Suppress unstable-trait notes under -Zforce-unstable-if-unmarked

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -5608,15 +5608,17 @@ pub(super) fn get_explanation_based_on_obligation<'tcx>(
             None => String::new(),
         };
         if let ty::PredicatePolarity::Positive = trait_predicate.polarity() {
+            // If the trait in question is unstable, mention that fact in the diagnostic.
+            // But if we're building with `-Zforce-unstable-if-unmarked` then _any_ trait
+            // not explicitly marked stable is considered unstable, so the extra text is
+            // unhelpful noise. See <https://github.com/rust-lang/rust/issues/152692>.
+            let mention_unstable = !tcx.sess.opts.unstable_opts.force_unstable_if_unmarked
+                && try { tcx.lookup_stability(trait_predicate.def_id())?.level.is_stable() }
+                    == Some(false);
+            let unstable = if mention_unstable { "nightly-only, unstable " } else { "" };
+
             format!(
-                "{pre_message}the {}trait `{}` is not implemented for{desc} `{}`",
-                if tcx.lookup_stability(trait_predicate.def_id()).map(|s| s.level.is_stable())
-                    == Some(false)
-                {
-                    "nightly-only, unstable "
-                } else {
-                    ""
-                },
+                "{pre_message}the {unstable}trait `{}` is not implemented for{desc} `{}`",
                 trait_predicate.print_modifiers_and_trait_path(),
                 tcx.short_string(trait_predicate.self_ty().skip_binder(), long_ty_path),
             )

tests/ui/traits/auxiliary/force_unstable.rs

@@ -0,0 +1,7 @@
+//@ edition: 2024
+//@ compile-flags: -Zforce-unstable-if-unmarked
+
+// Auxiliary crate that uses `-Zforce-unstable-if-unmarked` to export an
+// "unstable" trait.
+
+pub trait ForeignTrait {}

tests/ui/traits/nightly-only-unstable.force.stderr

@@ -0,0 +1,36 @@
+error[E0277]: the trait bound `(): LocalTrait` is not satisfied
+  --> $DIR/nightly-only-unstable.rs:25:21
+   |
+LL |     use_local_trait(());
+   |     --------------- ^^ the trait `LocalTrait` is not implemented for `()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/nightly-only-unstable.rs:14:1
+   |
+LL | trait LocalTrait {}
+   | ^^^^^^^^^^^^^^^^
+note: required by a bound in `use_local_trait`
+  --> $DIR/nightly-only-unstable.rs:16:28
+   |
+LL | fn use_local_trait(_: impl LocalTrait) {}
+   |                            ^^^^^^^^^^ required by this bound in `use_local_trait`
+
+error[E0277]: the trait bound `(): ForeignTrait` is not satisfied
+  --> $DIR/nightly-only-unstable.rs:31:23
+   |
+LL |     use_foreign_trait(());
+   |     ----------------- ^^ the trait `ForeignTrait` is not implemented for `()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `use_foreign_trait`
+  --> $DIR/nightly-only-unstable.rs:20:30
+   |
+LL | fn use_foreign_trait(_: impl force_unstable::ForeignTrait) {}
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `use_foreign_trait`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/nightly-only-unstable.normal.stderr

@@ -0,0 +1,36 @@
+error[E0277]: the trait bound `(): LocalTrait` is not satisfied
+  --> $DIR/nightly-only-unstable.rs:25:21
+   |
+LL |     use_local_trait(());
+   |     --------------- ^^ the trait `LocalTrait` is not implemented for `()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/nightly-only-unstable.rs:14:1
+   |
+LL | trait LocalTrait {}
+   | ^^^^^^^^^^^^^^^^
+note: required by a bound in `use_local_trait`
+  --> $DIR/nightly-only-unstable.rs:16:28
+   |
+LL | fn use_local_trait(_: impl LocalTrait) {}
+   |                            ^^^^^^^^^^ required by this bound in `use_local_trait`
+
+error[E0277]: the trait bound `(): ForeignTrait` is not satisfied
+  --> $DIR/nightly-only-unstable.rs:31:23
+   |
+LL |     use_foreign_trait(());
+   |     ----------------- ^^ the nightly-only, unstable trait `ForeignTrait` is not implemented for `()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `use_foreign_trait`
+  --> $DIR/nightly-only-unstable.rs:20:30
+   |
+LL | fn use_foreign_trait(_: impl force_unstable::ForeignTrait) {}
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `use_foreign_trait`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/nightly-only-unstable.rs

@@ -0,0 +1,36 @@
+//@ revisions: normal force
+//@ edition: 2024
+//@ aux-crate: force_unstable=force_unstable.rs
+//@[force] compile-flags: -Zforce-unstable-if-unmarked
+
+#![feature(rustc_private)]
+
+// Regression test for <https://github.com/rust-lang/rust/issues/152692>.
+//
+// When building a crate with `-Zforce-unstable-if-unmarked` (e.g. the compiler or stdlib),
+// it's unhelpful to mention that a not-implemented trait is unstable, because that will
+// be true of every local and foreign trait that isn't explicitly marked stable.
+
+trait LocalTrait {}
+
+fn use_local_trait(_: impl LocalTrait) {}
+//~^ NOTE required by a bound in `use_local_trait`
+//~| NOTE required by this bound in `use_local_trait`
+
+fn use_foreign_trait(_: impl force_unstable::ForeignTrait) {}
+//~^ NOTE required by a bound in `use_foreign_trait`
+//~| NOTE required by this bound in `use_foreign_trait`
+
+fn main() {
+    use_local_trait(());
+    //~^ ERROR the trait bound `(): LocalTrait` is not satisfied
+    //[normal]~| NOTE the trait `LocalTrait` is not implemented for `()`
+    //[force]~| NOTE the trait `LocalTrait` is not implemented for `()`
+    //~| NOTE required by a bound introduced by this call
+
+    use_foreign_trait(());
+    //~^ ERROR the trait bound `(): ForeignTrait` is not satisfied
+    //[normal]~| NOTE the nightly-only, unstable trait `ForeignTrait` is not implemented for `()`
+    //[force]~| NOTE the trait `ForeignTrait` is not implemented for `()`
+    //~| NOTE required by a bound introduced by this call
+}