do not use <: in subtyping overflow msg

Author: lcnr

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

@@ -239,13 +239,26 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             }
             OverflowCause::TraitSolver(predicate) => {
                 let predicate = self.resolve_vars_if_possible(predicate);
-                let pred_str = with_short_path(self.tcx, predicate);
-                struct_span_code_err!(
-                    self.dcx(),
-                    span,
-                    E0275,
-                    "overflow evaluating the requirement `{pred_str}`",
-                )
+                match predicate.kind().skip_binder() {
+                    ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ })
+                    | ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
+                        struct_span_code_err!(
+                            self.dcx(),
+                            span,
+                            E0275,
+                            "overflow setting `{a}` to a subtype of `{b}`",
+                        )
+                    }
+                    _ => {
+                        let pred_str = with_short_path(self.tcx, predicate);
+                        struct_span_code_err!(
+                            self.dcx(),
+                            span,
+                            E0275,
+                            "overflow evaluating the requirement `{pred_str}`",
+                        )
+                    }
+                }
             }
         };
 

tests/ui/impl-trait/issues/issue-84073.rs

@@ -29,5 +29,5 @@ where
 }
 
 fn main() {
-    Race::new(|race| race.when()); //~ ERROR overflow evaluating the requirement `_ <: Option<_>`
+    Race::new(|race| race.when()); //~ ERROR overflow setting `_` to a subtype of `Option<_>`
 }

tests/ui/impl-trait/issues/issue-84073.stderr

@@ -1,4 +1,4 @@
-error[E0275]: overflow evaluating the requirement `_ <: Option<_>`
+error[E0275]: overflow setting `_` to a subtype of `Option<_>`
   --> $DIR/issue-84073.rs:32:22
    |
 LL |     Race::new(|race| race.when());

tests/ui/infinite/infinite-autoderef.rs

@@ -14,7 +14,7 @@ pub fn main() {
     let mut x;
     loop {
         x = Box::new(x);
-        //~^ ERROR overflow evaluating the requirement `Box<_> <: _`
+        //~^ ERROR overflow setting `Box<_>` to a subtype of `_`
         x.foo;
         x.bar();
     }

tests/ui/infinite/infinite-autoderef.stderr

@@ -1,4 +1,4 @@
-error[E0275]: overflow evaluating the requirement `Box<_> <: _`
+error[E0275]: overflow setting `Box<_>` to a subtype of `_`
   --> $DIR/infinite-autoderef.rs:16:13
    |
 LL |         x = Box::new(x);

tests/ui/occurs-check-2.rs

@@ -5,5 +5,5 @@ fn main() {
 
     g = f;
     f = Box::new(g);
-    //~^ ERROR overflow evaluating the requirement `Box<_> <: _`
+    //~^ ERROR  overflow setting `Box<_>` to a subtype of `_`
 }

tests/ui/occurs-check-2.stderr

@@ -1,4 +1,4 @@
-error[E0275]: overflow evaluating the requirement `Box<_> <: _`
+error[E0275]: overflow setting `Box<_>` to a subtype of `_`
   --> $DIR/occurs-check-2.rs:7:9
    |
 LL |     f = Box::new(g);

tests/ui/occurs-check-3.rs

@@ -4,7 +4,7 @@ enum Clam<T> { A(T) }
 fn main() {
     let c;
     c = Clam::A(c);
-    //~^ ERROR overflow evaluating the requirement `Clam<_> <: _`
+    //~^ ERROR overflow setting `Clam<_>` to a subtype of `_`
     match c {
         Clam::A::<isize>(_) => { }
     }

tests/ui/occurs-check-3.stderr

@@ -1,4 +1,4 @@
-error[E0275]: overflow evaluating the requirement `Clam<_> <: _`
+error[E0275]: overflow setting `Clam<_>` to a subtype of `_`
   --> $DIR/occurs-check-3.rs:6:9
    |
 LL |     c = Clam::A(c);

tests/ui/occurs-check.rs

@@ -1,5 +1,5 @@
 fn main() {
     let f;
     f = Box::new(f);
-    //~^ ERROR overflow evaluating the requirement `Box<_> <: _`
+    //~^ ERROR overflow setting `Box<_>` to a subtype of `_`
 }