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diff --git a/third_party/rust/serde_json/src/lexical/rounding.rs b/third_party/rust/serde_json/src/lexical/rounding.rs
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+++ b/third_party/rust/serde_json/src/lexical/rounding.rs
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+// Adapted from https://github.com/Alexhuszagh/rust-lexical.
+
+//! Defines rounding schemes for floating-point numbers.
+
+use super::float::ExtendedFloat;
+use super::num::*;
+use super::shift::*;
+use core::mem;
+
+// MASKS
+
+/// Calculate a scalar factor of 2 above the halfway point.
+#[inline]
+pub(crate) fn nth_bit(n: u64) -> u64 {
+    let bits: u64 = mem::size_of::<u64>() as u64 * 8;
+    debug_assert!(n < bits, "nth_bit() overflow in shl.");
+
+    1 << n
+}
+
+/// Generate a bitwise mask for the lower `n` bits.
+#[inline]
+pub(crate) fn lower_n_mask(n: u64) -> u64 {
+    let bits: u64 = mem::size_of::<u64>() as u64 * 8;
+    debug_assert!(n <= bits, "lower_n_mask() overflow in shl.");
+
+    if n == bits {
+        u64::max_value()
+    } else {
+        (1 << n) - 1
+    }
+}
+
+/// Calculate the halfway point for the lower `n` bits.
+#[inline]
+pub(crate) fn lower_n_halfway(n: u64) -> u64 {
+    let bits: u64 = mem::size_of::<u64>() as u64 * 8;
+    debug_assert!(n <= bits, "lower_n_halfway() overflow in shl.");
+
+    if n == 0 {
+        0
+    } else {
+        nth_bit(n - 1)
+    }
+}
+
+/// Calculate a bitwise mask with `n` 1 bits starting at the `bit` position.
+#[inline]
+pub(crate) fn internal_n_mask(bit: u64, n: u64) -> u64 {
+    let bits: u64 = mem::size_of::<u64>() as u64 * 8;
+    debug_assert!(bit <= bits, "internal_n_halfway() overflow in shl.");
+    debug_assert!(n <= bits, "internal_n_halfway() overflow in shl.");
+    debug_assert!(bit >= n, "internal_n_halfway() overflow in sub.");
+
+    lower_n_mask(bit) ^ lower_n_mask(bit - n)
+}
+
+// NEAREST ROUNDING
+
+// Shift right N-bytes and round to the nearest.
+//
+// Return if we are above halfway and if we are halfway.
+#[inline]
+pub(crate) fn round_nearest(fp: &mut ExtendedFloat, shift: i32) -> (bool, bool) {
+    // Extract the truncated bits using mask.
+    // Calculate if the value of the truncated bits are either above
+    // the mid-way point, or equal to it.
+    //
+    // For example, for 4 truncated bytes, the mask would be b1111
+    // and the midway point would be b1000.
+    let mask: u64 = lower_n_mask(shift as u64);
+    let halfway: u64 = lower_n_halfway(shift as u64);
+
+    let truncated_bits = fp.mant & mask;
+    let is_above = truncated_bits > halfway;
+    let is_halfway = truncated_bits == halfway;
+
+    // Bit shift so the leading bit is in the hidden bit.
+    overflowing_shr(fp, shift);
+
+    (is_above, is_halfway)
+}
+
+// Tie rounded floating point to event.
+#[inline]
+pub(crate) fn tie_even(fp: &mut ExtendedFloat, is_above: bool, is_halfway: bool) {
+    // Extract the last bit after shifting (and determine if it is odd).
+    let is_odd = fp.mant & 1 == 1;
+
+    // Calculate if we need to roundup.
+    // We need to roundup if we are above halfway, or if we are odd
+    // and at half-way (need to tie-to-even).
+    if is_above || (is_odd && is_halfway) {
+        fp.mant += 1;
+    }
+}
+
+// Shift right N-bytes and round nearest, tie-to-even.
+//
+// Floating-point arithmetic uses round to nearest, ties to even,
+// which rounds to the nearest value, if the value is halfway in between,
+// round to an even value.
+#[inline]
+pub(crate) fn round_nearest_tie_even(fp: &mut ExtendedFloat, shift: i32) {
+    let (is_above, is_halfway) = round_nearest(fp, shift);
+    tie_even(fp, is_above, is_halfway);
+}
+
+// DIRECTED ROUNDING
+
+// Shift right N-bytes and round towards a direction.
+//
+// Return if we have any truncated bytes.
+#[inline]
+fn round_toward(fp: &mut ExtendedFloat, shift: i32) -> bool {
+    let mask: u64 = lower_n_mask(shift as u64);
+    let truncated_bits = fp.mant & mask;
+
+    // Bit shift so the leading bit is in the hidden bit.
+    overflowing_shr(fp, shift);
+
+    truncated_bits != 0
+}
+
+// Round down.
+#[inline]
+fn downard(_: &mut ExtendedFloat, _: bool) {}
+
+// Shift right N-bytes and round toward zero.
+//
+// Floating-point arithmetic defines round toward zero, which rounds
+// towards positive zero.
+#[inline]
+pub(crate) fn round_downward(fp: &mut ExtendedFloat, shift: i32) {
+    // Bit shift so the leading bit is in the hidden bit.
+    // No rounding schemes, so we just ignore everything else.
+    let is_truncated = round_toward(fp, shift);
+    downard(fp, is_truncated);
+}
+
+// ROUND TO FLOAT
+
+// Shift the ExtendedFloat fraction to the fraction bits in a native float.
+//
+// Floating-point arithmetic uses round to nearest, ties to even,
+// which rounds to the nearest value, if the value is halfway in between,
+// round to an even value.
+#[inline]
+pub(crate) fn round_to_float<F, Algorithm>(fp: &mut ExtendedFloat, algorithm: Algorithm)
+where
+    F: Float,
+    Algorithm: FnOnce(&mut ExtendedFloat, i32),
+{
+    // Calculate the difference to allow a single calculation
+    // rather than a loop, to minimize the number of ops required.
+    // This does underflow detection.
+    let final_exp = fp.exp + F::DEFAULT_SHIFT;
+    if final_exp < F::DENORMAL_EXPONENT {
+        // We would end up with a denormal exponent, try to round to more
+        // digits. Only shift right if we can avoid zeroing out the value,
+        // which requires the exponent diff to be < M::BITS. The value
+        // is already normalized, so we shouldn't have any issue zeroing
+        // out the value.
+        let diff = F::DENORMAL_EXPONENT - fp.exp;
+        if diff <= u64::FULL {
+            // We can avoid underflow, can get a valid representation.
+            algorithm(fp, diff);
+        } else {
+            // Certain underflow, assign literal 0s.
+            fp.mant = 0;
+            fp.exp = 0;
+        }
+    } else {
+        algorithm(fp, F::DEFAULT_SHIFT);
+    }
+
+    if fp.mant & F::CARRY_MASK == F::CARRY_MASK {
+        // Roundup carried over to 1 past the hidden bit.
+        shr(fp, 1);
+    }
+}
+
+// AVOID OVERFLOW/UNDERFLOW
+
+// Avoid overflow for large values, shift left as needed.
+//
+// Shift until a 1-bit is in the hidden bit, if the mantissa is not 0.
+#[inline]
+pub(crate) fn avoid_overflow<F>(fp: &mut ExtendedFloat)
+where
+    F: Float,
+{
+    // Calculate the difference to allow a single calculation
+    // rather than a loop, minimizing the number of ops required.
+    if fp.exp >= F::MAX_EXPONENT {
+        let diff = fp.exp - F::MAX_EXPONENT;
+        if diff <= F::MANTISSA_SIZE {
+            // Our overflow mask needs to start at the hidden bit, or at
+            // `F::MANTISSA_SIZE+1`, and needs to have `diff+1` bits set,
+            // to see if our value overflows.
+            let bit = (F::MANTISSA_SIZE + 1) as u64;
+            let n = (diff + 1) as u64;
+            let mask = internal_n_mask(bit, n);
+            if (fp.mant & mask) == 0 {
+                // If we have no 1-bit in the hidden-bit position,
+                // which is index 0, we need to shift 1.
+                let shift = diff + 1;
+                shl(fp, shift);
+            }
+        }
+    }
+}
+
+// ROUND TO NATIVE
+
+// Round an extended-precision float to a native float representation.
+#[inline]
+pub(crate) fn round_to_native<F, Algorithm>(fp: &mut ExtendedFloat, algorithm: Algorithm)
+where
+    F: Float,
+    Algorithm: FnOnce(&mut ExtendedFloat, i32),
+{
+    // Shift all the way left, to ensure a consistent representation.
+    // The following right-shifts do not work for a non-normalized number.
+    fp.normalize();
+
+    // Round so the fraction is in a native mantissa representation,
+    // and avoid overflow/underflow.
+    round_to_float::<F, _>(fp, algorithm);
+    avoid_overflow::<F>(fp);
+}