Use hexponent to implement hex float parsing in wcstod

This teaches wcstod to parse hex floats like 0x1.5p3 via a forked
version of hexponent. This support is necessary for printf.

fish-rust/Cargo.lock

@@ -374,6 +374,7 @@ dependencies = [
  "cxx-gen",
  "errno",
  "fast-float",
+ "hexponent",
  "inventory",
  "libc",
  "lru",
@@ -462,6 +463,11 @@ version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fed44880c466736ef9a5c5b5facefb5ed0785676d0c02d612db14e54f0d84286"
 
+[[package]]
+name = "hexponent"
+version = "0.3.1"
+source = "git+https://github.com/fish-shell/hexponent?branch=fish#d2b97417d34adc9ea3ec954c69accc59828cbdb4"
+
 [[package]]
 name = "humantime"
 version = "2.1.0"

fish-rust/Cargo.toml

@@ -8,6 +8,7 @@ rust-version = "1.67"
 widestring-suffix = { path = "./widestring-suffix/" }
 pcre2 = { git = "https://github.com/fish-shell/rust-pcre2", branch = "master", default-features = false, features = ["utf32"] }
 fast-float = { git = "https://github.com/fish-shell/fast-float-rust", branch="fish" }
+hexponent = { git = "https://github.com/fish-shell/hexponent", branch="fish" }
 printf-compat = { git = "https://github.com/fish-shell/printf-compat.git", branch="fish" }
 
 autocxx = "0.23.1"

fish-rust/src/wutil/wcstod.rs

@@ -39,6 +39,16 @@ where
         }
     }
 
+    // If it's a hex float, use hexponent.
+    if is_hex_float(chars.clone()) {
+        let mut n = 0;
+        let res = hexponent::parse_hex_float(chars, decimal_sep, &mut n);
+        if res.is_ok() {
+            *consumed = whitespace_skipped + n;
+        }
+        return res.map_err(hexponent_error);
+    }
+
     let ret = parse_partial_iter(chars.clone().fuse(), decimal_sep);
     if ret.is_err() {
         *consumed = 0;
@@ -63,6 +73,39 @@ where
     Ok(val)
 }
 
+/// Check if a character iterator appears to be a hex float.
+/// That is, an optional + or -, followed by 0x or 0X, and a hex digit.
+pub fn is_hex_float<Chars: Iterator<Item = char>>(mut chars: Chars) -> bool {
+    match chars.next() {
+        Some('+' | '-') => {
+            if chars.next() != Some('0') {
+                return false;
+            }
+        }
+        Some('0') => (),
+        _ => return false,
+    };
+    match chars.next() {
+        Some('x') | Some('X') => (),
+        _ => return false,
+    };
+    match chars.next() {
+        Some(c) => c.is_ascii_hexdigit(),
+        None => false,
+    }
+}
+
+// Convert a a hexponent error to our error type.
+fn hexponent_error(e: hexponent::ParseError) -> Error {
+    use hexponent::ParseErrorKind;
+    match e.kind {
+        ParseErrorKind::MissingPrefix
+        | ParseErrorKind::MissingDigits
+        | ParseErrorKind::MissingExponent => Error::InvalidChar,
+        ParseErrorKind::ExponentOverflow => Error::Overflow,
+    }
+}
+
 #[cfg(test)]
 mod test {
     #![allow(overflowing_literals)]
@@ -101,6 +144,9 @@ mod test {
         test_consumed("0.y", Ok(0.0), 2);
         test_consumed(".0y", Ok(0.0), 2);
         test_consumed("000,,,e1", Ok(0.0), 3);
+        test_consumed("0x1", Ok(1.0), 3);
+        test_consumed("0X1p2", Ok(4.0), 5);
+        test_consumed("0X1P3", Ok(8.0), 5);
         test("000e1", Ok(0.0));
         test_consumed("000,1e1", Ok(0.0), 3);
         test("0", Ok(0.0));