Deserialize small numbers as integers in arbitrary_precision

src/de.rs

@@ -864,6 +864,15 @@ impl<'de, R: Read<'de>> Deserializer<R> {
             buf.push('-');
         }
         self.scan_integer(&mut buf)?;
+        if positive {
+            if let Ok(unsigned) = buf.parse() {
+                return Ok(ParserNumber::U64(unsigned));
+            }
+        } else {
+            if let Ok(signed) = buf.parse() {
+                return Ok(ParserNumber::I64(signed));
+            }
+        }
         Ok(ParserNumber::String(buf))
     }
 

src/lib.rs

@@ -303,6 +303,7 @@
 #![doc(html_root_url = "https://docs.rs/serde_json/1.0.74")]
 // Ignored clippy lints
 #![allow(
+    clippy::collapsible_else_if,
     clippy::comparison_chain,
     clippy::deprecated_cfg_attr,
     clippy::doc_markdown,

tests/regression/issue845.rs

@@ -0,0 +1,72 @@
+use serde::{Deserialize, Deserializer};
+use std::convert::TryFrom;
+use std::fmt::{self, Display};
+use std::marker::PhantomData;
+use std::str::FromStr;
+
+pub struct NumberVisitor<T> {
+    marker: PhantomData<T>,
+}
+
+impl<'de, T> serde::de::Visitor<'de> for NumberVisitor<T>
+where
+    T: TryFrom<u64> + TryFrom<i64> + FromStr,
+    <T as TryFrom<u64>>::Error: Display,
+    <T as TryFrom<i64>>::Error: Display,
+    <T as FromStr>::Err: Display,
+{
+    type Value = T;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("an integer or string")
+    }
+
+    fn visit_u64<E>(self, v: u64) -> Result<Self::Value, E>
+    where
+        E: serde::de::Error,
+    {
+        T::try_from(v).map_err(serde::de::Error::custom)
+    }
+
+    fn visit_i64<E>(self, v: i64) -> Result<Self::Value, E>
+    where
+        E: serde::de::Error,
+    {
+        T::try_from(v).map_err(serde::de::Error::custom)
+    }
+
+    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+    where
+        E: serde::de::Error,
+    {
+        v.parse().map_err(serde::de::Error::custom)
+    }
+}
+
+fn deserialize_integer_or_string<'de, D, T>(deserializer: D) -> Result<T, D::Error>
+where
+    D: Deserializer<'de>,
+    T: TryFrom<u64> + TryFrom<i64> + FromStr,
+    <T as TryFrom<u64>>::Error: Display,
+    <T as TryFrom<i64>>::Error: Display,
+    <T as FromStr>::Err: Display,
+{
+    deserializer.deserialize_any(NumberVisitor {
+        marker: PhantomData,
+    })
+}
+
+#[derive(Deserialize, Debug)]
+pub struct Struct {
+    #[serde(deserialize_with = "deserialize_integer_or_string")]
+    pub i: i64,
+}
+
+#[test]
+fn test() {
+    let j = r#" {"i":100} "#;
+    println!("{:?}", serde_json::from_str::<Struct>(j).unwrap());
+
+    let j = r#" {"i":"100"} "#;
+    println!("{:?}", serde_json::from_str::<Struct>(j).unwrap());
+}

tests/test.rs

@@ -715,11 +715,7 @@ fn test_parse_char() {
         ),
         (
             "10",
-            if cfg!(feature = "arbitrary_precision") {
-                "invalid type: number, expected a character at line 1 column 2"
-            } else {
-                "invalid type: integer `10`, expected a character at line 1 column 2"
-            },
+            "invalid type: integer `10`, expected a character at line 1 column 2",
         ),
     ]);
 
@@ -1203,11 +1199,7 @@ fn test_parse_struct() {
     test_parse_err::<Outer>(&[
         (
             "5",
-            if cfg!(feature = "arbitrary_precision") {
-                "invalid type: number, expected struct Outer at line 1 column 1"
-            } else {
-                "invalid type: integer `5`, expected struct Outer at line 1 column 1"
-            },
+            "invalid type: integer `5`, expected struct Outer at line 1 column 1",
         ),
         (
             "\"hello\"",