPredicate<'a>> MyTrait<'a> for T {}
    | ---------------------------------------------- first implementation here
 LL | impl<'a, T> MyTrait<'a> for &'a T {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&_`
+   |
+   = note: downstream crates may implement trait `MyPredicate<'_>` for type `&_`
 
 error: aborting due to 1 previous error
 

tests/ui/coherence/coherence-negative-outlives-lifetimes.with_negative_coherence.stderr

@@ -5,6 +5,8 @@ LL | impl<'a, T: MyPredicate<'a>> MyTrait<'a> for T {}
    | ---------------------------------------------- first implementation here
 LL | impl<'a, T> MyTrait<'a> for &'a T {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&_`
+   |
+   = note: downstream crates may implement trait `MyPredicate<'_>` for type `&_`
 
 error: aborting due to 1 previous error
 

tests/ui/coherence/coherence-overlap-downstream-inherent.rs

@@ -1,6 +1,3 @@
-//@ revisions: old next
-//@[next] compile-flags: -Znext-solver
-
 // Tests that we consider `T: Sugar + Fruit` to be ambiguous, even
 // though no impls are found.
 

tests/ui/coherence/coherence-overlap-downstream-inherent.next.stderr → tests/ui/coherence/coherence-overlap-downstream-inherent.stderr

@@ -1,5 +1,5 @@
 error[E0592]: duplicate definitions with name `dummy`
-  --> $DIR/coherence-overlap-downstream-inherent.rs:10:26
+  --> $DIR/coherence-overlap-downstream-inherent.rs:7:26
    |
 LL | impl<T:Sugar> Sweet<T> { fn dummy(&self) { } }
    |                          ^^^^^^^^^^^^^^^ duplicate definitions for `dummy`
@@ -8,7 +8,7 @@ LL | impl<T:Fruit> Sweet<T> { fn dummy(&self) { } }
    |                          --------------- other definition for `dummy`
 
 error[E0592]: duplicate definitions with name `f`
-  --> $DIR/coherence-overlap-downstream-inherent.rs:16:38
+  --> $DIR/coherence-overlap-downstream-inherent.rs:13:38
    |
 LL | impl<X, T> A<T, X> where T: Bar<X> { fn f(&self) {} }
    |                                      ^^^^^^^^^^^ duplicate definitions for `f`

tests/ui/coherence/coherence-overlap-downstream.rs

@@ -1,6 +1,3 @@
-//@ revisions: old next
-//@[next] compile-flags: -Znext-solver
-
 // Tests that we consider `T: Sugar + Fruit` to be ambiguous, even
 // though no impls are found.
 

tests/ui/coherence/coherence-overlap-downstream.old.stderr → tests/ui/coherence/coherence-overlap-downstream.stderr

@@ -1,13 +1,13 @@
 error[E0119]: conflicting implementations of trait `Sweet`
-  --> $DIR/coherence-overlap-downstream.rs:11:1
+  --> $DIR/coherence-overlap-downstream.rs:8:1
    |
 LL | impl<T:Sugar> Sweet for T { }
    | ------------------------- first implementation here
 LL | impl<T:Fruit> Sweet for T { }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
 
 error[E0119]: conflicting implementations of trait `Foo<_>` for type `i32`
-  --> $DIR/coherence-overlap-downstream.rs:17:1
+  --> $DIR/coherence-overlap-downstream.rs:14:1
    |
 LL | impl<X, T> Foo<X> for T where T: Bar<X> {}
    | --------------------------------------- first implementation here

tests/ui/coherence/coherence-overlap-issue-23516-inherent.rs

@@ -1,6 +1,3 @@
-//@ revisions: old next
-//@[next] compile-flags: -Znext-solver
-
 // Tests that we consider `Box<U>: !Sugar` to be ambiguous, even
 // though we see no impl of `Sugar` for `Box`. Therefore, an overlap
 // error is reported for the following pair of impls (#23516).

tests/ui/coherence/coherence-overlap-issue-23516-inherent.next.stderr → tests/ui/coherence/coherence-overlap-issue-23516-inherent.stderr

@@ -1,5 +1,5 @@
 error[E0592]: duplicate definitions with name `dummy`
-  --> $DIR/coherence-overlap-issue-23516-inherent.rs:12:25
+  --> $DIR/coherence-overlap-issue-23516-inherent.rs:9:25
    |
 LL | impl<T:Sugar> Cake<T> { fn dummy(&self) { } }
    |                         ^^^^^^^^^^^^^^^ duplicate definitions for `dummy`

tests/ui/coherence/coherence-overlap-issue-23516.rs

@@ -1,6 +1,3 @@
-//@ revisions: old next
-//@[next] compile-flags: -Znext-solver
-
 // Tests that we consider `Box<U>: !Sugar` to be ambiguous, even
 // though we see no impl of `Sugar` for `Box`. Therefore, an overlap
 // error is reported for the following pair of impls (#23516).

tests/ui/coherence/coherence-overlap-issue-23516.next.stderr → tests/ui/coherence/coherence-overlap-issue-23516.stderr

@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `Sweet` for type `Box<_>`
-  --> $DIR/coherence-overlap-issue-23516.rs:11:1
+  --> $DIR/coherence-overlap-issue-23516.rs:8:1
    |
 LL | impl<T:Sugar> Sweet for T { }
    | ------------------------- first implementation here

tests/ui/coherence/coherence-overlap-negate-not-use-feature-gate.stderr

@@ -5,6 +5,8 @@ LL | impl<T: DerefMut> Foo for T {}
    | --------------------------- first implementation here
 LL | impl<U> Foo for &U {}
    | ^^^^^^^^^^^^^^^^^^ conflicting implementation for `&_`
+   |
+   = note: downstream crates may implement trait `std::ops::DerefMut` for type `&_`
 
 error: aborting due to 1 previous error
 

tests/ui/coherence/coherence-overlap-unnormalizable-projection-0.rs

@@ -2,9 +2,6 @@
 // "Coherence incorrectly considers `unnormalizable_projection: Trait` to not hold even if it could"
 #![crate_type = "lib"]
 
-//@ revisions: classic next
-//@[next] compile-flags: -Znext-solver
-
 trait WhereBound {}
 impl WhereBound for () {}
 

tests/ui/coherence/coherence-overlap-unnormalizable-projection-0.next.stderr → tests/ui/coherence/coherence-overlap-unnormalizable-projection-0.stderr

@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `Trait` for type `Box<_>`
-  --> $DIR/coherence-overlap-unnormalizable-projection-0.rs:27:1
+  --> $DIR/coherence-overlap-unnormalizable-projection-0.rs:24:1
    |
 LL | / impl<T> Trait for T
 LL | | where