Add serde_spanned::Spanned deserialization

Provide deserialization of serde_spanned::Spanned<T>, which tracks
the position of the value in the original JSON document.

Author: gnosek

Cargo.toml

@@ -17,6 +17,7 @@ itoa = "1.0"
 memchr = { version = "2", default-features = false }
 ryu = "1.0"
 serde = { version = "1.0.194", default-features = false }
+serde_spanned = { version = "0.6.8", features = ["serde"], optional = true }
 
 [dev-dependencies]
 automod = "1.0.11"
@@ -89,3 +90,7 @@ raw_value = []
 # overflow the stack after deserialization has completed, including, but not
 # limited to, Display and Debug and Drop impls.
 unbounded_depth = []
+
+# Provide deserialization of serde_spanned::Spanned<T>, which tracks the position
+# of the value in the original JSON document
+serde_spanned = ["dep:serde_spanned"]

src/de.rs

@@ -67,6 +67,11 @@ where
             disable_recursion_limit: false,
         }
     }
+
+    #[cfg(feature = "serde_spanned")]
+    pub(crate) fn byte_offset(&self) -> usize {
+        self.read.byte_offset()
+    }
 }
 
 #[cfg(feature = "std")]
@@ -1817,20 +1822,28 @@ impl<'de, R: Read<'de>> de::Deserializer<'de> for &mut Deserializer<R> {
 
     fn deserialize_struct<V>(
         self,
-        _name: &'static str,
-        _fields: &'static [&'static str],
+        name: &'static str,
+        fields: &'static [&'static str],
         visitor: V,
     ) -> Result<V::Value>
     where
         V: de::Visitor<'de>,
     {
+        let _ = name;
+        let _ = fields;
+
         let peek = match tri!(self.parse_whitespace()) {
             Some(b) => b,
             None => {
                 return Err(self.peek_error(ErrorCode::EofWhileParsingValue));
             }
         };
 
+        #[cfg(feature = "serde_spanned")]
+        if serde_spanned::__unstable::is_spanned(name, fields) {
+            return visitor.visit_map(crate::spanned::SpannedDeserializer::new(self));
+        }
+
         let value = match peek {
             b'[' => {
                 check_recursion! {

src/lib.rs

@@ -436,3 +436,6 @@ mod read;
 
 #[cfg(feature = "raw_value")]
 mod raw;
+
+#[cfg(feature = "serde_spanned")]
+pub(crate) mod spanned;

src/spanned.rs

@@ -0,0 +1,87 @@
+use crate::de::{Deserializer, Read};
+use serde::de::value::BorrowedStrDeserializer;
+use serde::de::IntoDeserializer as _;
+
+pub(crate) enum SpannedDeserializer<'d, R> {
+    Start {
+        value_deserializer: &'d mut Deserializer<R>,
+    },
+    Value {
+        value_deserializer: &'d mut Deserializer<R>,
+    },
+    End {
+        end_pos: usize,
+    },
+    Done,
+}
+
+impl<'d, R> SpannedDeserializer<'d, R> {
+    pub fn new(value_deserializer: &'d mut Deserializer<R>) -> Self {
+        Self::Start { value_deserializer }
+    }
+}
+
+impl<'d, 'de, R> serde::de::MapAccess<'de> for SpannedDeserializer<'d, R>
+where
+    R: Read<'de>,
+{
+    type Error = <&'d mut Deserializer<R> as serde::de::Deserializer<'de>>::Error;
+
+    fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error>
+    where
+        K: serde::de::DeserializeSeed<'de>,
+    {
+        let key = match self {
+            Self::Start { .. } => serde_spanned::__unstable::START_FIELD,
+            Self::End { .. } => serde_spanned::__unstable::END_FIELD,
+            Self::Value { .. } => serde_spanned::__unstable::VALUE_FIELD,
+            Self::Done => return Ok(None),
+        };
+
+        seed.deserialize(BorrowedStrDeserializer::new(key))
+            .map(Some)
+    }
+
+    fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
+    where
+        V: serde::de::DeserializeSeed<'de>,
+    {
+        match self {
+            Self::Start { .. } => {
+                let prev = std::mem::replace(self, Self::Done);
+                let Self::Start { value_deserializer } = prev else {
+                    unreachable!()
+                };
+
+                let start = value_deserializer.byte_offset();
+                *self = Self::Value { value_deserializer };
+                seed.deserialize(start.into_deserializer())
+            }
+
+            Self::Value { .. } => {
+                let prev = std::mem::replace(self, Self::Done);
+                let Self::Value { value_deserializer } = prev else {
+                    unreachable!()
+                };
+
+                let val = seed.deserialize(&mut *value_deserializer);
+                *self = Self::End {
+                    end_pos: value_deserializer.byte_offset(),
+                };
+                val
+            }
+
+            Self::End { .. } => {
+                let prev = std::mem::replace(self, Self::Done);
+                let Self::End { end_pos } = prev else {
+                    unreachable!()
+                };
+                seed.deserialize(end_pos.into_deserializer())
+            }
+
+            Self::Done => {
+                panic!("should not get here");
+            }
+        }
+    }
+}

tests/crate/Cargo.toml

@@ -20,3 +20,4 @@ float_roundtrip = ["serde_json/float_roundtrip"]
 arbitrary_precision = ["serde_json/arbitrary_precision"]
 raw_value = ["serde_json/raw_value"]
 unbounded_depth = ["serde_json/unbounded_depth"]
+serde_spanned = ["serde_json/serde_spanned"]

tests/test.rs

@@ -33,7 +33,7 @@ use serde_json::{
     to_vec, Deserializer, Number, Value,
 };
 use std::collections::BTreeMap;
-#[cfg(feature = "raw_value")]
+#[cfg(any(feature = "raw_value", feature = "serde_spanned"))]
 use std::collections::HashMap;
 use std::fmt::{self, Debug};
 use std::hash::BuildHasher;
@@ -43,6 +43,8 @@ use std::io;
 use std::iter;
 use std::marker::PhantomData;
 use std::mem;
+#[cfg(feature = "serde_spanned")]
+use std::ops::Range;
 use std::str::FromStr;
 use std::{f32, f64};
 
@@ -2558,3 +2560,138 @@ fn test_control_character_search() {
         "control character (\\u0000-\\u001F) found while parsing a string at line 1 column 2",
     )]);
 }
+
+#[cfg(feature = "serde_spanned")]
+fn format_span(json: &str, span: Range<usize>) -> String {
+    format!(
+        "{}\n{}{}",
+        json,
+        " ".repeat(span.start),
+        "^".repeat(span.end - span.start)
+    )
+}
+
+#[cfg(feature = "serde_spanned")]
+#[track_caller]
+fn assert_span_eq(json: &str, expected: Range<usize>, actual: Range<usize>) {
+    let expected_str = format_span(json, expected.clone());
+    let actual_str = format_span(json, actual.clone());
+
+    assert_eq!(
+        expected, actual,
+        "Expected span:\n{}\nActual span:\n{}",
+        expected_str, actual_str
+    );
+}
+
+#[cfg(feature = "serde_spanned")]
+#[test]
+fn test_spanned_string() {
+    use serde_spanned::Spanned;
+
+    #[derive(Deserialize)]
+    struct SpannedStruct {
+        field: Spanned<String>,
+    }
+
+    let json = r#"{"field": "value"}"#;
+    let result: SpannedStruct = serde_json::from_str(json).unwrap();
+    assert_eq!(result.field.as_ref(), "value");
+    assert_span_eq(json, result.field.span(), 10..17);
+}
+
+#[cfg(feature = "serde_spanned")]
+#[test]
+fn test_spanned_number() {
+    use serde_spanned::Spanned;
+
+    #[derive(Deserialize)]
+    struct SpannedStruct {
+        field: Spanned<f64>,
+    }
+
+    let json = r#"{"field": -2.718e28}"#;
+    let result: SpannedStruct = serde_json::from_str(json).unwrap();
+    assert_eq!(*result.field.as_ref(), -2.718e28);
+    assert_span_eq(json, result.field.span(), 10..19);
+}
+
+#[cfg(feature = "serde_spanned")]
+#[test]
+fn test_spanned_whole_array() {
+    use serde_spanned::Spanned;
+
+    #[derive(Deserialize)]
+    struct SpannedStruct {
+        field: Spanned<Vec<i32>>,
+    }
+
+    let json = r#"{"field": [1, 2, 3, 4]}"#;
+    let result: SpannedStruct = serde_json::from_str(json).unwrap();
+    assert_eq!(result.field.as_ref(), &[1, 2, 3, 4]);
+    assert_span_eq(json, result.field.span(), 10..22);
+}
+
+#[cfg(feature = "serde_spanned")]
+#[test]
+fn test_spanned_array_items() {
+    use serde_spanned::Spanned;
+
+    #[derive(Deserialize)]
+    struct SpannedStruct {
+        field: Vec<Spanned<i32>>,
+    }
+
+    let json = r#"{"field": [1, 2, 3, 4]}"#;
+    let result: SpannedStruct = serde_json::from_str(json).unwrap();
+    assert_eq!(result.field.len(), 4);
+
+    assert_eq!(*result.field[0].as_ref(), 1);
+    assert_span_eq(json, result.field[0].span(), 11..12);
+
+    assert_eq!(*result.field[1].as_ref(), 2);
+    assert_span_eq(json, result.field[1].span(), 14..15);
+
+    assert_eq!(*result.field[2].as_ref(), 3);
+    assert_span_eq(json, result.field[2].span(), 17..18);
+
+    assert_eq!(*result.field[3].as_ref(), 4);
+    assert_span_eq(json, result.field[3].span(), 20..21);
+}
+
+#[cfg(feature = "serde_spanned")]
+#[test]
+fn test_spanned_whole_map() {
+    use serde_spanned::Spanned;
+
+    #[derive(Deserialize)]
+    struct SpannedStruct {
+        field: Spanned<HashMap<i32, String>>,
+    }
+
+    let json = r#"{"field": {"1": "one", "2": "two"}}"#;
+    let result: SpannedStruct = serde_json::from_str(json).unwrap();
+    assert_span_eq(json, result.field.span(), 10..34);
+    let mut map = result.field.into_inner();
+    let one = map.remove(&1).unwrap();
+    assert_eq!(one, "one");
+    let two = map.remove(&2).unwrap();
+    assert_eq!(two, "two");
+    assert!(map.is_empty());
+}
+
+#[cfg(feature = "serde_spanned")]
+#[test]
+fn test_spanned_whitespace() {
+    use serde_spanned::Spanned;
+
+    #[derive(Deserialize)]
+    struct SpannedStruct {
+        field: Spanned<f64>,
+    }
+
+    let json = r#"{"field":      -2.718e28   }"#;
+    let result: SpannedStruct = serde_json::from_str(json).unwrap();
+    assert_eq!(*result.field.as_ref(), -2.718e28);
+    assert_span_eq(json, result.field.span(), 15..24);
+}