Rollup merge of #129059 - compiler-errors:subtyping-correct-type, r=lcnr

Record the correct target type when coercing fn items/closures to pointers

Self-explanatory. We were previously not recording the *target* type of a coercion as the output of an adjustment. This should remedy that.

We must also modify the function pointer casts in MIR typeck to use subtyping, since those broke since #118247.

r? lcnr

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1989,9 +1989,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
                         let ty_fn_ptr_from = Ty::new_fn_ptr(tcx, fn_sig);
 
-                        if let Err(terr) = self.eq_types(
-                            *ty,
+                        if let Err(terr) = self.sub_types(
                             ty_fn_ptr_from,
+                            *ty,
                             location.to_locations(),
                             ConstraintCategory::Cast { unsize_to: None },
                         ) {
@@ -2014,9 +2014,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                         let ty_fn_ptr_from =
                             Ty::new_fn_ptr(tcx, tcx.signature_unclosure(sig, *safety));
 
-                        if let Err(terr) = self.eq_types(
-                            *ty,
+                        if let Err(terr) = self.sub_types(
                             ty_fn_ptr_from,
+                            *ty,
                             location.to_locations(),
                             ConstraintCategory::Cast { unsize_to: None },
                         ) {

compiler/rustc_hir_typeck/src/coercion.rs

@@ -137,7 +137,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 at.lub(DefineOpaqueTypes::Yes, b, a)
             } else {
                 at.sup(DefineOpaqueTypes::Yes, b, a)
-                    .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
+                    .map(|InferOk { value: (), obligations }| InferOk { value: b, obligations })
             };
 
             // In the new solver, lazy norm may allow us to shallowly equate

tests/mir-opt/build_correct_coerce.main.built.after.mir

@@ -0,0 +1,18 @@
+// MIR for `main` after built
+
+fn main() -> () {
+    let mut _0: ();
+    let _1: for<'a> fn(&'a (), &'a ());
+    scope 1 {
+        debug x => _1;
+    }
+
+    bb0: {
+        StorageLive(_1);
+        _1 = foo as for<'a> fn(&'a (), &'a ()) (PointerCoercion(ReifyFnPointer));
+        FakeRead(ForLet(None), _1);
+        _0 = const ();
+        StorageDead(_1);
+        return;
+    }
+}

tests/mir-opt/build_correct_coerce.rs

@@ -0,0 +1,12 @@
+// skip-filecheck
+
+// Validate that we record the target for the `as` coercion as `for<'a> fn(&'a (), &'a ())`,
+// and not `for<'a, 'b>(&'a (), &'b ())`. We previously did the latter due to a bug in
+// the code that records adjustments in HIR typeck.
+
+fn foo<'a, 'b>(_: &'a (), _: &'b ()) {}
+
+// EMIT_MIR build_correct_coerce.main.built.after.mir
+fn main() {
+    let x = foo as for<'a> fn(&'a (), &'a ());
+}

tests/ui/higher-ranked/subtyping-fn-ptr-coercion.rs

@@ -0,0 +1,10 @@
+//@ check-pass
+
+// Check that we use subtyping when reifying a closure into a function pointer.
+
+fn foo(x: &str) {}
+
+fn main() {
+    let c = |_: &str| {};
+    let x = c as fn(&'static str);
+}

tests/ui/impl-trait/recursive-ice-101862.stderr

@@ -11,13 +11,13 @@ LL |     vec![].append(&mut ice(x.as_ref()));
    = note: `#[warn(unconditional_recursion)]` on by default
 
 error[E0792]: expected generic type parameter, found `&str`
-  --> $DIR/recursive-ice-101862.rs:6:5
+  --> $DIR/recursive-ice-101862.rs:6:19
    |
 LL | pub fn ice(x: impl AsRef<str>) -> impl IntoIterator<Item = ()> {
    |               --------------- this generic parameter must be used with a generic type parameter
 LL |
 LL |     vec![].append(&mut ice(x.as_ref()));
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                   ^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error; 1 warning emitted
 

tests/ui/traits/next-solver/alias-bound-unsound.rs

@@ -27,5 +27,6 @@ fn main() {
     //~| ERROR overflow evaluating the requirement `&<() as Foo>::Item well-formed`
     //~| ERROR overflow evaluating the requirement `<() as Foo>::Item == _`
     //~| ERROR overflow evaluating the requirement `<() as Foo>::Item == _`
+    //~| ERROR overflow evaluating the requirement `<() as Foo>::Item == _`
     println!("{x}");
 }

tests/ui/traits/next-solver/alias-bound-unsound.stderr

@@ -44,6 +44,12 @@ LL |     drop(<() as Foo>::copy_me(&x));
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 6 previous errors
+error[E0275]: overflow evaluating the requirement `<() as Foo>::Item == _`
+  --> $DIR/alias-bound-unsound.rs:24:31
+   |
+LL |     drop(<() as Foo>::copy_me(&x));
+   |                               ^^
+
+error: aborting due to 7 previous errors
 
 For more information about this error, try `rustc --explain E0275`.