refactor: use macro for special key char def

Reduce verbosity of const definitions. Define a dedicated const for the
base of the special key encoding range. This range is 256 bytes wide, so
by defining consts via an u8 offset from the base, we can guarantee that
the consts fall into the allocated range. Ideally, we would also check
for collisions, but Rust's const capabilities don't allow for that as
far as I'm aware.

Having `SPECIAL_KEY_ENCODE_BASE` in the `rust-widestring` crate allows
getting rid of the dependency on `key::Backspace` in the
`fish_reserved_codepoint` function, which unblocks code extraction.

Part of #12625

src/key.rs

@@ -7,31 +7,50 @@
 use fish_common::{EscapeFlags, EscapeStringStyle};
 use fish_fallback::fish_wcwidth;
 use fish_feature_flags::{FeatureFlag, feature_test};
-use fish_widestring::{L, WExt as _, WString, decode_byte_from_char, wstr};
+use fish_widestring::{
+    L, SPECIAL_KEY_ENCODE_BASE, WExt as _, WString, char_offset, decode_byte_from_char, wstr,
+};
 
-pub(crate) const Backspace: char = '\u{F500}'; // below ENCODE_DIRECT_BASE
-pub(crate) const Delete: char = '\u{F501}';
-pub(crate) const Escape: char = '\u{F502}';
-pub(crate) const Enter: char = '\u{F503}';
-pub(crate) const Up: char = '\u{F504}';
-pub(crate) const Down: char = '\u{F505}';
-pub(crate) const Left: char = '\u{F506}';
-pub(crate) const Right: char = '\u{F507}';
-pub(crate) const PageUp: char = '\u{F508}';
-pub(crate) const PageDown: char = '\u{F509}';
-pub(crate) const Home: char = '\u{F50A}';
-pub(crate) const End: char = '\u{F50B}';
-pub(crate) const Insert: char = '\u{F50C}';
-pub(crate) const Tab: char = '\u{F50D}';
-pub(crate) const Space: char = '\u{F50E}';
-pub(crate) const Menu: char = '\u{F50F}';
-pub(crate) const PrintScreen: char = '\u{F510}';
-pub(crate) const MAX_FUNCTION_KEY: u32 = 12;
-pub(crate) fn function_key(n: u32) -> char {
-    assert!((1..=MAX_FUNCTION_KEY).contains(&n));
-    char::from_u32(u32::from('\u{F5FF}') - MAX_FUNCTION_KEY + (n - 1)).unwrap()
+const fn special_key_char(offset: u8) -> char {
+    // TODO: use `u32::from(offset)` once that's available in const fn
+    char_offset(SPECIAL_KEY_ENCODE_BASE, offset as u32)
+}
+
+macro_rules! define_special_keys {
+    ($($key_name:ident: $offset:expr)*) => {
+        $(
+            pub(crate) const $key_name: char = special_key_char($offset);
+        )*
+    };
+}
+
+define_special_keys! {
+    Backspace: 0
+    Delete: 1
+    Escape: 2
+    Enter: 3
+    Up: 4
+    Down: 5
+    Left: 6
+    Right: 7
+    PageUp: 8
+    PageDown: 9
+    Home: 10
+    End: 11
+    Insert: 12
+    Tab: 13
+    Space: 14
+    Menu: 15
+    PrintScreen: 16
+
+    Invalid: 255
+}
+
+pub(crate) const MAX_FUNCTION_KEY: u8 = 12;
+pub(crate) fn function_key(n: u8) -> char {
+    assert!((1..=MAX_FUNCTION_KEY).contains(&n));
+    special_key_char(254 - MAX_FUNCTION_KEY + n)
 }
-pub(crate) const Invalid: char = '\u{F5FF}';
 
 pub(crate) const KEY_NAMES: &[(char, &wstr)] = &[
     ('-', L!("minus")),
@@ -297,7 +316,7 @@ pub(crate) fn parse_keys(value: &wstr) -> Result<Vec<Key>, WString> {
                 let num = key_name.strip_prefix('f').unwrap();
                 let codepoint = match fish_wcstoul(num) {
                     Ok(n) if (1..=u64::from(MAX_FUNCTION_KEY)).contains(&n) => {
-                        function_key(u32::try_from(n).unwrap())
+                        function_key(u8::try_from(n).unwrap())
                     }
                     _ => {
                         return Err(wgettext_fmt!(