Merge rust-bitcoin/rust-bitcoin#1942: witness: clean up Debug implementation

e30c492faf witness: clean up Debug implementation (Andrew Poelstra)

Pull request description:

  The previous code seems to have been rebased/iterated on too many times, and had room for significant simplification. By inlining the indentation logic we can eliminate 40 LOC and also clean up the output by removing trailing spaces.

  Fixes #1937

  It is not good form to add unit tests for debug output but you can test this locally with the patch
  ```
  diff --git a/bitcoin/src/blockdata/witness.rs b/bitcoin/src/blockdata/witness.rs
  index d0b7408c..a2c38af0 100644
  --- a/bitcoin/src/blockdata/witness.rs
  +++ b/bitcoin/src/blockdata/witness.rs
  @@ -619,6 +619,9 @@ mod test {
               "304402207c800d698f4b0298c5aac830b822f011bb02df41eb114ade9a6702f364d5e39c0220366900d2a60cab903e77ef7dd415d46509b1f78ac78906e3296f495aa1b1b54101")
               ];
           assert_eq!(witness.to_vec(), expected_witness);
  +
  +            println!("{:?}", witness);
  +            panic!();
       }

       #[test]
  ```
  And by sticking `{:#?}` in there to see the alternate output.

ACKs for top commit:
  tcharding:
    tACK e30c492faf
  RCasatta:
    ACK e30c492faf

Tree-SHA512: 0ec07885f5c75f3f34965852cf5b42b63290295d1f56e9fef7d5b3610b8ac8d318cbf8f184da5b8a9ed5b352bb2c0402797b41714cb9d5488e93c2e290340c2a

bitcoin/src/blockdata/witness.rs

@@ -63,10 +63,8 @@ fn fmt_debug(w: &Witness, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error>
     };
 
     f.write_str("Witness: { ")?;
-
-    f.write_str(&indices_str(w))?;
-    f.write_str(&indices_start_str(w))?;
-
+    write!(f, "indices: {}, ", w.witness_elements)?;
+    write!(f, "indices_start: {}, ", w.indices_start)?;
     f.write_str("witnesses: [")?;
 
     let instructions = w.iter();
@@ -79,73 +77,35 @@ fn fmt_debug(w: &Witness, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error>
         f.write_str("[")?;
 
         for (j, byte) in bytes.enumerate() {
-            f.write_str(&byte_str(*byte))?;
+            write!(f, "{:#04x}", byte)?;
             f.write_str(comma_or_close(j, last_byte))?;
         }
 
         f.write_str(comma_or_close(i, last_instruction))?;
     }
 
-    f.write_str(" }")?;
-
-    Ok(())
+    f.write_str(" }")
 }
 
 fn fmt_debug_pretty(w: &Witness, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
-    let mut indent = 0;
+    f.write_str("Witness: {\n")?;
+    writeln!(f, "    indices: {},", w.witness_elements)?;
+    writeln!(f, "    indices_start: {},", w.indices_start)?;
+    f.write_str("    witnesses: [\n")?;
 
-    writeln(f, indent, "Witness: { ")?;
-    indent += 1;
-
-    writeln(f, indent, &indices_str(w))?;
-    writeln(f, indent, &indices_start_str(w))?;
-
-    writeln(f, indent, "witnesses: [ ")?;
-    indent += 1;
-
-    let instructions = w.iter();
-
-    for instruction in instructions {
-        let bytes = instruction.iter();
-        let last_byte = bytes.len() - 1;
-
-        write(f, indent, "[")?;
-
-        for (j, byte) in bytes.enumerate() {
-            f.write_str(&byte_str(*byte))?;
-            if j == last_byte {
-                f.write_str("],\n")?;
-            } else {
+    for instruction in w.iter() {
+        f.write_str("        [")?;
+        for (j, byte) in instruction.iter().enumerate() {
+            if j > 0 {
                 f.write_str(", ")?;
             }
+            write!(f, "{:#04x}", byte)?;
         }
+        f.write_str("],\n")?;
     }
 
-    indent -= 1;
-    writeln(f, indent, "],")?;
-
-    indent -= 1;
-    write(f, indent, "}")?;
-
-    Ok(())
-}
-
-fn indices_str(w: &Witness) -> String { format!("indices: {}, ", w.witness_elements) }
-
-fn indices_start_str(w: &Witness) -> String { format!("indices_start: {}, ", w.indices_start) }
-
-fn byte_str(byte: u8) -> String { format!("{:#04x}", byte) }
-
-fn writeln(f: &mut fmt::Formatter<'_>, indent: usize, s: &str) -> Result<(), fmt::Error> {
-    write(f, indent, s)?;
-    f.write_str("\n")
-}
-
-fn write(f: &mut fmt::Formatter<'_>, indent: usize, s: &str) -> Result<(), fmt::Error> {
-    for _ in 0..indent {
-        f.write_str("    ")?;
-    }
-    f.write_str(s)
+    writeln!(f, "    ],")?;
+    writeln!(f, "}}")
 }
 
 /// An iterator returning individual witness elements.