borrowck: avoid ICE describing fields on generic params

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -537,9 +537,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     Some(self.infcx.tcx.hir_name(var_id).to_string())
                 }
                 _ => {
-                    // Might need a revision when the fields in trait RFC is implemented
-                    // (https://github.com/rust-lang/rfcs/pull/1546)
-                    bug!("End-user description not implemented for field access on `{:?}`", ty);
+                    // This can happen for field accesses on `Box<T>`: the field is
+                    // described from the boxed type, which may have no named fields
+                    Some(field.index().to_string())
                 }
             }
         }

tests/ui/borrowck/borrowck-describe-field-generic-param-owned-box.rs

@@ -0,0 +1,56 @@
+// Regression test for #155344.
+// Borrowck diagnostics should not ICE when describing a field access on a generic parameter.
+
+//@ edition: 2024
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
+#[lang = "sized"]
+pub trait Sized: MetaSized {}
+
+#[lang = "legacy_receiver"]
+pub trait LegacyReceiver {}
+
+impl<T: PointeeSized> LegacyReceiver for &T {}
+impl<T: PointeeSized> LegacyReceiver for &mut T {}
+
+#[lang = "copy"]
+pub trait Copy {}
+
+impl Copy for *mut () {}
+
+#[lang = "drop"]
+pub trait Drop {
+    fn drop(&mut self);
+}
+
+unsafe extern "C" {
+    fn free(_: *mut ());
+}
+
+unsafe fn transmute<T, U>(_: T) -> U {
+    loop {}
+}
+
+#[repr(transparent)]
+pub struct NonNull<T: ?Sized>(pub *const T);
+
+#[lang = "owned_box"]
+pub struct Box<T: ?Sized, A = ()>(NonNull<T>, A);
+
+impl<T: ?Sized, A> Drop for Box<T, A> {
+    fn drop(&mut self) {
+        unsafe {
+            free(transmute::<NonNull<T>, *mut _>(self.0));
+            //~^ ERROR cannot move out of `self.0` which is behind a mutable reference
+        }
+    }
+}

tests/ui/borrowck/borrowck-describe-field-generic-param-owned-box.stderr

@@ -0,0 +1,9 @@
+error[E0507]: cannot move out of `self.0` which is behind a mutable reference
+  --> $DIR/borrowck-describe-field-generic-param-owned-box.rs:52:50
+   |
+LL |             free(transmute::<NonNull<T>, *mut _>(self.0));
+   |                                                  ^^^^^^ move occurs because `self.0` has type `NonNull<T>`, which does not implement the `Copy` trait
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0507`.