psbt: docs for Display and LowerHex improved, also support "1" version

psbt/src/data.rs

@@ -541,25 +541,39 @@ mod display_from_str {
     /// - by default, it uses version specified in the PSBT itself;
     /// - if zero `{:0}` is given and no width (`{:0}`) or a zero width (`{:00}`) is provided, than
     ///   the PSBT is encoded as V0 even if the structure itself uses V2;
+    /// - if a width equal to one is given like in `{:1}`, than zero flag is ignored (so `{:01}`
+    ///   also works that way) and PSBT is encoded as V0 even if the structure itself uses V2;
     /// - if a width equal to two is given like in `{:2}`, than zero flag is ignored (so `{:02}`
     ///   also works that way) and PSBT is encoded as V2 even if the structure itself uses V1;
     /// - all other flags has no effect on the display.
     impl Display for Psbt {
         fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
             let ver = match (f.width(), f.sign_aware_zero_pad()) {
                 (None, true) => PsbtVer::V0,
+                (Some(1), _) => PsbtVer::V0,
                 (Some(ver), _) => PsbtVer::try_from(ver).map_err(|_| fmt::Error)?,
                 _ => self.version,
             };
             write!(f, "{}", Base64Display::new(&self.serialize(ver), &BASE64_STANDARD))
         }
     }
 
+    /// PSBT is formatted like hex-encoded string. The selection of the version if the following:
+    /// - by default, it uses version specified in the PSBT itself;
+    /// - if zero `{:0}` is given and no width (`{:0}`) or a zero width (`{:00}`) is provided, than
+    ///   the PSBT is encoded as V0 even if the structure itself uses V2;
+    /// - if a width equal to one is given like in `{:1}`, than zero flag is ignored (so `{:01}`
+    ///   also works that way) and PSBT is encoded as V0 even if the structure itself uses V2;
+    /// - if a width equal to two is given like in `{:2}`, than zero flag is ignored (so `{:02}`
+    ///   also works that way) and PSBT is encoded as V2 even if the structure itself uses V1;
+    /// - all other flags has no effect on the display.
     impl LowerHex for Psbt {
         fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-            let ver = match f.width() {
-                Some(ver) => PsbtVer::try_from(ver).map_err(|_| fmt::Error)?,
-                None => self.version,
+            let ver = match (f.width(), f.sign_aware_zero_pad()) {
+                (None, true) => PsbtVer::V0,
+                (Some(1), _) => PsbtVer::V0,
+                (Some(ver), _) => PsbtVer::try_from(ver).map_err(|_| fmt::Error)?,
+                _ => self.version,
             };
             f.write_str(&self.to_base16_ver(ver))
         }