Auto merge of #88087 - jesyspa:issue-87935-box, r=jackh726

Check that a box expression's type is Sized

This resolves [issue 87935](https://github.com/rust-lang/rust/issues/87935).

This makes E0161 (move from an unsized rvalue) much less common.  I've replaced the test to use [this case](https://github.com/rust-lang/rust/blob/master/src/test/ui/object-safety/object-safety-by-value-self-use.rs), when a boxed `dyn` trait is passed by value, but that isn't an error when `unsized_locals` is enabled.  I think it may be possible to get rid of E0161 entirely by checking that case earlier, but I'm not sure if that's desirable?

1 files changed, 21 insertions, 5 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0161.md b/compiler/rustc_error_codes/src/error_codes/E0161.md
index c2e2f0240f4..ebd2c97698b 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0161.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0161.md
@@ -4,11 +4,18 @@ Erroneous code example:
 
 ```compile_fail,E0161
 #![feature(box_syntax)]
+trait Bar {
+    fn f(self);
+}
+
+impl Bar for i32 {
+    fn f(self) {}
+}
 
 fn main() {
-    let array: &[isize] = &[1, 2, 3];
-    let _x: Box<[isize]> = box *array;
-    // error: cannot move a value of type [isize]: the size of [isize] cannot
+    let b: Box<dyn Bar> = box (0 as i32);
+    b.f();
+    // error: cannot move a value of type dyn Bar: the size of dyn Bar cannot
     //        be statically determined
 }
 ```
@@ -22,8 +29,17 @@ it around as usual. Example:
 ```
 #![feature(box_syntax)]
 
+trait Bar {
+    fn f(&self);
+}
+
+impl Bar for i32 {
+    fn f(&self) {}
+}
+
 fn main() {
-    let array: &[isize] = &[1, 2, 3];
-    let _x: Box<&[isize]> = box array; // ok!
+    let b: Box<dyn Bar> = box (0 as i32);
+    b.f();
+    // ok!
 }
 ```