Add Claims::new_expires_in() (#97)

* Add Claims::new_expires_in()

* Add test for Claims::neW_expires_in()

src/claims.rs

@@ -1,6 +1,7 @@
 #![cfg_attr(docsrs, doc(cfg(feature = "std")))]
 
 use crate::errors::Error;
+use core::convert::TryFrom;
 use serde_json::Value;
 use std::collections::HashMap;
 use time::format_description::well_known::Rfc3339;
@@ -40,6 +41,30 @@ impl Claims {
         Ok(claims)
     }
 
+    /// Create a new `Claims` instance expiring in `duration`, setting:
+    /// - `iat`, `nbf` to current UTC time
+    /// - `iat + duration`
+    ///
+    /// Errors:
+    /// - If adding current time with `duration` would overflow
+    /// - If `core::time::Duration` failed to convert to `time::Duration`
+    pub fn new_expires_in(duration: &core::time::Duration) -> Result<Self, Error> {
+        let iat = OffsetDateTime::now_utc();
+        let nbf = iat;
+        let mut exp = iat;
+        exp += Duration::try_from(*duration).map_err(|_| Error::InvalidClaim)?;
+
+        let mut claims = Self {
+            list_of: HashMap::new(),
+        };
+
+        claims.issued_at(&iat.format(&Rfc3339).map_err(|_| Error::InvalidClaim)?)?;
+        claims.not_before(&nbf.format(&Rfc3339).map_err(|_| Error::InvalidClaim)?)?;
+        claims.expiration(&exp.format(&Rfc3339).map_err(|_| Error::InvalidClaim)?)?;
+
+        Ok(claims)
+    }
+
     /// Removes the `exp` claim, indicating a token that never expires.
     pub fn non_expiring(&mut self) {
         if self.contains_claim("exp") {
@@ -731,4 +756,24 @@ mod test {
         claims.list_of.remove("exp");
         assert!(claims_validation.validate_claims(&claims).is_err());
     }
+
+    #[test]
+    fn test_custom_expiration_duration() {
+        // Duration of 3 hours instead of 1.
+        let duration = core::time::Duration::new(10800, 0);
+        let mut claims = Claims::new_expires_in(&duration).unwrap();
+
+        let claims_validation = ClaimsValidationRules::new();
+        assert!(claims_validation.validate_claims(&claims).is_ok());
+
+        claims
+            .list_of
+            .insert("exp".to_string(), "2019-01-01T00:00:00+00:00".into())
+            .unwrap();
+        // Expired
+        assert_eq!(
+            claims_validation.validate_claims(&claims).unwrap_err(),
+            Error::ClaimValidation
+        );
+    }
 }