Rollup merge of rust-lang#80394 - RalfJung:const-err-future, r=oli-obk

make const_err a future incompat lint

This is the first step for rust-lang#71800: make const_err a future-incompat lint. I also rewrote the const_err lint description as the old one seemed wrong.

This has the unfortunate side-effect of making const-eval error even more verbose by making the const_err message longer without fixing the redundancy caused by additionally emitting an error on each use site of the constant. We cannot fix that redundancy until const_err is a *hard* error (at that point the error-on-use-site can be turned into a `delay_span_bug!` for uses of monomorphic consts, and into a nicely rendered error for [lazily / post-monomorhization evaluated] associated consts).

~~The one annoying effect of this PR is that `let _x = &(1/(1-1));` now also shows the future-incompat warning, even though of course we will *not* make this a hard error. We'll instead (hopefully) stop promoting it -- see rust-lang/rfcs#3027. The only way I see to avoid the future-incompat warning is to use a different lint for "failure to evaluate promoted".~~

Cc `@rust-lang/wg-const-eval`

Author: m-ou-se

compiler/rustc_lint_defs/src/builtin.rs

@@ -255,6 +255,10 @@ declare_lint! {
     pub CONST_ERR,
     Deny,
     "constant evaluation encountered erroneous expression",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #71800 <https://github.com/rust-lang/rust/issues/71800>",
+        edition: None,
+    };
     report_in_external_macro
 }
 

src/test/ui/array-slice-vec/array_const_index-0.rs

@@ -2,6 +2,7 @@ const A: &'static [i32] = &[];
 const B: i32 = (&A)[1];
 //~^ index out of bounds: the length is 0 but the index is 1
 //~| ERROR any use of this value will cause an error
+//~| WARN this was previously accepted by the compiler but is being phased out
 
 fn main() {
     let _ = B;

src/test/ui/array-slice-vec/array_const_index-0.stderr

@@ -7,6 +7,8 @@ LL | const B: i32 = (&A)[1];
    |                index out of bounds: the length is 0 but the index is 1
    |
    = 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
 

src/test/ui/array-slice-vec/array_const_index-1.rs

@@ -2,6 +2,7 @@ const A: [i32; 0] = [];
 const B: i32 = A[1];
 //~^ index out of bounds: the length is 0 but the index is 1
 //~| ERROR any use of this value will cause an error
+//~| WARN this was previously accepted by the compiler but is being phased out
 
 fn main() {
     let _ = B;

src/test/ui/array-slice-vec/array_const_index-1.stderr

@@ -7,6 +7,8 @@ LL | const B: i32 = A[1];
    |                index out of bounds: the length is 0 but the index is 1
    |
    = 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
 

src/test/ui/associated-consts/defaults-not-assumed-fail.rs

@@ -7,6 +7,7 @@ trait Tr {
     // `Self::A` must not be assumed to hold inside the trait.
     const B: u8 = Self::A + 1;
     //~^ ERROR any use of this value will cause an error
+    //~| WARN this was previously accepted by the compiler but is being phased out
 }
 
 // An impl that doesn't override any constant will NOT cause a const eval error
@@ -33,6 +34,7 @@ fn main() {
     assert_eq!(<() as Tr>::B, 0);    // causes the error above
     //~^ ERROR evaluation of constant value failed
     //~| ERROR erroneous constant used
+    //~| WARN this was previously accepted by the compiler but is being phased out
 
     assert_eq!(<u8 as Tr>::A, 254);
     assert_eq!(<u8 as Tr>::B, 255);

src/test/ui/associated-consts/defaults-not-assumed-fail.stderr

@@ -7,19 +7,23 @@ LL |     const B: u8 = Self::A + 1;
    |                   attempt to compute `u8::MAX + 1_u8`, which would overflow
    |
    = 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[E0080]: evaluation of constant value failed
-  --> $DIR/defaults-not-assumed-fail.rs:33:16
+  --> $DIR/defaults-not-assumed-fail.rs:34:16
    |
 LL |     assert_eq!(<() as Tr>::B, 0);    // causes the error above
    |                ^^^^^^^^^^^^^ referenced constant has errors
 
 error: erroneous constant used
-  --> $DIR/defaults-not-assumed-fail.rs:33:5
+  --> $DIR/defaults-not-assumed-fail.rs:34:5
    |
 LL |     assert_eq!(<() as Tr>::B, 0);    // causes the error above
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
    |
+   = 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>
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 3 previous errors

src/test/ui/const-ptr/out_of_bounds_read.stderr

@@ -15,6 +15,8 @@ LL |     const _READ: u32 = unsafe { ptr::read(PAST_END_PTR) };
    |     ------------------------------------------------------
    |
    = 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: any use of this value will cause an error
   --> $SRC_DIR/core/src/intrinsics.rs:LL:COL
@@ -32,6 +34,9 @@ LL |     unsafe { copy_nonoverlapping(src, dst, count) }
    |
 LL |     const _CONST_READ: u32 = unsafe { PAST_END_PTR.read() };
    |     --------------------------------------------------------
+   |
+   = 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: any use of this value will cause an error
   --> $SRC_DIR/core/src/intrinsics.rs:LL:COL
@@ -49,6 +54,9 @@ LL |     unsafe { copy_nonoverlapping(src, dst, count) }
    |
 LL |     const _MUT_READ: u32 = unsafe { (PAST_END_PTR as *mut u32).read() };
    |     --------------------------------------------------------------------
+   |
+   = 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 3 previous errors
 

src/test/ui/consts/assoc_const_generic_impl.rs

@@ -9,6 +9,7 @@ trait ZeroSized: Sized {
 
 impl<T: Sized> ZeroSized for T {
     const I_AM_ZERO_SIZED: ()  = [()][std::mem::size_of::<Self>()]; //~ WARN any use of this value
+    //~| WARN this was previously accepted by the compiler but is being phased out
     fn requires_zero_size(self) {
         let () = Self::I_AM_ZERO_SIZED; //~ ERROR erroneous constant encountered
         println!("requires_zero_size called");

src/test/ui/consts/assoc_const_generic_impl.stderr

@@ -11,9 +11,11 @@ note: the lint level is defined here
    |
 LL | #![warn(const_err)]
    |         ^^^^^^^^^
+   = 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: erroneous constant encountered
-  --> $DIR/assoc_const_generic_impl.rs:13:18
+  --> $DIR/assoc_const_generic_impl.rs:14:18
    |
 LL |         let () = Self::I_AM_ZERO_SIZED;
    |                  ^^^^^^^^^^^^^^^^^^^^^