Type checking verifies that values match the types required by a program. During JSON deserialization, a Rust struct makes those expectations explicit: `name` must be a `String`, while `value` must be an `i32`. section 6.3
Type mismatches appear at different stages:
- The compiler requests a type annotation when `serde_json::from_str` cannot infer the result type. section 6.3
- The compiler rejects deserialization until the struct implements `Deserialize` and supports the requested formatting. section 6.3
- Serde reports an input mismatch at runtime when JSON supplies a string where the struct requires an integer. section 6.3
A mismatched JSON field
Changing `Data.name` from `String` to `i32` leaves the JSON input unchanged but causes deserialization to fail: `invalid type: string "Sharpe Oliver", expected i32`, with line and column information. section 6.3
Listing 6.6 JSON parsing code with a run-time type error
#[derive(Debug, serde::Deserialize)]
struct Data {
name: i32, #1
value: i32,
}
fn main() {
let input = "{ \"name\": \"Sharpe Oliver\", \"value\": 134087 }"; #2
let parsed: Data = serde_json::from_str(input).unwrap();
println!("{:?}", parsed);
}
Inspect the complete Rust example that introduces the mismatched field and demonstrates type checking during JSON parsing. section 6.3
