make NOP dyn casts not require anything about the vtable

3 files changed, 38 insertions, 20 deletions

diff --git a/src/test/ui/consts/const-eval/ub-wide-ptr.32bit.stderr b/src/test/ui/consts/const-eval/ub-wide-ptr.32bit.stderr
index 345ead48151..9431fb33c53 100644
--- a/src/test/ui/consts/const-eval/ub-wide-ptr.32bit.stderr
+++ b/src/test/ui/consts/const-eval/ub-wide-ptr.32bit.stderr
@@ -278,26 +278,36 @@ LL | const TRAIT_OBJ_CONTENT_INVALID: &dyn Trait = unsafe { mem::transmute::<_,
                ╾allocN─╼ ╾allocN─╼                         │ ╾──╼╾──╼
            }
 
-error[E0080]: evaluation of constant value failed
-  --> $DIR/ub-wide-ptr.rs:147:62
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/ub-wide-ptr.rs:147:1
    |
 LL | const RAW_TRAIT_OBJ_VTABLE_NULL: *const dyn Trait = unsafe { mem::transmute((&92u8, 0usize)) };
-   |                                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer use: null pointer is a dangling pointer (it has no provenance)
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered null pointer, but expected a vtable pointer
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: 8, align: 4) {
+               ╾allocN─╼ 00 00 00 00                         │ ╾──╼....
+           }
 
-error[E0080]: evaluation of constant value failed
-  --> $DIR/ub-wide-ptr.rs:150:65
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/ub-wide-ptr.rs:149:1
    |
 LL | const RAW_TRAIT_OBJ_VTABLE_INVALID: *const dyn Trait = unsafe { mem::transmute((&92u8, &3u64)) };
-   |                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using allocN as vtable pointer but it does not point to a vtable
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered allocN, but expected a vtable pointer
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: 8, align: 4) {
+               ╾allocN─╼ ╾allocN─╼                         │ ╾──╼╾──╼
+           }
 
 error[E0080]: could not evaluate static initializer
-  --> $DIR/ub-wide-ptr.rs:157:5
+  --> $DIR/ub-wide-ptr.rs:155:5
    |
 LL |     mem::transmute::<_, &dyn Trait>((&92u8, 0usize))
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer use: null pointer is a dangling pointer (it has no provenance)
 
 error[E0080]: could not evaluate static initializer
-  --> $DIR/ub-wide-ptr.rs:161:5
+  --> $DIR/ub-wide-ptr.rs:159:5
    |
 LL |     mem::transmute::<_, &dyn Trait>((&92u8, &3u64))
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using allocN as vtable pointer but it does not point to a vtable
diff --git a/src/test/ui/consts/const-eval/ub-wide-ptr.64bit.stderr b/src/test/ui/consts/const-eval/ub-wide-ptr.64bit.stderr
index 501932cb95c..15ef703024a 100644
--- a/src/test/ui/consts/const-eval/ub-wide-ptr.64bit.stderr
+++ b/src/test/ui/consts/const-eval/ub-wide-ptr.64bit.stderr
@@ -278,26 +278,36 @@ LL | const TRAIT_OBJ_CONTENT_INVALID: &dyn Trait = unsafe { mem::transmute::<_,
                ╾──────allocN───────╼ ╾──────allocN───────╼ │ ╾──────╼╾──────╼
            }
 
-error[E0080]: evaluation of constant value failed
-  --> $DIR/ub-wide-ptr.rs:147:62
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/ub-wide-ptr.rs:147:1
    |
 LL | const RAW_TRAIT_OBJ_VTABLE_NULL: *const dyn Trait = unsafe { mem::transmute((&92u8, 0usize)) };
-   |                                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer use: null pointer is a dangling pointer (it has no provenance)
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered null pointer, but expected a vtable pointer
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: 16, align: 8) {
+               ╾──────allocN───────╼ 00 00 00 00 00 00 00 00 │ ╾──────╼........
+           }
 
-error[E0080]: evaluation of constant value failed
-  --> $DIR/ub-wide-ptr.rs:150:65
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/ub-wide-ptr.rs:149:1
    |
 LL | const RAW_TRAIT_OBJ_VTABLE_INVALID: *const dyn Trait = unsafe { mem::transmute((&92u8, &3u64)) };
-   |                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using allocN as vtable pointer but it does not point to a vtable
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered allocN, but expected a vtable pointer
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: 16, align: 8) {
+               ╾──────allocN───────╼ ╾──────allocN───────╼ │ ╾──────╼╾──────╼
+           }
 
 error[E0080]: could not evaluate static initializer
-  --> $DIR/ub-wide-ptr.rs:157:5
+  --> $DIR/ub-wide-ptr.rs:155:5
    |
 LL |     mem::transmute::<_, &dyn Trait>((&92u8, 0usize))
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer use: null pointer is a dangling pointer (it has no provenance)
 
 error[E0080]: could not evaluate static initializer
-  --> $DIR/ub-wide-ptr.rs:161:5
+  --> $DIR/ub-wide-ptr.rs:159:5
    |
 LL |     mem::transmute::<_, &dyn Trait>((&92u8, &3u64))
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using allocN as vtable pointer but it does not point to a vtable
diff --git a/src/test/ui/consts/const-eval/ub-wide-ptr.rs b/src/test/ui/consts/const-eval/ub-wide-ptr.rs
index a0377ab1efd..2da694a8bc4 100644
--- a/src/test/ui/consts/const-eval/ub-wide-ptr.rs
+++ b/src/test/ui/consts/const-eval/ub-wide-ptr.rs
@@ -145,11 +145,9 @@ const TRAIT_OBJ_CONTENT_INVALID: &dyn Trait = unsafe { mem::transmute::<_, &bool
 
 // # raw trait object
 const RAW_TRAIT_OBJ_VTABLE_NULL: *const dyn Trait = unsafe { mem::transmute((&92u8, 0usize)) };
-//~^ ERROR evaluation of constant value failed
-//~| null pointer
+//~^ ERROR it is undefined behavior to use this value
 const RAW_TRAIT_OBJ_VTABLE_INVALID: *const dyn Trait = unsafe { mem::transmute((&92u8, &3u64)) };
-//~^ ERROR evaluation of constant value failed
-//~| does not point to a vtable
+//~^ ERROR it is undefined behavior to use this value
 const RAW_TRAIT_OBJ_CONTENT_INVALID: *const dyn Trait = unsafe { mem::transmute::<_, &bool>(&3u8) } as *const dyn Trait; // ok because raw
 
 // Const eval fails for these, so they need to be statics to error.