Auto merge of rust-lang#86194 - RalfJung:const-ub-hard-error, r=oli-obk

make UB during CTFE a hard error

This is a next step for rust-lang#71800. `const_err` has been a future-incompatibility lint for 4 months now since rust-lang#80394 (and err-by-default for many years before that), so I think we could try making it a proper hard error at least in some situations.

I didn't yet adjust the tests, since I first want to gauge the fall-out via crater.
Cc `@rust-lang/wg-const-eval`

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -525,6 +525,7 @@ impl InterpError<'_> {
         use InterpError::*;
         match *self {
             MachineStop(ref err) => err.is_hard_err(),
+            InterpError::UndefinedBehavior(_) => true,
             _ => false,
         }
     }

src/test/ui/consts/const-eval/dangling.rs

@@ -5,9 +5,8 @@ use std::mem;
 // Make sure we error with the right kind of error on a too large slice.
 const TEST: () = { unsafe {
     let slice: *const [u8] = mem::transmute((1usize, usize::MAX));
-    let _val = &*slice; //~ ERROR: any use of this value will cause an error
+    let _val = &*slice; //~ ERROR: evaluation of constant value failed
     //~| slice is bigger than largest supported object
-    //~| WARN this was previously accepted by the compiler but is being phased out
 } };
 
 fn main() {}

src/test/ui/consts/const-eval/dangling.stderr

@@ -1,18 +1,9 @@
-error: any use of this value will cause an error
+error[E0080]: evaluation of constant value failed
   --> $DIR/dangling.rs:8:16
    |
-LL | / const TEST: () = { unsafe {
-LL | |     let slice: *const [u8] = mem::transmute((1usize, usize::MAX));
-LL | |     let _val = &*slice;
-   | |                ^^^^^^^ invalid metadata in wide pointer: slice is bigger than largest supported object
-LL | |
-LL | |
-LL | | } };
-   | |____-
-   |
-   = note: `#[deny(const_err)]` on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+LL |     let _val = &*slice;
+   |                ^^^^^^^ invalid metadata in wide pointer: slice is bigger than largest supported object
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/const-eval/heap/alloc_intrinsic_errors.rs

@@ -8,8 +8,7 @@ const FOO: i32 = foo();
 const fn foo() -> i32 {
     unsafe {
         let _ = intrinsics::const_allocate(4, 3) as * mut i32;
-        //~^ error: any use of this value will cause an error [const_err]
-        //~| WARN this was previously accepted by the compiler but is being phased out
+        //~^ error: evaluation of constant value failed
     }
     1
 

src/test/ui/consts/const-eval/heap/alloc_intrinsic_errors.stderr

@@ -1,19 +1,15 @@
-error: any use of this value will cause an error
+error[E0080]: evaluation of constant value failed
   --> $DIR/alloc_intrinsic_errors.rs:10:17
    |
 LL | const FOO: i32 = foo();
-   | -----------------------
+   |                  ----- inside `FOO` at $DIR/alloc_intrinsic_errors.rs:7:18
 ...
 LL |         let _ = intrinsics::const_allocate(4, 3) as * mut i32;
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                 |
    |                 align has to be a power of 2, `3` is not a power of 2
    |                 inside `foo` at $DIR/alloc_intrinsic_errors.rs:10:17
-   |                 inside `FOO` at $DIR/alloc_intrinsic_errors.rs:7:18
-   |
-   = note: `#[deny(const_err)]` on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/const-eval/issue-49296.rs

@@ -17,8 +17,7 @@ const fn wat(x: u64) -> &'static u64 {
     unsafe { transmute(&x) }
 }
 const X: u64 = *wat(42);
-//~^ ERROR any use of this value will cause an error
-//~| WARN this was previously accepted by the compiler but is being phased out
+//~^ ERROR evaluation of constant value failed
 
 fn main() {
     println!("{}", X);

src/test/ui/consts/const-eval/issue-49296.stderr

@@ -1,14 +1,9 @@
-error: any use of this value will cause an error
+error[E0080]: evaluation of constant value failed
   --> $DIR/issue-49296.rs:19:16
    |
 LL | const X: u64 = *wat(42);
-   | ---------------^^^^^^^^-
-   |                |
-   |                pointer to alloc1 was dereferenced after this allocation got freed
-   |
-   = note: `#[deny(const_err)]` on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+   |                ^^^^^^^^ pointer to alloc1 was dereferenced after this allocation got freed
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/const-eval/ub-incorrect-vtable.32bit.stderr

@@ -1,30 +1,17 @@
-error: any use of this value will cause an error
+error[E0080]: evaluation of constant value failed
   --> $DIR/ub-incorrect-vtable.rs:19:14
    |
-LL | / const INVALID_VTABLE_ALIGNMENT: &dyn Trait =
-LL | |     unsafe { std::mem::transmute((&92u8, &[0usize, 1usize, 1000usize])) };
-   | |______________^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^__-
-   |                |
-   |                invalid vtable: alignment `1000` is not a power of 2
-   |
-   = note: `#[deny(const_err)]` on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+LL |     unsafe { std::mem::transmute((&92u8, &[0usize, 1usize, 1000usize])) };
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid vtable: alignment `1000` is not a power of 2
 
-error: any use of this value will cause an error
-  --> $DIR/ub-incorrect-vtable.rs:25:14
-   |
-LL | / const INVALID_VTABLE_SIZE: &dyn Trait =
-LL | |     unsafe { std::mem::transmute((&92u8, &[1usize, usize::MAX, 1usize])) };
-   | |______________^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^__-
-   |                |
-   |                invalid vtable: size is bigger than largest supported object
+error[E0080]: evaluation of constant value failed
+  --> $DIR/ub-incorrect-vtable.rs:24:14
    |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+LL |     unsafe { std::mem::transmute((&92u8, &[1usize, usize::MAX, 1usize])) };
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid vtable: size is bigger than largest supported object
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-incorrect-vtable.rs:36:1
+  --> $DIR/ub-incorrect-vtable.rs:34:1
    |
 LL | / const INVALID_VTABLE_ALIGNMENT_UB: W<&dyn Trait> =
 LL | |     unsafe { std::mem::transmute((&92u8, &(drop_me as fn(*mut usize), 1usize, 1000usize))) };
@@ -36,7 +23,7 @@ LL | |     unsafe { std::mem::transmute((&92u8, &(drop_me as fn(*mut usize), 1us
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-incorrect-vtable.rs:41:1
+  --> $DIR/ub-incorrect-vtable.rs:39:1
    |
 LL | / const INVALID_VTABLE_SIZE_UB: W<&dyn Trait> =
 LL | |     unsafe { std::mem::transmute((&92u8, &(drop_me as fn(*mut usize), usize::MAX, 1usize))) };

src/test/ui/consts/const-eval/ub-incorrect-vtable.64bit.stderr

@@ -1,30 +1,17 @@
-error: any use of this value will cause an error
+error[E0080]: evaluation of constant value failed
   --> $DIR/ub-incorrect-vtable.rs:19:14
    |
-LL | / const INVALID_VTABLE_ALIGNMENT: &dyn Trait =
-LL | |     unsafe { std::mem::transmute((&92u8, &[0usize, 1usize, 1000usize])) };
-   | |______________^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^__-
-   |                |
-   |                invalid vtable: alignment `1000` is not a power of 2
-   |
-   = note: `#[deny(const_err)]` on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+LL |     unsafe { std::mem::transmute((&92u8, &[0usize, 1usize, 1000usize])) };
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid vtable: alignment `1000` is not a power of 2
 
-error: any use of this value will cause an error
-  --> $DIR/ub-incorrect-vtable.rs:25:14
-   |
-LL | / const INVALID_VTABLE_SIZE: &dyn Trait =
-LL | |     unsafe { std::mem::transmute((&92u8, &[1usize, usize::MAX, 1usize])) };
-   | |______________^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^__-
-   |                |
-   |                invalid vtable: size is bigger than largest supported object
+error[E0080]: evaluation of constant value failed
+  --> $DIR/ub-incorrect-vtable.rs:24:14
    |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+LL |     unsafe { std::mem::transmute((&92u8, &[1usize, usize::MAX, 1usize])) };
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid vtable: size is bigger than largest supported object
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-incorrect-vtable.rs:36:1
+  --> $DIR/ub-incorrect-vtable.rs:34:1
    |
 LL | / const INVALID_VTABLE_ALIGNMENT_UB: W<&dyn Trait> =
 LL | |     unsafe { std::mem::transmute((&92u8, &(drop_me as fn(*mut usize), 1usize, 1000usize))) };
@@ -36,7 +23,7 @@ LL | |     unsafe { std::mem::transmute((&92u8, &(drop_me as fn(*mut usize), 1us
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-incorrect-vtable.rs:41:1
+  --> $DIR/ub-incorrect-vtable.rs:39:1
    |
 LL | / const INVALID_VTABLE_SIZE_UB: W<&dyn Trait> =
 LL | |     unsafe { std::mem::transmute((&92u8, &(drop_me as fn(*mut usize), usize::MAX, 1usize))) };

src/test/ui/consts/const-eval/ub-incorrect-vtable.rs

@@ -17,14 +17,12 @@ trait Trait {}
 
 const INVALID_VTABLE_ALIGNMENT: &dyn Trait =
     unsafe { std::mem::transmute((&92u8, &[0usize, 1usize, 1000usize])) };
-//~^ ERROR any use of this value will cause an error
-//~| WARNING this was previously accepted by the compiler
+//~^ ERROR evaluation of constant value failed
 //~| invalid vtable: alignment `1000` is not a power of 2
 
 const INVALID_VTABLE_SIZE: &dyn Trait =
     unsafe { std::mem::transmute((&92u8, &[1usize, usize::MAX, 1usize])) };
-//~^ ERROR any use of this value will cause an error
-//~| WARNING this was previously accepted by the compiler
+//~^ ERROR evaluation of constant value failed
 //~| invalid vtable: size is bigger than largest supported object
 
 #[repr(transparent)]

src/test/ui/consts/const-eval/ub-nonnull.32bit.stderr

@@ -9,29 +9,14 @@ LL | const NULL_PTR: NonNull<u8> = unsafe { mem::transmute(0usize) };
                00 00 00 00                                     │ ....
            }
 
-error: any use of this value will cause an error
+error[E0080]: evaluation of constant value failed
   --> $DIR/ub-nonnull.rs:19:30
    |
-LL | / const OUT_OF_BOUNDS_PTR: NonNull<u8> = { unsafe {
-LL | |     let ptr: &[u8; 256] = mem::transmute(&0u8); // &0 gets promoted so it does not dangle
-LL | |     // Use address-of-element for pointer arithmetic. This could wrap around to null!
-LL | |     let out_of_bounds_ptr = &ptr[255];
-   | |                              ^^^^^^^^ memory access failed: pointer must be in-bounds at offset 256, but is outside bounds of alloc10 which has size 1
-LL | |
-LL | |     mem::transmute(out_of_bounds_ptr)
-LL | | } };
-   | |____-
-   |
-note: the lint level is defined here
-  --> $DIR/ub-nonnull.rs:15:8
-   |
-LL | #[deny(const_err)] // this triggers a `const_err` so validation does not even happen
-   |        ^^^^^^^^^
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+LL |     let out_of_bounds_ptr = &ptr[255];
+   |                              ^^^^^^^^ memory access failed: pointer must be in-bounds at offset 256, but is outside bounds of alloc10 which has size 1
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:24:1
+  --> $DIR/ub-nonnull.rs:23:1
    |
 LL | const NULL_U8: NonZeroU8 = unsafe { mem::transmute(0u8) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
@@ -42,7 +27,7 @@ LL | const NULL_U8: NonZeroU8 = unsafe { mem::transmute(0u8) };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:26:1
+  --> $DIR/ub-nonnull.rs:25:1
    |
 LL | const NULL_USIZE: NonZeroUsize = unsafe { mem::transmute(0usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
@@ -53,7 +38,7 @@ LL | const NULL_USIZE: NonZeroUsize = unsafe { mem::transmute(0usize) };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:34:1
+  --> $DIR/ub-nonnull.rs:33:1
    |
 LL | const UNINIT: NonZeroU8 = unsafe { MaybeUninit { uninit: () }.init };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .0: encountered uninitialized bytes, but expected initialized plain (non-pointer) bytes
@@ -64,7 +49,7 @@ LL | const UNINIT: NonZeroU8 = unsafe { MaybeUninit { uninit: () }.init };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:42:1
+  --> $DIR/ub-nonnull.rs:41:1
    |
 LL | const BAD_RANGE1: RestrictedRange1 = unsafe { RestrictedRange1(42) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 42, but expected something in the range 10..=30
@@ -75,7 +60,7 @@ LL | const BAD_RANGE1: RestrictedRange1 = unsafe { RestrictedRange1(42) };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:48:1
+  --> $DIR/ub-nonnull.rs:47:1
    |
 LL | const BAD_RANGE2: RestrictedRange2 = unsafe { RestrictedRange2(20) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 20, but expected something less or equal to 10, or greater or equal to 30

src/test/ui/consts/const-eval/ub-nonnull.64bit.stderr

@@ -9,29 +9,14 @@ LL | const NULL_PTR: NonNull<u8> = unsafe { mem::transmute(0usize) };
                00 00 00 00 00 00 00 00                         │ ........
            }
 
-error: any use of this value will cause an error
+error[E0080]: evaluation of constant value failed
   --> $DIR/ub-nonnull.rs:19:30
    |
-LL | / const OUT_OF_BOUNDS_PTR: NonNull<u8> = { unsafe {
-LL | |     let ptr: &[u8; 256] = mem::transmute(&0u8); // &0 gets promoted so it does not dangle
-LL | |     // Use address-of-element for pointer arithmetic. This could wrap around to null!
-LL | |     let out_of_bounds_ptr = &ptr[255];
-   | |                              ^^^^^^^^ memory access failed: pointer must be in-bounds at offset 256, but is outside bounds of alloc10 which has size 1
-LL | |
-LL | |     mem::transmute(out_of_bounds_ptr)
-LL | | } };
-   | |____-
-   |
-note: the lint level is defined here
-  --> $DIR/ub-nonnull.rs:15:8
-   |
-LL | #[deny(const_err)] // this triggers a `const_err` so validation does not even happen
-   |        ^^^^^^^^^
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+LL |     let out_of_bounds_ptr = &ptr[255];
+   |                              ^^^^^^^^ memory access failed: pointer must be in-bounds at offset 256, but is outside bounds of alloc10 which has size 1
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:24:1
+  --> $DIR/ub-nonnull.rs:23:1
    |
 LL | const NULL_U8: NonZeroU8 = unsafe { mem::transmute(0u8) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
@@ -42,7 +27,7 @@ LL | const NULL_U8: NonZeroU8 = unsafe { mem::transmute(0u8) };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:26:1
+  --> $DIR/ub-nonnull.rs:25:1
    |
 LL | const NULL_USIZE: NonZeroUsize = unsafe { mem::transmute(0usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
@@ -53,7 +38,7 @@ LL | const NULL_USIZE: NonZeroUsize = unsafe { mem::transmute(0usize) };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:34:1
+  --> $DIR/ub-nonnull.rs:33:1
    |
 LL | const UNINIT: NonZeroU8 = unsafe { MaybeUninit { uninit: () }.init };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .0: encountered uninitialized bytes, but expected initialized plain (non-pointer) bytes
@@ -64,7 +49,7 @@ LL | const UNINIT: NonZeroU8 = unsafe { MaybeUninit { uninit: () }.init };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:42:1
+  --> $DIR/ub-nonnull.rs:41:1
    |
 LL | const BAD_RANGE1: RestrictedRange1 = unsafe { RestrictedRange1(42) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 42, but expected something in the range 10..=30
@@ -75,7 +60,7 @@ LL | const BAD_RANGE1: RestrictedRange1 = unsafe { RestrictedRange1(42) };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:48:1
+  --> $DIR/ub-nonnull.rs:47:1
    |
 LL | const BAD_RANGE2: RestrictedRange2 = unsafe { RestrictedRange2(20) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 20, but expected something less or equal to 10, or greater or equal to 30

src/test/ui/consts/const-eval/ub-nonnull.rs

@@ -16,8 +16,7 @@ const NULL_PTR: NonNull<u8> = unsafe { mem::transmute(0usize) };
 const OUT_OF_BOUNDS_PTR: NonNull<u8> = { unsafe {
     let ptr: &[u8; 256] = mem::transmute(&0u8); // &0 gets promoted so it does not dangle
     // Use address-of-element for pointer arithmetic. This could wrap around to null!
-    let out_of_bounds_ptr = &ptr[255]; //~ ERROR any use of this value will cause an error
-    //~| WARN this was previously accepted by the compiler but is being phased out
+    let out_of_bounds_ptr = &ptr[255]; //~ ERROR evaluation of constant value failed
     mem::transmute(out_of_bounds_ptr)
 } };