Initial impl of unnecessary_first_then_check

Fixes #11212

Author: lukaslueg

CHANGELOG.md

@@ -6021,6 +6021,7 @@ Released 2018-09-13
 [`unnecessary_fallible_conversions`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_fallible_conversions
 [`unnecessary_filter_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_filter_map
 [`unnecessary_find_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_find_map
+[`unnecessary_first_then_check`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_first_then_check
 [`unnecessary_fold`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_fold
 [`unnecessary_get_then_check`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_get_then_check
 [`unnecessary_join`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_join

clippy_lints/src/declared_lints.rs

@@ -467,6 +467,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
  crate::methods::UNNECESSARY_FALLIBLE_CONVERSIONS_INFO,
  crate::methods::UNNECESSARY_FILTER_MAP_INFO,
  crate::methods::UNNECESSARY_FIND_MAP_INFO,
+ crate::methods::UNNECESSARY_FIRST_THEN_CHECK_INFO,
  crate::methods::UNNECESSARY_FOLD_INFO,
  crate::methods::UNNECESSARY_GET_THEN_CHECK_INFO,
  crate::methods::UNNECESSARY_JOIN_INFO,

clippy_lints/src/methods/mod.rs

@@ -111,6 +111,7 @@ mod uninit_assumed_init;
 mod unit_hash;
 mod unnecessary_fallible_conversions;
 mod unnecessary_filter_map;
+mod unnecessary_first_then_check;
 mod unnecessary_fold;
 mod unnecessary_get_then_check;
 mod unnecessary_iter_cloned;
@@ -4137,6 +4138,34 @@ declare_clippy_lint! {
  "use of `map` returning the original item"
 }
 
+declare_clippy_lint! {
+ /// ### What it does
+ /// Checks the usage of `.first().is_some()` or `.first().is_none()` to check if a slice is
+ /// empty.
+ ///
+ /// ### Why is this bad?
+ /// Using `.is_empty()` is shorter and better communicates the intention.
+ ///
+ /// ### Example
+ /// ```no_run
+ /// let v = vec![1, 2, 3];
+ /// if v.first().is_none() {
+ /// // The vector is empty...
+ /// }
+ /// ```
+ /// Use instead:
+ /// ```no_run
+ /// let v = vec![1, 2, 3];
+ /// if v.is_empty() {
+ /// // The vector is empty...
+ /// }
+ /// ```
+ #[clippy::version = "1.83.0"]
+ pub UNNECESSARY_FIRST_THEN_CHECK,
+ complexity,
+ "calling `.first().is_some()` or `.first().is_none()` instead of `.is_empty()`"
+}
+
 pub struct Methods {
  avoid_breaking_exported_api: bool,
  msrv: Msrv,
@@ -4294,6 +4323,7 @@ impl_lint_pass!(Methods => [
  UNNECESSARY_RESULT_MAP_OR_ELSE,
  MANUAL_C_STR_LITERALS,
  UNNECESSARY_GET_THEN_CHECK,
+ UNNECESSARY_FIRST_THEN_CHECK,
  NEEDLESS_CHARACTER_ITERATION,
  MANUAL_INSPECT,
  UNNECESSARY_MIN_OR_MAX,
@@ -5066,6 +5096,9 @@ fn check_is_some_is_none(cx: &LateContext<'_>, expr: &Expr<'_>, recv: &Expr<'_>,
  Some(("get", f_recv, [arg], _, _)) => {
  unnecessary_get_then_check::check(cx, call_span, recv, f_recv, arg, is_some);
  },
+ Some(("first", f_recv, [], _, _)) => {
+ unnecessary_first_then_check::check(cx, call_span, recv, f_recv, is_some);
+ },
  _ => {},
  }
 }

clippy_lints/src/methods/unnecessary_first_then_check.rs

@@ -0,0 +1,56 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::SpanRangeExt;
+
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_span::Span;
+
+use super::UNNECESSARY_FIRST_THEN_CHECK;
+
+pub(super) fn check(
+ cx: &LateContext<'_>,
+ call_span: Span,
+ first_call: &Expr<'_>,
+ first_caller: &Expr<'_>,
+ is_some: bool,
+) {
+ if !cx
+ .typeck_results()
+ .expr_ty_adjusted(first_caller)
+ .peel_refs()
+ .is_slice()
+ {
+ return;
+ }
+
+ let ExprKind::MethodCall(_, _, _, first_call_span) = first_call.kind else {
+ return;
+ };
+
+ let both_calls_span = first_call_span.with_hi(call_span.hi());
+ if let Some(both_calls_snippet) = both_calls_span.get_source_text(cx)
+ && let Some(first_caller_snippet) = first_caller.span.get_source_text(cx)
+ {
+ let (sugg_span, suggestion) = if is_some {
+ (
+ first_caller.span.with_hi(call_span.hi()),
+ format!("!{first_caller_snippet}.is_empty()"),
+ )
+ } else {
+ (both_calls_span, "is_empty()".to_owned())
+ };
+ span_lint_and_sugg(
+ cx,
+ UNNECESSARY_FIRST_THEN_CHECK,
+ sugg_span,
+ format!(
+ "unnecessary use of `{both_calls_snippet}` to check if slice {}",
+ if is_some { "is not empty" } else { "is empty" }
+ ),
+ "replace this with",
+ suggestion,
+ Applicability::MaybeIncorrect,
+ );
+ }
+}

tests/ui/unnecessary_first_then_check.fixed

@@ -0,0 +1,22 @@
+#![warn(clippy::unnecessary_first_then_check)]
+#![allow(clippy::useless_vec, clippy::const_is_empty)]
+
+fn main() {
+ let s = [1, 2, 3];
+ let _: bool = !s.is_empty();
+ let _: bool = s.is_empty();
+
+ let v = vec![1, 2, 3];
+ let _: bool = !v.is_empty();
+
+ let n = [[1, 2, 3], [4, 5, 6]];
+ let _: bool = !n[0].is_empty();
+ let _: bool = n[0].is_empty();
+
+ struct Foo {
+ bar: &'static [i32],
+ }
+ let f = [Foo { bar: &[] }];
+ let _: bool = !f[0].bar.is_empty();
+ let _: bool = f[0].bar.is_empty();
+}

tests/ui/unnecessary_first_then_check.rs

@@ -0,0 +1,22 @@
+#![warn(clippy::unnecessary_first_then_check)]
+#![allow(clippy::useless_vec, clippy::const_is_empty)]
+
+fn main() {
+ let s = [1, 2, 3];
+ let _: bool = s.first().is_some();
+ let _: bool = s.first().is_none();
+
+ let v = vec![1, 2, 3];
+ let _: bool = v.first().is_some();
+
+ let n = [[1, 2, 3], [4, 5, 6]];
+ let _: bool = n[0].first().is_some();
+ let _: bool = n[0].first().is_none();
+
+ struct Foo {
+ bar: &'static [i32],
+ }
+ let f = [Foo { bar: &[] }];
+ let _: bool = f[0].bar.first().is_some();
+ let _: bool = f[0].bar.first().is_none();
+}

tests/ui/unnecessary_first_then_check.stderr

@@ -0,0 +1,47 @@
+error: unnecessary use of `first().is_some()` to check if slice is not empty
+ --> tests/ui/unnecessary_first_then_check.rs:6:19
+ |
+LL | let _: bool = s.first().is_some();
+ | ^^^^^^^^^^^^^^^^^^^ help: replace this with: `!s.is_empty()`
+ |
+ = note: `-D clippy::unnecessary-first-then-check` implied by `-D warnings`
+ = help: to override `-D warnings` add `#[allow(clippy::unnecessary_first_then_check)]`
+
+error: unnecessary use of `first().is_none()` to check if slice is empty
+ --> tests/ui/unnecessary_first_then_check.rs:7:21
+ |
+LL | let _: bool = s.first().is_none();
+ | ^^^^^^^^^^^^^^^^^ help: replace this with: `is_empty()`
+
+error: unnecessary use of `first().is_some()` to check if slice is not empty
+ --> tests/ui/unnecessary_first_then_check.rs:10:19
+ |
+LL | let _: bool = v.first().is_some();
+ | ^^^^^^^^^^^^^^^^^^^ help: replace this with: `!v.is_empty()`
+
+error: unnecessary use of `first().is_some()` to check if slice is not empty
+ --> tests/ui/unnecessary_first_then_check.rs:13:19
+ |
+LL | let _: bool = n[0].first().is_some();
+ | ^^^^^^^^^^^^^^^^^^^^^^ help: replace this with: `!n[0].is_empty()`
+
+error: unnecessary use of `first().is_none()` to check if slice is empty
+ --> tests/ui/unnecessary_first_then_check.rs:14:24
+ |
+LL | let _: bool = n[0].first().is_none();
+ | ^^^^^^^^^^^^^^^^^ help: replace this with: `is_empty()`
+
+error: unnecessary use of `first().is_some()` to check if slice is not empty
+ --> tests/ui/unnecessary_first_then_check.rs:20:19
+ |
+LL | let _: bool = f[0].bar.first().is_some();
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace this with: `!f[0].bar.is_empty()`
+
+error: unnecessary use of `first().is_none()` to check if slice is empty
+ --> tests/ui/unnecessary_first_then_check.rs:21:28
+ |
+LL | let _: bool = f[0].bar.first().is_none();
+ | ^^^^^^^^^^^^^^^^^ help: replace this with: `is_empty()`
+
+error: aborting due to 7 previous errors
+