Fix const-eval of shared generic reborrows

compiler/rustc_const_eval/src/interpret/step.rs

@@ -230,9 +230,15 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 })?;
             }
 
-            Reborrow(_, _, place) => {
-                let op = self.eval_place_to_op(place, Some(dest.layout))?;
-                self.copy_op(&op, &dest)?;
+            Reborrow(_, mutability, place) => {
+                let op = self.eval_place_to_op(place, None)?;
+                if mutability.is_not() {
+                    // Shared generic reborrows use `CoerceShared`: a bitwise copy into a
+                    // distinct same-layout target ADT.
+                    self.copy_op_allow_transmute(&op, &dest)?;
+                } else {
+                    self.copy_op(&op, &dest)?;
+                }
             }
 
             RawPtr(kind, place) => {

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -580,11 +580,7 @@ impl<'tcx> Validator<'_, 'tcx> {
                 self.validate_ref(*kind, place)?;
             }
 
-            Rvalue::Reborrow(_, _, place) => {
-                // FIXME(reborrow): should probably have a place_simplified like above.
-                let op = &Operand::Copy(*place);
-                self.validate_operand(op)?
-            }
+            Rvalue::Reborrow(..) => return Err(Unpromotable),
 
             Rvalue::Aggregate(_, operands) => {
                 for o in operands {

src/tools/miri/tests/pass/reborrow-coerce-shared.rs

@@ -0,0 +1,48 @@
+// Regression test for the Miri reproducer in rust-lang/rust#156313.
+//
+// The issue's exact recursive example ICEd while evaluating the argument
+// conversion before the recursion mattered. This keeps that same
+// `CustomMut`-to-`CustomRef` call path, but terminates after one recursive
+// call so it can be a pass test once the ICE is fixed.
+
+#![feature(reborrow)]
+
+use std::marker::{CoerceShared, Reborrow};
+
+#[allow(unused)]
+struct CustomMut<'a, T>(&'a mut T);
+impl<'a, T> Reborrow for CustomMut<'a, T> {}
+impl<'a, T> CoerceShared<CustomRef<'a, T>> for CustomMut<'a, T> {}
+
+struct CustomRef<'a, T>(&'a T);
+
+impl<'a, T> Clone for CustomRef<'a, T> {
+    fn clone(&self) -> Self {
+        Self(self.0)
+    }
+}
+impl<'a, T> Copy for CustomRef<'a, T> {}
+
+fn method(_a: CustomRef<'_, ()>) {}
+
+fn recursive_method(_a: CustomRef<'_, ()>, recurse: bool) {
+    if recurse {
+        let a = CustomMut(&mut ());
+        recursive_method(a, false);
+    }
+}
+
+fn issue_156313_runtime_reproducer() {
+    let a = CustomMut(&mut ());
+    method(a);
+}
+
+fn issue_156313_recursive_call_reproducer() {
+    let a = CustomMut(&mut ());
+    recursive_method(a, true);
+}
+
+fn main() {
+    issue_156313_runtime_reproducer();
+    issue_156313_recursive_call_reproducer();
+}

tests/ui/reborrow/coerce_shared_consteval.rs

@@ -0,0 +1,28 @@
+//@ run-pass
+// Regression test for the const-eval reproducer in rust-lang/rust#156313.
+
+#![feature(reborrow)]
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+use std::marker::{CoerceShared, Reborrow};
+
+pub struct MyMut<'a>(&'a u8);
+
+impl Reborrow for MyMut<'_> {}
+
+#[derive(Clone, Copy)]
+pub struct MyRef<'a>(&'a u8);
+
+impl<'a> CoerceShared<MyRef<'a>> for MyMut<'a> {}
+
+const fn consteval_reproducer() {
+    let value = 1;
+    foo(MyMut(&value));
+}
+
+const fn foo(x: MyRef<'_>) {}
+
+fn main() {
+    const { consteval_reproducer(); }
+}

tests/ui/reborrow/reborrow-promotion-rejected.rs

@@ -0,0 +1,21 @@
+//@ check-fail
+
+#![feature(reborrow)]
+
+use std::marker::{CoerceShared, Reborrow};
+
+struct MyMut<'a>(&'a u8);
+impl Reborrow for MyMut<'_> {}
+
+#[derive(Clone, Copy)]
+struct MyRef<'a>(&'a u8);
+impl<'a> CoerceShared<MyRef<'a>> for MyMut<'a> {}
+
+const fn coerce(x: MyRef<'_>) -> MyRef<'_> {
+    x
+}
+
+static BAD: &'static MyRef<'static> = &coerce(MyMut(&1));
+//~^ ERROR temporary value dropped while borrowed
+
+fn main() {}

tests/ui/reborrow/reborrow-promotion-rejected.stderr

@@ -0,0 +1,13 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/reborrow-promotion-rejected.rs:18:47
+   |
+LL | static BAD: &'static MyRef<'static> = &coerce(MyMut(&1));
+   |                                       --------^^^^^^^^^-
+   |                                       |       |        |
+   |                                       |       |        temporary value is freed at the end of this statement
+   |                                       |       creates a temporary value which is freed while still in use
+   |                                       using this value as a static requires that borrow lasts for `'static`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0716`.