test: add tests for 'unchecked_duration_subtraction' lint

tests/ui/unchecked_duration_subtraction.rs

@@ -0,0 +1,16 @@
+#![warn(clippy::unchecked_duration_subtraction)]
+
+use std::time::{Duration, Instant};
+
+fn main() {
+    let _first = Instant::now();
+    let second = Duration::from_secs(3);
+
+    let _ = _first - second;
+
+    let _ = Instant::now() - Duration::from_secs(5);
+
+    let _ = _first - Duration::from_secs(5);
+
+    let _ = Instant::now() - second;
+}

tests/ui/unchecked_duration_subtraction.stderr

@@ -0,0 +1,32 @@
+[clippy_lints/src/unchecked_duration_subtraction.rs:75] def = std::time::Instant
+error: unchecked subtraction of a 'Duration' from an 'Instant'
+  --> $DIR/unchecked_duration_subtraction.rs:9:13
+   |
+LL |     let _ = _first - second;
+   |             ^^^^^^^^^^^^^^^ help: try: `_first.checked_sub(second).unwrap();`
+   |
+   = note: `-D clippy::unchecked-duration-subtraction` implied by `-D warnings`
+
+[clippy_lints/src/unchecked_duration_subtraction.rs:75] def = std::time::Instant
+error: unchecked subtraction of a 'Duration' from an 'Instant'
+  --> $DIR/unchecked_duration_subtraction.rs:11:13
+   |
+LL |     let _ = Instant::now() - Duration::from_secs(5);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `Instant::now().checked_sub(Duration::from_secs(5)).unwrap();`
+
+[clippy_lints/src/unchecked_duration_subtraction.rs:75] def = std::time::Instant
+error: unchecked subtraction of a 'Duration' from an 'Instant'
+  --> $DIR/unchecked_duration_subtraction.rs:13:13
+   |
+LL |     let _ = _first - Duration::from_secs(5);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `_first.checked_sub(Duration::from_secs(5)).unwrap();`
+
+[clippy_lints/src/unchecked_duration_subtraction.rs:75] def = std::time::Instant
+error: unchecked subtraction of a 'Duration' from an 'Instant'
+  --> $DIR/unchecked_duration_subtraction.rs:15:13
+   |
+LL |     let _ = Instant::now() - second;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^ help: try: `Instant::now().checked_sub(second).unwrap();`
+
+error: aborting due to 4 previous errors
+