Auto merge of rust-lang#88139 - lcnr:marker-trait-attr, r=nikomatsakis

marker_traits: require `EvaluatedToOk` during winnowing

closes rust-lang#84955, while it doesn't really fix it in a way that makes me happy it should prevent the issue for now and this
test can't be reproduced anyways, so it doesn't make much sense to keep it open.

fixes rust-lang#84917 as only one of the impls depends on regions, so we now drop the ambiguous one instead of the correct one.

cc https://rust-lang.zulipchat.com/#narrow/stream/144729-wg-traits/topic/winnowing.20soundly/near/247899832

r? `@nikomatsakis`

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

@@ -1586,12 +1586,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // See if we can toss out `victim` based on specialization.
                 // This requires us to know *for sure* that the `other` impl applies
                 // i.e., `EvaluatedToOk`.
+                //
+                // FIXME(@lcnr): Using `modulo_regions` here seems kind of scary
+                // to me but is required for `std` to compile, so I didn't change it
+                // for now.
+                let tcx = self.tcx();
                 if other.evaluation.must_apply_modulo_regions() {
-                    let tcx = self.tcx();
                     if tcx.specializes((other_def, victim_def)) {
                         return true;
                     }
-                    return match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
+                }
+
+                if other.evaluation.must_apply_considering_regions() {
+                    match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
                         Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
                             // Subtle: If the predicate we are evaluating has inference
                             // variables, do *not* allow discarding candidates due to
@@ -1636,7 +1643,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         }
                         Some(_) => true,
                         None => false,
-                    };
+                    }
                 } else {
                     false
                 }

src/test/ui/marker_trait_attr/overlapping-impl-1-modulo-regions.rs

@@ -0,0 +1,9 @@
+// check-pass
+#![feature(marker_trait_attr)]
+
+#[marker]
+pub trait F {}
+impl<T> F for T where T: Copy {}
+impl<T> F for T where T: 'static {}
+
+fn main() {}

src/test/ui/marker_trait_attr/region-overlap.rs

@@ -0,0 +1,8 @@
+#![feature(marker_trait_attr)]
+
+#[marker]
+trait A {}
+impl<'a> A for (&'static (), &'a ()) {} //~ ERROR type annotations needed
+impl<'a> A for (&'a (), &'static ()) {} //~ ERROR type annotations needed
+
+fn main() {}

src/test/ui/marker_trait_attr/region-overlap.stderr

@@ -0,0 +1,29 @@
+error[E0283]: type annotations needed
+  --> $DIR/region-overlap.rs:5:10
+   |
+LL | impl<'a> A for (&'static (), &'a ()) {}
+   |          ^ cannot infer type for tuple `(&'static (), &'a ())`
+   |
+   = note: cannot satisfy `(&'static (), &'a ()): A`
+note: required by a bound in `A`
+  --> $DIR/region-overlap.rs:4:1
+   |
+LL | trait A {}
+   | ^^^^^^^ required by this bound in `A`
+
+error[E0283]: type annotations needed
+  --> $DIR/region-overlap.rs:6:10
+   |
+LL | impl<'a> A for (&'a (), &'static ()) {}
+   |          ^ cannot infer type for tuple `(&'a (), &'static ())`
+   |
+   = note: cannot satisfy `(&'a (), &'static ()): A`
+note: required by a bound in `A`
+  --> $DIR/region-overlap.rs:4:1
+   |
+LL | trait A {}
+   | ^^^^^^^ required by this bound in `A`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0283`.

src/test/ui/marker_trait_attr/unsound-overlap.rs

@@ -0,0 +1,25 @@
+#![feature(marker_trait_attr)]
+
+#[marker]
+trait A {}
+
+trait B {}
+
+impl<T: A> B for T {}
+impl<T: B> A for T {}
+impl A for &str {}
+impl<T: A + B> A for (T,) {}
+trait TraitWithAssoc {
+    type Assoc;
+}
+
+impl<T: A> TraitWithAssoc for T {
+    type Assoc = T;
+}
+
+impl TraitWithAssoc for ((&str,),) {
+    //~^ ERROR conflicting implementations
+    type Assoc = ((&'static str,),);
+}
+
+fn main() {}

src/test/ui/marker_trait_attr/unsound-overlap.stderr

@@ -0,0 +1,12 @@
+error[E0119]: conflicting implementations of trait `TraitWithAssoc` for type `((&str,),)`
+  --> $DIR/unsound-overlap.rs:20:1
+   |
+LL | impl<T: A> TraitWithAssoc for T {
+   | ------------------------------- first implementation here
+...
+LL | impl TraitWithAssoc for ((&str,),) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `((&str,),)`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.