Clarify constructor failures and borrowed API contracts

crates/ferese-ipc/src/activities.rs

@@ -4,6 +4,7 @@ use serde::{Deserialize, Serialize};
 
 pub const VERSION: u32 = 1;
 
+#[must_use]
 pub fn now_ms() -> u64 {
     let mut time = libc::timespec {
         tv_sec: 0,

crates/ferese-ipc/src/events.rs

@@ -63,6 +63,7 @@ impl Event {
 }
 
 impl Snapshot {
+    #[must_use]
     pub fn changes_since(&self, previous: &Self) -> Vec<Change> {
         let mut changes = Vec::new();
         if self.outputs != previous.outputs {

crates/ferese-ipc/src/frame.rs

@@ -36,6 +36,7 @@ pub struct ResponseError {
 }
 
 impl Response {
+    #[must_use]
     pub fn success(id: u64, result: Value) -> Self {
         Self {
             version: VERSION,

crates/ferese-ipc/src/media.rs

@@ -1,5 +1,6 @@
 use serde::{Deserialize, Serialize};
 
+#[must_use]
 pub fn now_us() -> u64 {
     let mut time = libc::timespec {
         tv_sec: 0,
@@ -79,6 +80,7 @@ impl Default for Player {
 }
 
 impl Player {
+    #[must_use]
     pub fn label(&self) -> &str {
         if self.title.is_empty() {
             &self.identity
@@ -87,6 +89,7 @@ impl Player {
         }
     }
 
+    #[must_use]
     pub fn can_toggle(&self) -> bool {
         self.can_control
             && if self.status == Playback::Playing {
@@ -96,6 +99,7 @@ impl Player {
             }
     }
 
+    #[must_use]
     pub fn position_at(&self, now_us: u64) -> Option<u64> {
         let mut position = self.position_us? as f64;
         if self.status == Playback::Playing && self.rate.is_finite() {

crates/ferese-ipc/src/xwayland.rs

@@ -17,6 +17,7 @@ pub enum State {
 }
 
 impl State {
+    #[must_use]
     pub fn is_inactive(self) -> bool {
         matches!(self, Self::Disabled | Self::Idle | Self::Stopped)
     }
@@ -51,6 +52,7 @@ impl Status {
         serde_json::from_value(value)
     }
 
+    #[must_use]
     pub fn to_value(&self) -> Value {
         match serde_json::to_value(self) {
             Ok(value) => value,