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

Check that a box expression's type is Sized

This resolves [issue 87935](#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?

Author: bors

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!
 }
 ```

compiler/rustc_middle/src/traits/mod.rs

@@ -225,6 +225,8 @@ pub enum ObligationCauseCode<'tcx> {
     SizedReturnType,
     /// Yield type must be `Sized`.
     SizedYieldType,
+    /// Box expression result type must be `Sized`.
+    SizedBoxType,
     /// Inline asm operand type must be `Sized`.
     InlineAsmSized,
     /// `[T, ..n]` implies that `T` must be `Copy`.

compiler/rustc_mir/src/borrow_check/type_check/mod.rs

@@ -1893,9 +1893,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
                 // While this is located in `nll::typeck` this error is not
                 // an NLL error, it's a required check to prevent creation
-                // of unsized rvalues in certain cases:
-                // * operand of a box expression
-                // * callee in a call expression
+                // of unsized rvalues in a call expression.
                 diag.emit();
             }
         }

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2072,6 +2072,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             ObligationCauseCode::SizedYieldType => {
                 err.note("the yield type of a generator must have a statically known size");
             }
+            ObligationCauseCode::SizedBoxType => {
+                err.note("the type of a box expression must have a statically known size");
+            }
             ObligationCauseCode::AssignmentLhsSized => {
                 err.note("the left-hand-side of an assignment must have a statically known size");
             }

compiler/rustc_typeck/src/check/expr.rs

@@ -313,6 +313,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => NoExpectation,
         });
         let referent_ty = self.check_expr_with_expectation(expr, expected_inner);
+        self.require_type_is_sized(referent_ty, expr.span, traits::SizedBoxType);
         self.tcx.mk_box(referent_ty)
     }
 

src/test/ui/dst/dst-rvalue.rs

@@ -4,11 +4,9 @@
 
 pub fn main() {
     let _x: Box<str> = box *"hello world";
-    //~^ ERROR E0161
-    //~^^ ERROR cannot move out of a shared reference
+    //~^ ERROR E0277
 
     let array: &[isize] = &[1, 2, 3];
     let _x: Box<[isize]> = box *array;
-    //~^ ERROR E0161
-    //~^^ ERROR cannot move out of type `[isize]`, a non-copy slice
+    //~^ ERROR E0277
 }

src/test/ui/dst/dst-rvalue.stderr

@@ -1,31 +1,21 @@
-error[E0161]: cannot move a value of type str: the size of str cannot be statically determined
+error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> $DIR/dst-rvalue.rs:6:28
    |
 LL |     let _x: Box<str> = box *"hello world";
-   |                            ^^^^^^^^^^^^^^
-
-error[E0161]: cannot move a value of type [isize]: the size of [isize] cannot be statically determined
-  --> $DIR/dst-rvalue.rs:11:32
+   |                            ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-LL |     let _x: Box<[isize]> = box *array;
-   |                                ^^^^^^
+   = help: the trait `Sized` is not implemented for `str`
+   = note: the type of a box expression must have a statically known size
 
-error[E0507]: cannot move out of a shared reference
-  --> $DIR/dst-rvalue.rs:6:28
-   |
-LL |     let _x: Box<str> = box *"hello world";
-   |                            ^^^^^^^^^^^^^^ move occurs because value has type `str`, which does not implement the `Copy` trait
-
-error[E0508]: cannot move out of type `[isize]`, a non-copy slice
-  --> $DIR/dst-rvalue.rs:11:32
+error[E0277]: the size for values of type `[isize]` cannot be known at compilation time
+  --> $DIR/dst-rvalue.rs:10:32
    |
 LL |     let _x: Box<[isize]> = box *array;
-   |                                ^^^^^^
-   |                                |
-   |                                cannot move out of here
-   |                                move occurs because `*array` has type `[isize]`, which does not implement the `Copy` trait
+   |                                ^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[isize]`
+   = note: the type of a box expression must have a statically known size
 
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0161, E0507, E0508.
-For more information about an error, try `rustc --explain E0161`.
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/error-codes/E0161.edition.stderr

@@ -1,8 +1,8 @@
-error[E0161]: cannot move a value of type [i32]: the size of [i32] cannot be statically determined
-  --> $DIR/E0161.rs:22:9
+error[E0161]: cannot move a value of type dyn Bar: the size of dyn Bar cannot be statically determined
+  --> $DIR/E0161.rs:29:5
    |
-LL |     box *x;
-   |         ^^
+LL |     x.f();
+   |     ^
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0161.editionul.stderr

@@ -1,8 +1,8 @@
-error[E0161]: cannot move a value of type [i32]: the size of [i32] cannot be statically determined
-  --> $DIR/E0161.rs:22:9
+error[E0161]: cannot move a value of type dyn Bar: the size of dyn Bar cannot be statically determined
+  --> $DIR/E0161.rs:29:5
    |
-LL |     box *x;
-   |         ^^
+LL |     x.f();
+   |     ^
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0161.migrate.stderr

@@ -1,8 +1,8 @@
-error[E0161]: cannot move a value of type [i32]: the size of [i32] cannot be statically determined
-  --> $DIR/E0161.rs:22:9
+error[E0161]: cannot move a value of type dyn Bar: the size of dyn Bar cannot be statically determined
+  --> $DIR/E0161.rs:29:5
    |
-LL |     box *x;
-   |         ^^
+LL |     x.f();
+   |     ^
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0161.migrateul.stderr

@@ -8,6 +8,10 @@
 //[edition]edition:2018
 //[zflagsul]compile-flags: -Z borrowck=migrate
 //[editionul]edition:2018
+//[migrateul] check-pass
+//[nllul] check-pass
+//[zflagsul] check-pass
+//[editionul] check-pass
 
 #![allow(incomplete_features)]
 #![cfg_attr(nll, feature(nll))]
@@ -16,12 +20,14 @@
 #![cfg_attr(zflagsul, feature(unsized_locals))]
 #![cfg_attr(nllul, feature(unsized_locals))]
 #![cfg_attr(editionul, feature(unsized_locals))]
-#![feature(box_syntax)]
 
-fn foo(x: Box<[i32]>) {
-    box *x;
+trait Bar {
+    fn f(self);
+}
+
+fn foo(x: Box<dyn Bar>) {
+    x.f();
     //[migrate,nll,zflags,edition]~^ ERROR E0161
-    //[migrateul,nllul,zflagsul,editionul]~^^ ERROR E0161
 }
 
 fn main() {}

src/test/ui/error-codes/E0161.nll.stderr

@@ -1,8 +1,8 @@
-error[E0161]: cannot move a value of type [i32]: the size of [i32] cannot be statically determined
-  --> $DIR/E0161.rs:22:9
+error[E0161]: cannot move a value of type dyn Bar: the size of dyn Bar cannot be statically determined
+  --> $DIR/E0161.rs:29:5
    |
-LL |     box *x;
-   |         ^^
+LL |     x.f();
+   |     ^
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0161.nllul.stderr

@@ -0,0 +1,10 @@
+#![feature(box_syntax)]
+// Box expression needs to be movable, and hence has to be of a Sized type.
+fn main() {
+    let _x: Box<[u32]> = box { loop {} };
+    //~^ ERROR: the size for values of type `[u32]` cannot be known at compilation time
+
+    // Check that a deduced size does not cause issues.
+    let _y: Box<[u32]> = box [];
+    let _z: Box<[u32; 0]> = box { loop {} };
+}

src/test/ui/error-codes/E0161.rs

@@ -0,0 +1,12 @@
+error[E0277]: the size for values of type `[u32]` cannot be known at compilation time
+  --> $DIR/issue-87935-unsized-box-expr.rs:4:30
+   |
+LL |     let _x: Box<[u32]> = box { loop {} };
+   |                              ^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u32]`
+   = note: the type of a box expression must have a statically known size
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.