Get rid of the Scalar and ScalarPair variants of ConstValue...

src/test/ui/consts/const-err4.rs

@@ -16,7 +16,7 @@ union Foo {
 
 enum Bar {
     Boo = [unsafe { Foo { b: () }.a }; 4][3],
-    //~^ ERROR could not evaluate enum discriminant
+    //~^ ERROR constant evaluation of enum discriminant resulted in non-integer
 }
 
 fn main() {

src/test/ui/consts/const-err4.stderr

@@ -1,8 +1,8 @@
-error[E0080]: could not evaluate enum discriminant
+error[E0080]: constant evaluation of enum discriminant resulted in non-integer
   --> $DIR/const-err4.rs:18:11
    |
 LL |     Boo = [unsafe { Foo { b: () }.a }; 4][3],
-   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/consts/const-eval/const-pointer-values-in-various-types.rs

@@ -37,7 +37,7 @@ fn main() {
     //~^ ERROR this constant likely exhibits undefined behavior
 
     const I32_REF_U128_UNION: u128 = unsafe { Nonsense { int_32_ref: &3 }.uint_128 };
-    //~^ ERROR this constant cannot be used
+    //~^ ERROR this constant likely exhibits undefined behavior
 
     const I32_REF_I8_UNION: i8 = unsafe { Nonsense { int_32_ref: &3 }.int_8 };
     //~^ ERROR this constant cannot be used
@@ -52,7 +52,7 @@ fn main() {
     //~^ ERROR this constant likely exhibits undefined behavior
 
     const I32_REF_I128_UNION: i128 = unsafe { Nonsense { int_32_ref: &3 }.int_128 };
-    //~^ ERROR this constant cannot be used
+    //~^ ERROR this constant likely exhibits undefined behavior
 
     const I32_REF_F32_UNION: f32 = unsafe { Nonsense { int_32_ref: &3 }.float_32 };
     //~^ ERROR this constant cannot be used

src/test/ui/consts/const-eval/const-pointer-values-in-various-types.stderr

@@ -40,11 +40,13 @@ LL |     const I32_REF_U64_UNION: u64 = unsafe { Nonsense { int_32_ref: &3 }.uin
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
-error: this constant cannot be used
+error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/const-pointer-values-in-various-types.rs:39:5
    |
 LL |     const I32_REF_U128_UNION: u128 = unsafe { Nonsense { int_32_ref: &3 }.uint_128 };
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized bytes, but expected initialized plain bits
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error: this constant cannot be used
   --> $DIR/const-pointer-values-in-various-types.rs:42:5
@@ -78,11 +80,13 @@ LL |     const I32_REF_I64_UNION: i64 = unsafe { Nonsense { int_32_ref: &3 }.int
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
-error: this constant cannot be used
+error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/const-pointer-values-in-various-types.rs:54:5
    |
 LL |     const I32_REF_I128_UNION: i128 = unsafe { Nonsense { int_32_ref: &3 }.int_128 };
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized bytes, but expected initialized plain bits
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error: this constant cannot be used
   --> $DIR/const-pointer-values-in-various-types.rs:57:5

src/test/ui/consts/const-eval/union-const-eval-field.rs

@@ -34,7 +34,7 @@ const fn read_field2() -> Field2 {
 }
 
 const fn read_field3() -> Field3 {
-    const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR cannot be used
+    const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR likely exhibits undefined behavior
     FIELD3
 }
 

src/test/ui/consts/const-eval/union-const-eval-field.stderr

@@ -1,10 +1,11 @@
-error: this constant cannot be used
+error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-const-eval-field.rs:37:5
    |
-LL |     const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR cannot be used
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
+LL |     const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR likely exhibits undefined behavior
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized bytes, but expected initialized plain bits
    |
-   = note: #[deny(const_err)] on by default
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/const-eval/union-ice.rs

@@ -20,7 +20,7 @@ union DummyUnion {
 
 const UNION: DummyUnion = DummyUnion { field1: 1065353216 };
 
-const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR this constant cannot be used
+const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR likely exhibits undefined behavior
 
 const FIELD_PATH: Struct = Struct { //~ ERROR this constant likely exhibits undefined behavior
     a: 42,

src/test/ui/consts/const-eval/union-ice.stderr

@@ -1,10 +1,10 @@
-error: this constant cannot be used
+error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ice.rs:23:1
    |
-LL | const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR this constant cannot be used
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
+LL | const FIELD3: Field3 = unsafe { UNION.field3 }; //~ ERROR likely exhibits undefined behavior
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized bytes, but expected initialized plain bits
    |
-   = note: #[deny(const_err)] on by default
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ice.rs:25:1