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diff --git a/third_party/rust/serde_json/src/lexical/bhcomp.rs b/third_party/rust/serde_json/src/lexical/bhcomp.rs
new file mode 100644
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+++ b/third_party/rust/serde_json/src/lexical/bhcomp.rs
@@ -0,0 +1,218 @@
+// Adapted from https://github.com/Alexhuszagh/rust-lexical.
+
+//! Compare the mantissa to the halfway representation of the float.
+//!
+//! Compares the actual significant digits of the mantissa to the
+//! theoretical digits from `b+h`, scaled into the proper range.
+
+use super::bignum::*;
+use super::digit::*;
+use super::exponent::*;
+use super::float::*;
+use super::math::*;
+use super::num::*;
+use super::rounding::*;
+use core::{cmp, mem};
+
+// MANTISSA
+
+/// Parse the full mantissa into a big integer.
+///
+/// Max digits is the maximum number of digits plus one.
+fn parse_mantissa<F>(integer: &[u8], fraction: &[u8]) -> Bigint
+where
+    F: Float,
+{
+    // Main loop
+    let small_powers = POW10_LIMB;
+    let step = small_powers.len() - 2;
+    let max_digits = F::MAX_DIGITS - 1;
+    let mut counter = 0;
+    let mut value: Limb = 0;
+    let mut i: usize = 0;
+    let mut result = Bigint::default();
+
+    // Iteratively process all the data in the mantissa.
+    for &digit in integer.iter().chain(fraction) {
+        // We've parsed the max digits using small values, add to bignum
+        if counter == step {
+            result.imul_small(small_powers[counter]);
+            result.iadd_small(value);
+            counter = 0;
+            value = 0;
+        }
+
+        value *= 10;
+        value += as_limb(to_digit(digit).unwrap());
+
+        i += 1;
+        counter += 1;
+        if i == max_digits {
+            break;
+        }
+    }
+
+    // We will always have a remainder, as long as we entered the loop
+    // once, or counter % step is 0.
+    if counter != 0 {
+        result.imul_small(small_powers[counter]);
+        result.iadd_small(value);
+    }
+
+    // If we have any remaining digits after the last value, we need
+    // to add a 1 after the rest of the array, it doesn't matter where,
+    // just move it up. This is good for the worst-possible float
+    // representation. We also need to return an index.
+    // Since we already trimmed trailing zeros, we know there has
+    // to be a non-zero digit if there are any left.
+    if i < integer.len() + fraction.len() {
+        result.imul_small(10);
+        result.iadd_small(1);
+    }
+
+    result
+}
+
+// FLOAT OPS
+
+/// Calculate `b` from a a representation of `b` as a float.
+#[inline]
+pub(super) fn b_extended<F: Float>(f: F) -> ExtendedFloat {
+    ExtendedFloat::from_float(f)
+}
+
+/// Calculate `b+h` from a a representation of `b` as a float.
+#[inline]
+pub(super) fn bh_extended<F: Float>(f: F) -> ExtendedFloat {
+    // None of these can overflow.
+    let b = b_extended(f);
+    ExtendedFloat {
+        mant: (b.mant << 1) + 1,
+        exp: b.exp - 1,
+    }
+}
+
+// ROUNDING
+
+/// Custom round-nearest, tie-event algorithm for bhcomp.
+#[inline]
+fn round_nearest_tie_even(fp: &mut ExtendedFloat, shift: i32, is_truncated: bool) {
+    let (mut is_above, mut is_halfway) = round_nearest(fp, shift);
+    if is_halfway && is_truncated {
+        is_above = true;
+        is_halfway = false;
+    }
+    tie_even(fp, is_above, is_halfway);
+}
+
+// BHCOMP
+
+/// Calculate the mantissa for a big integer with a positive exponent.
+fn large_atof<F>(mantissa: Bigint, exponent: i32) -> F
+where
+    F: Float,
+{
+    let bits = mem::size_of::<u64>() * 8;
+
+    // Simple, we just need to multiply by the power of the radix.
+    // Now, we can calculate the mantissa and the exponent from this.
+    // The binary exponent is the binary exponent for the mantissa
+    // shifted to the hidden bit.
+    let mut bigmant = mantissa;
+    bigmant.imul_pow10(exponent as u32);
+
+    // Get the exact representation of the float from the big integer.
+    let (mant, is_truncated) = bigmant.hi64();
+    let exp = bigmant.bit_length() as i32 - bits as i32;
+    let mut fp = ExtendedFloat { mant, exp };
+    fp.round_to_native::<F, _>(|fp, shift| round_nearest_tie_even(fp, shift, is_truncated));
+    into_float(fp)
+}
+
+/// Calculate the mantissa for a big integer with a negative exponent.
+///
+/// This invokes the comparison with `b+h`.
+fn small_atof<F>(mantissa: Bigint, exponent: i32, f: F) -> F
+where
+    F: Float,
+{
+    // Get the significant digits and radix exponent for the real digits.
+    let mut real_digits = mantissa;
+    let real_exp = exponent;
+    debug_assert!(real_exp < 0);
+
+    // Get the significant digits and the binary exponent for `b+h`.
+    let theor = bh_extended(f);
+    let mut theor_digits = Bigint::from_u64(theor.mant);
+    let theor_exp = theor.exp;
+
+    // We need to scale the real digits and `b+h` digits to be the same
+    // order. We currently have `real_exp`, in `radix`, that needs to be
+    // shifted to `theor_digits` (since it is negative), and `theor_exp`
+    // to either `theor_digits` or `real_digits` as a power of 2 (since it
+    // may be positive or negative). Try to remove as many powers of 2
+    // as possible. All values are relative to `theor_digits`, that is,
+    // reflect the power you need to multiply `theor_digits` by.
+
+    // Can remove a power-of-two, since the radix is 10.
+    // Both are on opposite-sides of equation, can factor out a
+    // power of two.
+    //
+    // Example: 10^-10, 2^-10   -> ( 0, 10, 0)
+    // Example: 10^-10, 2^-15   -> (-5, 10, 0)
+    // Example: 10^-10, 2^-5    -> ( 5, 10, 0)
+    // Example: 10^-10, 2^5 -> (15, 10, 0)
+    let binary_exp = theor_exp - real_exp;
+    let halfradix_exp = -real_exp;
+    let radix_exp = 0;
+
+    // Carry out our multiplication.
+    if halfradix_exp != 0 {
+        theor_digits.imul_pow5(halfradix_exp as u32);
+    }
+    if radix_exp != 0 {
+        theor_digits.imul_pow10(radix_exp as u32);
+    }
+    if binary_exp > 0 {
+        theor_digits.imul_pow2(binary_exp as u32);
+    } else if binary_exp < 0 {
+        real_digits.imul_pow2(-binary_exp as u32);
+    }
+
+    // Compare real digits to theoretical digits and round the float.
+    match real_digits.compare(&theor_digits) {
+        cmp::Ordering::Greater => f.next_positive(),
+        cmp::Ordering::Less => f,
+        cmp::Ordering::Equal => f.round_positive_even(),
+    }
+}
+
+/// Calculate the exact value of the float.
+///
+/// Note: fraction must not have trailing zeros.
+pub(crate) fn bhcomp<F>(b: F, integer: &[u8], mut fraction: &[u8], exponent: i32) -> F
+where
+    F: Float,
+{
+    // Calculate the number of integer digits and use that to determine
+    // where the significant digits start in the fraction.
+    let integer_digits = integer.len();
+    let fraction_digits = fraction.len();
+    let digits_start = if integer_digits == 0 {
+        let start = fraction.iter().take_while(|&x| *x == b'0').count();
+        fraction = &fraction[start..];
+        start
+    } else {
+        0
+    };
+    let sci_exp = scientific_exponent(exponent, integer_digits, digits_start);
+    let count = F::MAX_DIGITS.min(integer_digits + fraction_digits - digits_start);
+    let scaled_exponent = sci_exp + 1 - count as i32;
+
+    let mantissa = parse_mantissa::<F>(integer, fraction);
+    if scaled_exponent >= 0 {
+        large_atof(mantissa, scaled_exponent)
+    } else {
+        small_atof(mantissa, scaled_exponent, b)
+    }
+}