miri validation: clarify valid values of 'char'

Author: RalfJung

src/librustc_mir/interpret/validity.rs

@@ -485,7 +485,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 try_validation!(
                     value.to_char(),
                     self.path,
-                    err_ub!(InvalidChar(..)) => { "{}", value } expected { "a valid unicode codepoint" },
+                    err_ub!(InvalidChar(..)) => { "{}", value } expected { "a valid unicode scalar value (in `0..=0x10FFFF` but not in `0xD800..=0xDFFF`)" },
                 );
                 Ok(true)
             }

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

@@ -82,7 +82,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-enum.rs:87:1
    |
 LL | const BAD_OPTION_CHAR: Option<(char, char)> = Some(('x', unsafe { mem::transmute(!0u32) }));
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0xffffffff at .<enum-variant(Some)>.0.1, but expected a valid unicode codepoint
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0xffffffff at .<enum-variant(Some)>.0.1, but expected a valid unicode scalar value (in `0..=0x10FFFF` but not in `0xD800..=0xDFFF`)
    |
    = 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.