do not use <: in subtyping overflow msg

Author: lcnr

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

@@ -246,13 +246,26 @@ 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 `_`
 }

tests/ui/occurs-check.stderr

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

tests/ui/traits/subtype-recursion-limit.rs

@@ -10,7 +10,7 @@ fn main() {
     let x = return;
     let y = return;
     let mut w = (x, y);
-    //~^ ERROR overflow evaluating the requirement
+    //~^ ERROR overflow setting `_` to a subtype of `*const _`
     // Avoid creating lifetimes, `Sized` bounds or function calls.
     let a = (ptr::addr_of!(y), ptr::addr_of!(x));
     w = a;

tests/ui/traits/subtype-recursion-limit.stderr

@@ -1,4 +1,4 @@
-error[E0275]: overflow evaluating the requirement `_ <: *const _`
+error[E0275]: overflow setting `_` to a subtype of `*const _`
   --> $DIR/subtype-recursion-limit.rs:12:17
    |
 LL |     let mut w = (x, y);

tests/ui/traits/well-formed-recursion-limit.rs

@@ -13,8 +13,8 @@ pub fn iso_un_option<A: 'static, B: 'static>(i: ISO<Option<A>, Option<B>>) -> IS
     //~^ ERROR no field `ab` on type
     //~| ERROR no field `ba` on type
     let left = move |o_a| match o_a {
-        //~^ ERROR overflow evaluating the requirement
-        None => panic!("absured"),
+        //~^ ERROR overflow setting `_` to a subtype of `Option<_>`
+        None => panic!("absurd"),
         Some(a) => a,
     };
     let right = move |o_b| match o_b {

tests/ui/traits/well-formed-recursion-limit.stderr

@@ -10,7 +10,7 @@ error[E0609]: no field `ba` on type `(Box<(dyn Fn(Option<A>) -> Option<B> + 'sta
 LL |     let (ab, ba) = (i.ab, i.ba);
    |                             ^^ unknown field
 
-error[E0275]: overflow evaluating the requirement `_ <: Option<_>`
+error[E0275]: overflow setting `_` to a subtype of `Option<_>`
   --> $DIR/well-formed-recursion-limit.rs:15:33
    |
 LL |     let left = move |o_a| match o_a {