[gcc r14-7626] gccrs: Fix cast site to not miss type-unifications

[gcc r14-7626] gccrs: Fix cast site to not miss type-unifications

[Arthur Cohen]

https://gcc.gnu.org/g:1b34e40e5e41bdcb227b40523cd3a64c6f2fde00

commit r14-7626-g1b34e40e5e41bdcb227b40523cd3a64c6f2fde00
Author: Philip Herron <herron.philip@googlemail.com>
Date:   Tue May 30 17:16:30 2023 +0100

    gccrs: Fix cast site to not miss type-unifications
    
    When attempting casts we can try a type coercion first, this is a
    TryCoercion which will return a result. In the event this is ok we need to
    perform a true coercion so that we don't leave missing infeence variable's
    ununified.
    
    Fixes #2195
    
    gcc/rust/ChangeLog:
    
            * typecheck/rust-casts.cc (TypeCastRules::check): do coercion
            (TypeCastRules::emit_cast_error): clang-format
    
    gcc/testsuite/ChangeLog:
    
            * rust/compile/issue-2195.rs: New test.
    
    Signed-off-by: Philip Herron <herron.philip@googlemail.com>

Diff:
---
 gcc/rust/typecheck/rust-casts.cc         | 9 +++++++--
 gcc/testsuite/rust/compile/issue-2195.rs | 8 ++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/gcc/rust/typecheck/rust-casts.cc b/gcc/rust/typecheck/rust-casts.cc
index 8e5e65eb22c..c9a25a03355 100644
--- a/gcc/rust/typecheck/rust-casts.cc
+++ b/gcc/rust/typecheck/rust-casts.cc
@@ -42,7 +42,12 @@ TypeCastRules::check ()
     = TypeCoercionRules::TryCoerce (from.get_ty (), to.get_ty (), locus,
 				    true /*allow-autoderef*/);
   if (!possible_coercion.is_error ())
-    return possible_coercion;
+    {
+      // given the attempt was ok we need to ensure we perform it so that any
+      // inference variables are unified correctly
+      return TypeCoercionRules::Coerce (from.get_ty (), to.get_ty (), locus,
+					true /*allow-autoderef*/);
+    }
 
   // try the simple cast rules
   auto simple_cast = cast_rules ();
@@ -303,4 +308,4 @@ TypeCastRules::emit_cast_error () const
 }
 
 } // namespace Resolver
-} // namespace Rust
\ No newline at end of file
+} // namespace Rust
diff --git a/gcc/testsuite/rust/compile/issue-2195.rs b/gcc/testsuite/rust/compile/issue-2195.rs
new file mode 100644
index 00000000000..c955df4f1c6
--- /dev/null
+++ b/gcc/testsuite/rust/compile/issue-2195.rs
@@ -0,0 +1,8 @@
+struct A<T> {
+    // { dg-warning "struct is never constructed" "" { target *-*-* } .-1 }
+    f: *const T,
+}
+
+pub fn cast<T>(a: A<T>) {
+    let z = a.f as *const ();
+}