From a041a7609740acc81c8437cb13d95b7320c44bc4 Mon Sep 17 00:00:00 2001
From: Benno Lossin <benno.lossin@proton.me>
Date: Tue, 21 Apr 2026 09:10:24 +0200
Subject: [PATCH] field representing types: add test for unsized types

---
 .../not-field-if-unsized.next.stderr          | 27 +++++++++++++++++++
 .../not-field-if-unsized.old.stderr           | 27 +++++++++++++++++++
 .../not-field-if-unsized.rs                   | 23 ++++++++++++++++
 3 files changed, 77 insertions(+)
 create mode 100644 tests/ui/field_representing_types/not-field-if-unsized.next.stderr
 create mode 100644 tests/ui/field_representing_types/not-field-if-unsized.old.stderr
 create mode 100644 tests/ui/field_representing_types/not-field-if-unsized.rs

diff --git a/tests/ui/field_representing_types/not-field-if-unsized.next.stderr b/tests/ui/field_representing_types/not-field-if-unsized.next.stderr
new file mode 100644
index 000000000000..e3cee8090772
--- /dev/null
+++ b/tests/ui/field_representing_types/not-field-if-unsized.next.stderr
@@ -0,0 +1,27 @@
+error[E0277]: the trait bound `field_of!(MyStruct, 0): std::field::Field` is not satisfied
+  --> $DIR/not-field-if-unsized.rs:17:20
+   |
+LL |     assert_field::<field_of!(MyStruct, 0)>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ the nightly-only, unstable trait `std::field::Field` is not implemented for `field_of!(MyStruct, 0)`
+   |
+note: required by a bound in `assert_field`
+  --> $DIR/not-field-if-unsized.rs:12:20
+   |
+LL | fn assert_field<F: Field>() {}
+   |                    ^^^^^ required by this bound in `assert_field`
+
+error[E0277]: the trait bound `field_of!(MyStruct, 1): std::field::Field` is not satisfied
+  --> $DIR/not-field-if-unsized.rs:21:20
+   |
+LL |     assert_field::<field_of!(MyStruct, 1)>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ the nightly-only, unstable trait `std::field::Field` is not implemented for `field_of!(MyStruct, 1)`
+   |
+note: required by a bound in `assert_field`
+  --> $DIR/not-field-if-unsized.rs:12:20
+   |
+LL | fn assert_field<F: Field>() {}
+   |                    ^^^^^ required by this bound in `assert_field`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/field_representing_types/not-field-if-unsized.old.stderr b/tests/ui/field_representing_types/not-field-if-unsized.old.stderr
new file mode 100644
index 000000000000..e3cee8090772
--- /dev/null
+++ b/tests/ui/field_representing_types/not-field-if-unsized.old.stderr
@@ -0,0 +1,27 @@
+error[E0277]: the trait bound `field_of!(MyStruct, 0): std::field::Field` is not satisfied
+  --> $DIR/not-field-if-unsized.rs:17:20
+   |
+LL |     assert_field::<field_of!(MyStruct, 0)>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ the nightly-only, unstable trait `std::field::Field` is not implemented for `field_of!(MyStruct, 0)`
+   |
+note: required by a bound in `assert_field`
+  --> $DIR/not-field-if-unsized.rs:12:20
+   |
+LL | fn assert_field<F: Field>() {}
+   |                    ^^^^^ required by this bound in `assert_field`
+
+error[E0277]: the trait bound `field_of!(MyStruct, 1): std::field::Field` is not satisfied
+  --> $DIR/not-field-if-unsized.rs:21:20
+   |
+LL |     assert_field::<field_of!(MyStruct, 1)>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ the nightly-only, unstable trait `std::field::Field` is not implemented for `field_of!(MyStruct, 1)`
+   |
+note: required by a bound in `assert_field`
+  --> $DIR/not-field-if-unsized.rs:12:20
+   |
+LL | fn assert_field<F: Field>() {}
+   |                    ^^^^^ required by this bound in `assert_field`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/field_representing_types/not-field-if-unsized.rs b/tests/ui/field_representing_types/not-field-if-unsized.rs
new file mode 100644
index 000000000000..739fc91c2942
--- /dev/null
+++ b/tests/ui/field_representing_types/not-field-if-unsized.rs
@@ -0,0 +1,23 @@
+//@ revisions: old next
+//@ [next] compile-flags: -Znext-solver
+#![expect(incomplete_features)]
+#![feature(field_projections)]
+
+use std::field::{Field, field_of};
+
+pub trait Trait {}
+
+pub struct MyStruct(usize, dyn Trait);
+
+fn assert_field<F: Field>() {}
+
+fn main() {
+    // FIXME(FRTs): this requires relaxing the `Base: ?Sized` bound in the
+    // `Field` trait & compiler changes.
+    assert_field::<field_of!(MyStruct, 0)>();
+    //~^ ERROR: the trait bound `field_of!(MyStruct, 0): std::field::Field` is not satisfied [E0277]
+
+    // FIXME(FRTs): improve this error message, point to the `dyn Trait` span.
+    assert_field::<field_of!(MyStruct, 1)>();
+    //~^ ERROR: the trait bound `field_of!(MyStruct, 1): std::field::Field` is not satisfied [E0277]
+}