chore: add vendor dependencies for kauma build

vendor/syn/src/bigint.rs

@@ -0,0 +1,66 @@
+use std::ops::{AddAssign, MulAssign};
+
+// For implementing base10_digits() accessor on LitInt.
+pub(crate) struct BigInt {
+    digits: Vec<u8>,
+}
+
+impl BigInt {
+    pub(crate) fn new() -> Self {
+        BigInt { digits: Vec::new() }
+    }
+
+    pub(crate) fn to_string(&self) -> String {
+        let mut repr = String::with_capacity(self.digits.len());
+
+        let mut has_nonzero = false;
+        for digit in self.digits.iter().rev() {
+            has_nonzero |= *digit != 0;
+            if has_nonzero {
+                repr.push((*digit + b'0') as char);
+            }
+        }
+
+        if repr.is_empty() {
+            repr.push('0');
+        }
+
+        repr
+    }
+
+    fn reserve_two_digits(&mut self) {
+        let len = self.digits.len();
+        let desired =
+            len + !self.digits.ends_with(&[0, 0]) as usize + !self.digits.ends_with(&[0]) as usize;
+        self.digits.resize(desired, 0);
+    }
+}
+
+impl AddAssign<u8> for BigInt {
+    // Assumes increment <16.
+    fn add_assign(&mut self, mut increment: u8) {
+        self.reserve_two_digits();
+
+        let mut i = 0;
+        while increment > 0 {
+            let sum = self.digits[i] + increment;
+            self.digits[i] = sum % 10;
+            increment = sum / 10;
+            i += 1;
+        }
+    }
+}
+
+impl MulAssign<u8> for BigInt {
+    // Assumes base <=16.
+    fn mul_assign(&mut self, base: u8) {
+        self.reserve_two_digits();
+
+        let mut carry = 0;
+        for digit in &mut self.digits {
+            let prod = *digit * base + carry;
+            *digit = prod % 10;
+            carry = prod / 10;
+        }
+    }
+}