Use SystemTime::now() for Invoice creation time

For std builds, Invoice::created_at can be automatically set upon
construction using SystemTime::now() offset by SystemTime::UNIX_EPOCH.
Change InvoiceRequest::respond_with and Refund::respond_with to only
take a created_at parameter in no-std builds.

lightning/src/offers/invoice_request.rs

@@ -323,8 +323,10 @@ impl InvoiceRequest {
 	/// Creates an [`Invoice`] for the request with the given required fields.
 	///
 	/// Unless [`InvoiceBuilder::relative_expiry`] is set, the invoice will expire two hours after
-	/// `created_at`. The caller is expected to remember the preimage of `payment_hash` in order to
-	/// claim a payment for the invoice.
+	/// calling this method (or after `created_at` in `no-std` builds).
+	///
+	/// The caller is expected to remember the preimage of `payment_hash` in order to claim a payment
+	/// for the invoice.
 	///
 	/// The `payment_paths` parameter is useful for maintaining the payment recipient's privacy. It
 	/// must contain one or more elements.
@@ -333,13 +335,19 @@ impl InvoiceRequest {
 	///
 	/// [`Invoice`]: crate::offers::invoice::Invoice
 	pub fn respond_with(
-		&self, payment_paths: Vec<(BlindedPath, BlindedPayInfo)>, created_at: Duration,
-		payment_hash: PaymentHash
+		&self, payment_paths: Vec<(BlindedPath, BlindedPayInfo)>, payment_hash: PaymentHash,
+		#[cfg(not(feature = "std"))]
+		created_at: Duration
 	) -> Result<InvoiceBuilder, SemanticError> {
 		if self.features().requires_unknown_bits() {
 			return Err(SemanticError::UnknownRequiredFeatures);
 		}
 
+		#[cfg(feature = "std")]
+		let created_at = std::time::SystemTime::now()
+			.duration_since(std::time::SystemTime::UNIX_EPOCH)
+			.expect("SystemTime::now() should come after SystemTime::UNIX_EPOCH");
+
 		InvoiceBuilder::for_offer(self, payment_paths, created_at, payment_hash)
 	}
 

lightning/src/offers/refund.rs

@@ -297,7 +297,9 @@ impl Refund {
 	/// Creates an [`Invoice`] for the refund with the given required fields.
 	///
 	/// Unless [`InvoiceBuilder::relative_expiry`] is set, the invoice will expire two hours after
-	/// `created_at`. The caller is expected to remember the preimage of `payment_hash` in order to
+	/// calling this method (or after `created_at` in `no-std` builds).
+	///
+	/// The caller is expected to remember the preimage of `payment_hash` in order to
 	/// claim a payment for the invoice.
 	///
 	/// The `signing_pubkey` is required to sign the invoice since refunds are not in response to an
@@ -310,13 +312,20 @@ impl Refund {
 	///
 	/// [`Invoice`]: crate::offers::invoice::Invoice
 	pub fn respond_with(
-		&self, payment_paths: Vec<(BlindedPath, BlindedPayInfo)>, created_at: Duration,
-		payment_hash: PaymentHash, signing_pubkey: PublicKey
+		&self, payment_paths: Vec<(BlindedPath, BlindedPayInfo)>, payment_hash: PaymentHash,
+		signing_pubkey: PublicKey,
+		#[cfg(not(feature = "std"))]
+		created_at: Duration
 	) -> Result<InvoiceBuilder, SemanticError> {
 		if self.features().requires_unknown_bits() {
 			return Err(SemanticError::UnknownRequiredFeatures);
 		}
 
+		#[cfg(feature = "std")]
+		let created_at = std::time::SystemTime::now()
+			.duration_since(std::time::SystemTime::UNIX_EPOCH)
+			.expect("SystemTime::now() should come after SystemTime::UNIX_EPOCH");
+
 		InvoiceBuilder::for_refund(self, payment_paths, created_at, payment_hash, signing_pubkey)
 	}