use the char trick to avoid allocation

src/util/address.rs

@@ -388,23 +388,34 @@ impl fmt::Display for Address {
                 version: ver,
                 program: ref prog,
             } => {
-                if fmt.alternate() {
-                    //TODO format without allocation when alternate uppercase is in bech32
-                    let lower = self.to_string();
-                    write!(fmt, "{}", lower.to_ascii_uppercase())
-                } else {
                 let hrp = match self.network {
                     Network::Bitcoin => "bc",
                     Network::Testnet | Network::Signet => "tb",
                     Network::Regtest => "bcrt",
                 };
-                    let mut bech32_writer = bech32::Bech32Writer::new(hrp, fmt)?;
+                let is_alternate = fmt.alternate();
+                let mut opt_up_writer = OptionallyUpperWriter(fmt, is_alternate);
+                let mut bech32_writer = bech32::Bech32Writer::new(hrp, &mut opt_up_writer)?;
                 bech32::WriteBase32::write_u5(&mut bech32_writer, ver)?;
                 bech32::ToBase32::write_base32(&prog, &mut bech32_writer)
             }
         }
     }
 }
+
+struct OptionallyUpperWriter<W: fmt::Write>(W, bool);
+
+impl<W: fmt::Write> fmt::Write for OptionallyUpperWriter<W> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        if self.1 {
+            for c in s.chars() {
+                self.0.write_char(c.to_ascii_uppercase())?;
+            }
+        } else {
+            self.0.write_str(s)?;
+        }
+        Ok(())
+    }
 }
 
 /// Extract the bech32 prefix.