add test for #121413

Fixes #121413

tests/ui/consts/const_refs_to_static-ice-121413.rs

@@ -0,0 +1,16 @@
+// ICE: ImmTy { imm: Scalar(alloc1), ty: *const dyn Sync } input to a fat-to-thin cast (*const dyn Sync -> *const usize
+// or with -Zextra-const-ub-checks: expected wide pointer extra data (e.g. slice length or trait object vtable)
+// issue: rust-lang/rust#121413
+//@ compile-flags: -Zextra-const-ub-checks
+// ignore-tidy-linelength
+#![feature(const_refs_to_static)]
+const REF_INTERIOR_MUT: &usize = {
+    static FOO: Sync = AtomicUsize::new(0);
+    //~^ ERROR failed to resolve: use of undeclared type `AtomicUsize`
+    //~| WARN trait objects without an explicit `dyn` are deprecated
+    //~| ERROR the size for values of type `(dyn Sync + 'static)` cannot be known at compilation time
+    //~| ERROR the size for values of type `(dyn Sync + 'static)` cannot be known at compilation time
+    //~| WARN this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+    unsafe { &*(&FOO as *const _ as *const usize) }
+};
+pub fn main() {}

tests/ui/consts/const_refs_to_static-ice-121413.stderr

@@ -0,0 +1,46 @@
+error[E0433]: failed to resolve: use of undeclared type `AtomicUsize`
+  --> $DIR/const_refs_to_static-ice-121413.rs:8:24
+   |
+LL |     static FOO: Sync = AtomicUsize::new(0);
+   |                        ^^^^^^^^^^^ use of undeclared type `AtomicUsize`
+   |
+help: consider importing this struct
+   |
+LL + use std::sync::atomic::AtomicUsize;
+   |
+
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/const_refs_to_static-ice-121413.rs:8:17
+   |
+LL |     static FOO: Sync = AtomicUsize::new(0);
+   |                 ^^^^
+   |
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
+   = note: `#[warn(bare_trait_objects)]` on by default
+help: if this is an object-safe trait, use `dyn`
+   |
+LL |     static FOO: dyn Sync = AtomicUsize::new(0);
+   |                 +++
+
+error[E0277]: the size for values of type `(dyn Sync + 'static)` cannot be known at compilation time
+  --> $DIR/const_refs_to_static-ice-121413.rs:8:17
+   |
+LL |     static FOO: Sync = AtomicUsize::new(0);
+   |                 ^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `(dyn Sync + 'static)`
+
+error[E0277]: the size for values of type `(dyn Sync + 'static)` cannot be known at compilation time
+  --> $DIR/const_refs_to_static-ice-121413.rs:8:24
+   |
+LL |     static FOO: Sync = AtomicUsize::new(0);
+   |                        ^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `(dyn Sync + 'static)`
+   = note: constant expressions must have a statically known size
+
+error: aborting due to 3 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0277, E0433.
+For more information about an error, try `rustc --explain E0277`.