unify handling of thin and fat pointers by moving primitive type handling out of aggregate handling

Also, make enum variant handling a bit nicer

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

@@ -2,7 +2,7 @@ error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/ub-nonnull.rs:17:1
    |
 LL | const NULL_PTR: NonNull<u8> = unsafe { mem::transmute(0usize) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something in the range 1..=18446744073709551615
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
    |
    = 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
 
@@ -10,7 +10,7 @@ error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/ub-nonnull.rs:20:1
    |
 LL | const NULL_U8: NonZeroU8 = unsafe { mem::transmute(0u8) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something in the range 1..=255
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
    |
    = 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
 
@@ -18,7 +18,7 @@ error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/ub-nonnull.rs:22:1
    |
 LL | const NULL_USIZE: NonZeroUsize = unsafe { mem::transmute(0usize) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something in the range 1..=18446744073709551615
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
    |
    = 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
 

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

@@ -15,6 +15,9 @@ use std::mem;
 const UNALIGNED: &u16 = unsafe { mem::transmute(&[0u8; 4]) };
 //~^ ERROR this constant likely exhibits undefined behavior
 
+const NULL: &u16 = unsafe { mem::transmute(0usize) };
+//~^ ERROR this constant likely exhibits undefined behavior
+
 const REF_AS_USIZE: usize = unsafe { mem::transmute(&0) };
 //~^ ERROR this constant likely exhibits undefined behavior
 

src/test/ui/consts/const-eval/ub-ref.stderr

@@ -9,19 +9,27 @@ LL | const UNALIGNED: &u16 = unsafe { mem::transmute(&[0u8; 4]) };
 error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/ub-ref.rs:18:1
    |
-LL | const REF_AS_USIZE: usize = unsafe { mem::transmute(&0) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer, but expected initialized plain bits
+LL | const NULL: &u16 = unsafe { mem::transmute(0usize) };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
    |
    = 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/ub-ref.rs:21:1
    |
+LL | const REF_AS_USIZE: usize = unsafe { mem::transmute(&0) };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer, 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[E0080]: this constant likely exhibits undefined behavior
+  --> $DIR/ub-ref.rs:24:1
+   |
 LL | const USIZE_AS_REF: &'static u8 = unsafe { mem::transmute(1337usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered integer pointer in non-ZST reference
    |
    = 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 3 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0080`.

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

@@ -13,7 +13,7 @@ LL | / const FIELD_PATH: Struct = Struct { //~ ERROR this constant likely exhibi
 LL | |     a: 42,
 LL | |     b: unsafe { UNION.field3 },
 LL | | };
-   | |__^ type validation failed: encountered uninitialized bytes at .b, but expected something in the range 0..=18446744073709551615
+   | |__^ type validation failed: encountered uninitialized bytes at .b, 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
 

src/test/ui/consts/const-eval/union-ub-fat-ptr.stderr

@@ -2,7 +2,7 @@ error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ub-fat-ptr.rs:87:1
    |
 LL | const B: &str = unsafe { SliceTransmute { repr: SliceRepr { ptr: &42, len: 999 } }.str};
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered dangling reference (not entirely in bounds)
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered dangling (not entirely in bounds) reference
    |
    = 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
 
@@ -26,7 +26,7 @@ error[E0080]: this constant likely exhibits undefined behavior
   --> $DIR/union-ub-fat-ptr.rs:99:1
    |
 LL | const B2: &[u8] = unsafe { SliceTransmute { repr: SliceRepr { ptr: &42, len: 999 } }.slice};
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered dangling reference (not entirely in bounds)
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered dangling (not entirely in bounds) reference
    |
    = 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