Merge pull request #1206 from dtolnay/hasnext

Improve performance of seq/map peeks

Author: dtolnay

src/de.rs

@@ -1930,30 +1930,26 @@ impl<'de, 'a, R: Read<'de> + 'a> de::SeqAccess<'de> for SeqAccess<'a, R> {
             seq: &mut SeqAccess<'a, R>,
         ) -> Result<bool> {
             let peek = match tri!(seq.de.parse_whitespace()) {
-                Some(b']') => {
-                    return Ok(false);
-                }
-                Some(b',') if !seq.first => {
-                    seq.de.eat_char();
-                    tri!(seq.de.parse_whitespace())
-                }
-                Some(b) => {
-                    if seq.first {
-                        seq.first = false;
-                        Some(b)
-                    } else {
-                        return Err(seq.de.peek_error(ErrorCode::ExpectedListCommaOrEnd));
-                    }
-                }
+                Some(b) => b,
                 None => {
                     return Err(seq.de.peek_error(ErrorCode::EofWhileParsingList));
                 }
             };
 
-            match peek {
-                Some(b']') => Err(seq.de.peek_error(ErrorCode::TrailingComma)),
-                Some(_) => Ok(true),
-                None => Err(seq.de.peek_error(ErrorCode::EofWhileParsingValue)),
+            if peek == b']' {
+                Ok(false)
+            } else if seq.first {
+                seq.first = false;
+                Ok(true)
+            } else if peek == b',' {
+                seq.de.eat_char();
+                match tri!(seq.de.parse_whitespace()) {
+                    Some(b']') => Err(seq.de.peek_error(ErrorCode::TrailingComma)),
+                    Some(_) => Ok(true),
+                    None => Err(seq.de.peek_error(ErrorCode::EofWhileParsingValue)),
+                }
+            } else {
+                Err(seq.de.peek_error(ErrorCode::ExpectedListCommaOrEnd))
             }
         }
 
@@ -1985,31 +1981,31 @@ impl<'de, 'a, R: Read<'de> + 'a> de::MapAccess<'de> for MapAccess<'a, R> {
     {
         fn has_next_key<'de, 'a, R: Read<'de> + 'a>(map: &mut MapAccess<'a, R>) -> Result<bool> {
             let peek = match tri!(map.de.parse_whitespace()) {
-                Some(b'}') => {
-                    return Ok(false);
-                }
-                Some(b',') if !map.first => {
-                    map.de.eat_char();
-                    tri!(map.de.parse_whitespace())
-                }
-                Some(b) => {
-                    if map.first {
-                        map.first = false;
-                        Some(b)
-                    } else {
-                        return Err(map.de.peek_error(ErrorCode::ExpectedObjectCommaOrEnd));
-                    }
-                }
+                Some(b) => b,
                 None => {
                     return Err(map.de.peek_error(ErrorCode::EofWhileParsingObject));
                 }
             };
 
-            match peek {
-                Some(b'"') => Ok(true),
-                Some(b'}') => Err(map.de.peek_error(ErrorCode::TrailingComma)),
-                Some(_) => Err(map.de.peek_error(ErrorCode::KeyMustBeAString)),
-                None => Err(map.de.peek_error(ErrorCode::EofWhileParsingValue)),
+            if peek == b'}' {
+                Ok(false)
+            } else if map.first {
+                map.first = false;
+                if peek == b'"' {
+                    Ok(true)
+                } else {
+                    Err(map.de.peek_error(ErrorCode::KeyMustBeAString))
+                }
+            } else if peek == b',' {
+                map.de.eat_char();
+                match tri!(map.de.parse_whitespace()) {
+                    Some(b'"') => Ok(true),
+                    Some(b'}') => Err(map.de.peek_error(ErrorCode::TrailingComma)),
+                    Some(_) => Err(map.de.peek_error(ErrorCode::KeyMustBeAString)),
+                    None => Err(map.de.peek_error(ErrorCode::EofWhileParsingValue)),
+                }
+            } else {
+                Err(map.de.peek_error(ErrorCode::ExpectedObjectCommaOrEnd))
             }
         }