Auto merge of #9264 - guerinoni:instant-elapsed, r=Alexendoo

Add `elapsed_instant` lint

Closes #8603

- \[x] Followed [lint naming conventions][lint_naming]
- \[x] Added passing UI tests (including committed `.stderr` file)
- \[x] `cargo test` passes locally
- \[x] Executed `cargo dev update_lints`
- \[x] Added lint documentation
- \[x] Run `cargo dev fmt`

---

changelog: [`elapsed_instant`]: Add lint

9 files changed, 146 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 646e1128e9a..84f06d98879 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3686,6 +3686,7 @@ Released 2018-09-13
 [`manual_find`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_find
 [`manual_find_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_find_map
 [`manual_flatten`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_flatten
+[`manual_instant_elapsed`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_instant_elapsed
 [`manual_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_map
 [`manual_memcpy`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_memcpy
 [`manual_non_exhaustive`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_non_exhaustive
diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index 430b8585f6d..c2bc4badc43 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -244,6 +244,7 @@ store.register_lints(&[
     manual_assert::MANUAL_ASSERT,
     manual_async_fn::MANUAL_ASYNC_FN,
     manual_bits::MANUAL_BITS,
+    manual_instant_elapsed::MANUAL_INSTANT_ELAPSED,
     manual_non_exhaustive::MANUAL_NON_EXHAUSTIVE,
     manual_ok_or::MANUAL_OK_OR,
     manual_rem_euclid::MANUAL_REM_EUCLID,
diff --git a/clippy_lints/src/lib.register_pedantic.rs b/clippy_lints/src/lib.register_pedantic.rs
index 4d4b89687d0..4250ee055e6 100644
--- a/clippy_lints/src/lib.register_pedantic.rs
+++ b/clippy_lints/src/lib.register_pedantic.rs
@@ -49,6 +49,7 @@ store.register_group(true, "clippy::pedantic", Some("clippy_pedantic"), vec![
     LintId::of(loops::EXPLICIT_ITER_LOOP),
     LintId::of(macro_use::MACRO_USE_IMPORTS),
     LintId::of(manual_assert::MANUAL_ASSERT),
+    LintId::of(manual_instant_elapsed::MANUAL_INSTANT_ELAPSED),
     LintId::of(manual_ok_or::MANUAL_OK_OR),
     LintId::of(matches::MATCH_BOOL),
     LintId::of(matches::MATCH_ON_VEC_ITEMS),
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 6ebf1983156..88c1a727f8d 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -273,6 +273,7 @@ mod main_recursion;
 mod manual_assert;
 mod manual_async_fn;
 mod manual_bits;
+mod manual_instant_elapsed;
 mod manual_non_exhaustive;
 mod manual_ok_or;
 mod manual_rem_euclid;
@@ -929,6 +930,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move || Box::new(operators::Operators::new(verbose_bit_mask_threshold)));
     store.register_late_pass(|| Box::new(invalid_utf8_in_unchecked::InvalidUtf8InUnchecked));
     store.register_late_pass(|| Box::new(std_instead_of_core::StdReexports::default()));
+    store.register_late_pass(|| Box::new(manual_instant_elapsed::ManualInstantElapsed));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 
diff --git a/clippy_lints/src/manual_instant_elapsed.rs b/clippy_lints/src/manual_instant_elapsed.rs
new file mode 100644
index 00000000000..331cda1db89
--- /dev/null
+++ b/clippy_lints/src/manual_instant_elapsed.rs
@@ -0,0 +1,69 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use rustc_errors::Applicability;
+use rustc_hir::{BinOpKind, Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::source_map::Spanned;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Lints subtraction between `Instant::now()` and another `Instant`.
+    ///
+    /// ### Why is this bad?
+    /// It is easy to accidentally write `prev_instant - Instant::now()`, which will always be 0ns
+    /// as `Instant` subtraction saturates.
+    ///
+    /// `prev_instant.elapsed()` also more clearly signals intention.
+    ///
+    /// ### Example
+    /// ```rust
+    /// use std::time::Instant;
+    /// let prev_instant = Instant::now();
+    /// let duration = Instant::now() - prev_instant;
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// use std::time::Instant;
+    /// let prev_instant = Instant::now();
+    /// let duration = prev_instant.elapsed();
+    /// ```
+    #[clippy::version = "1.64.0"]
+    pub MANUAL_INSTANT_ELAPSED,
+    pedantic,
+    "subtraction between `Instant::now()` and previous `Instant`"
+}
+
+declare_lint_pass!(ManualInstantElapsed => [MANUAL_INSTANT_ELAPSED]);
+
+impl LateLintPass<'_> for ManualInstantElapsed {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &'_ Expr<'_>) {
+        if let ExprKind::Binary(Spanned {node: BinOpKind::Sub, ..}, lhs, rhs) = expr.kind
+            && check_instant_now_call(cx, lhs)
+            && let ty_resolved = cx.typeck_results().expr_ty(rhs)
+            && let rustc_middle::ty::Adt(def, _) = ty_resolved.kind()
+            && clippy_utils::match_def_path(cx, def.did(), &clippy_utils::paths::INSTANT)
+            && let Some(sugg) = clippy_utils::sugg::Sugg::hir_opt(cx, rhs)
+        {
+            span_lint_and_sugg(
+                cx,
+                MANUAL_INSTANT_ELAPSED,
+                expr.span,
+                "manual implementation of `Instant::elapsed`",
+                "try",
+                format!("{}.elapsed()", sugg.maybe_par()),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+}
+
+fn check_instant_now_call(cx: &LateContext<'_>, expr_block: &'_ Expr<'_>) -> bool {
+    if let ExprKind::Call(fn_expr, []) = expr_block.kind
+        && let Some(fn_id) = clippy_utils::path_def_id(cx, fn_expr)
+        && clippy_utils::match_def_path(cx, fn_id, &clippy_utils::paths::INSTANT_NOW)
+    {
+        true
+    } else {
+        false
+    }
+}
diff --git a/clippy_utils/src/paths.rs b/clippy_utils/src/paths.rs
index 05429d05d9e..8d697a301c4 100644
--- a/clippy_utils/src/paths.rs
+++ b/clippy_utils/src/paths.rs
@@ -194,3 +194,5 @@ pub const VEC_RESIZE: [&str; 4] = ["alloc", "vec", "Vec", "resize"];
 pub const WEAK_ARC: [&str; 3] = ["alloc", "sync", "Weak"];
 pub const WEAK_RC: [&str; 3] = ["alloc", "rc", "Weak"];
 pub const PTR_NON_NULL: [&str; 4] = ["core", "ptr", "non_null", "NonNull"];
+pub const INSTANT_NOW: [&str; 4] = ["std", "time", "Instant", "now"];
+pub const INSTANT: [&str; 3] = ["std", "time", "Instant"];
diff --git a/tests/ui/manual_instant_elapsed.fixed b/tests/ui/manual_instant_elapsed.fixed
new file mode 100644
index 00000000000..0fa776b7b2e
--- /dev/null
+++ b/tests/ui/manual_instant_elapsed.fixed
@@ -0,0 +1,27 @@
+// run-rustfix
+#![warn(clippy::manual_instant_elapsed)]
+#![allow(clippy::unnecessary_operation)]
+#![allow(unused_variables)]
+#![allow(unused_must_use)]
+
+use std::time::Instant;
+
+fn main() {
+    let prev_instant = Instant::now();
+
+    {
+        // don't influence
+        let another_instant = Instant::now();
+    }
+
+    let duration = prev_instant.elapsed();
+
+    // don't catch
+    let duration = prev_instant.elapsed();
+
+    Instant::now() - duration;
+
+    let ref_to_instant = &Instant::now();
+
+    (*ref_to_instant).elapsed(); // to ensure parens are added correctly
+}
diff --git a/tests/ui/manual_instant_elapsed.rs b/tests/ui/manual_instant_elapsed.rs
new file mode 100644
index 00000000000..5b11b84535d
--- /dev/null
+++ b/tests/ui/manual_instant_elapsed.rs
@@ -0,0 +1,27 @@
+// run-rustfix
+#![warn(clippy::manual_instant_elapsed)]
+#![allow(clippy::unnecessary_operation)]
+#![allow(unused_variables)]
+#![allow(unused_must_use)]
+
+use std::time::Instant;
+
+fn main() {
+    let prev_instant = Instant::now();
+
+    {
+        // don't influence
+        let another_instant = Instant::now();
+    }
+
+    let duration = Instant::now() - prev_instant;
+
+    // don't catch
+    let duration = prev_instant.elapsed();
+
+    Instant::now() - duration;
+
+    let ref_to_instant = &Instant::now();
+
+    Instant::now() - *ref_to_instant; // to ensure parens are added correctly
+}
diff --git a/tests/ui/manual_instant_elapsed.stderr b/tests/ui/manual_instant_elapsed.stderr
new file mode 100644
index 00000000000..5537f5642a2
--- /dev/null
+++ b/tests/ui/manual_instant_elapsed.stderr
@@ -0,0 +1,16 @@
+error: manual implementation of `Instant::elapsed`
+  --> $DIR/manual_instant_elapsed.rs:17:20
+   |
+LL |     let duration = Instant::now() - prev_instant;
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `prev_instant.elapsed()`
+   |
+   = note: `-D clippy::manual-instant-elapsed` implied by `-D warnings`
+
+error: manual implementation of `Instant::elapsed`
+  --> $DIR/manual_instant_elapsed.rs:26:5
+   |
+LL |     Instant::now() - *ref_to_instant; // to ensure parens are added correctly
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `(*ref_to_instant).elapsed()`
+
+error: aborting due to 2 previous errors
+