Merge rust-bitcoin#3227: feat: remove zeroed vector by pushing front

441aac0 fix: vec! macro enabled only for test module (Chris Hyunhum Cho)
a050618 feat: remove zeroed vector by pushing front (Chris Hyunhum Cho)

Pull request description:

  Remove useless zeroed vector following rust-bitcoin#3131.
  1. Init vector with capacity not to realloc when pushing.
  2. If vector is empty(the first iteration), push data from the front.
  3. Iterate vector from the front, and copy it from the end.

ACKs for top commit:
  apoelstra:
    ACK 441aac0 successfully ran local tests
  jamillambert:
    ACK 441aac0
  Kixunil:
    ACK 441aac0

Tree-SHA512: dee934f675786e6977216de9516ecb7f03fa17304891f060b33d46ff3a02342b817628b68dc429c65730a52e1d99c3d0d5e9d47afee5b727b06d3c800767369e

base58/src/lib.rs

@@ -18,7 +18,6 @@
 #![allow(clippy::needless_question_mark)] // https://github.com/rust-bitcoin/rust-bitcoin/pull/2134
 #![allow(clippy::manual_range_contains)] // More readable than clippy's format.
 
-#[macro_use]
 extern crate alloc;
 
 #[cfg(bench)]
@@ -69,7 +68,7 @@ static BASE58_DIGITS: [Option<u8>; 128] = [
 /// Decodes a base58-encoded string into a byte vector.
 pub fn decode(data: &str) -> Result<Vec<u8>, InvalidCharacterError> {
     // 11/15 is just over log_256(58)
-    let mut scratch = vec![0u8; 1 + data.len() * 11 / 15];
+    let mut scratch = Vec::with_capacity(1 + data.len() * 11 / 15);
     // Build in base 256
     for d58 in data.bytes() {
         // Compute "X = X * 58 + next_digit" in base 256
@@ -82,18 +81,25 @@ pub fn decode(data: &str) -> Result<Vec<u8>, InvalidCharacterError> {
                 return Err(InvalidCharacterError { invalid: d58 });
             }
         };
-        for d256 in scratch.iter_mut().rev() {
-            carry += u32::from(*d256) * 58;
-            *d256 = carry as u8; // cast loses data intentionally
-            carry /= 256;
+        if scratch.is_empty() {
+            for _ in 0..scratch.capacity() {
+                scratch.push(carry as u8);
+                carry /= 256;
+            }
+        } else {
+            for d256 in scratch.iter_mut() {
+                carry += u32::from(*d256) * 58;
+                *d256 = carry as u8; // cast loses data intentionally
+                carry /= 256;
+            }
         }
         assert_eq!(carry, 0);
     }
 
     // Copy leading zeroes directly
     let mut ret: Vec<u8> = data.bytes().take_while(|&x| x == BASE58_CHARS[0]).map(|_| 0).collect();
     // Copy rest of string
-    ret.extend(scratch.into_iter().skip_while(|&x| x == 0));
+    ret.extend(scratch.into_iter().rev().skip_while(|&x| x == 0));
     Ok(ret)
 }
 
@@ -241,6 +247,7 @@ where
 
 #[cfg(test)]
 mod tests {
+    use alloc::vec;
     use hex::test_hex_unwrap as hex;
 
     use super::*;