Add iter_once and iter_empty lints

13 files changed, 383 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7ea7e76ee10..94f71eba0a2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3651,11 +3651,13 @@ Released 2018-09-13
 [`items_after_statements`]: https://rust-lang.github.io/rust-clippy/master/index.html#items_after_statements
 [`iter_cloned_collect`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_cloned_collect
 [`iter_count`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_count
+[`iter_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_empty
 [`iter_next_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_next_loop
 [`iter_next_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_next_slice
 [`iter_not_returning_iterator`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_not_returning_iterator
 [`iter_nth`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_nth
 [`iter_nth_zero`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_nth_zero
+[`iter_once`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_once
 [`iter_overeager_cloned`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_overeager_cloned
 [`iter_skip_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_skip_next
 [`iter_with_drain`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_with_drain
diff --git a/clippy_lints/src/iter_once_empty.rs b/clippy_lints/src/iter_once_empty.rs
new file mode 100644
index 00000000000..09896001112
--- /dev/null
+++ b/clippy_lints/src/iter_once_empty.rs
@@ -0,0 +1,164 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet;
+use rustc_ast::ast::{Expr, ExprKind};
+use rustc_errors::Applicability;
+use rustc_lint::{EarlyContext, EarlyLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for usage of:
+    ///
+    /// - `[foo].iter()`
+    /// - `[foo].iter_mut()`
+    /// - `[foo].into_iter()`
+    /// - `Some(foo).iter()`
+    /// - `Some(foo).iter_mut()`
+    /// - `Some(foo).into_iter()`
+    ///
+    /// ### Why is this bad?
+    ///
+    /// It is simpler to use the once function from the standard library:
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// let a = [123].iter();
+    /// let b = Some(123).into_iter();
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// use std::iter;
+    /// let a = iter::once(&123);
+    /// let b = iter::once(123);
+    /// ```
+    ///
+    /// ### Known problems
+    ///
+    /// The type of the resulting iterator might become incompatible with its usage
+    #[clippy::version = "1.64.0"]
+    pub ITER_ONCE,
+    nursery,
+    "Iterator for array of length 1"
+}
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for usage of:
+    ///
+    /// - `[].iter()`
+    /// - `[].iter_mut()`
+    /// - `[].into_iter()`
+    /// - `None.iter()`
+    /// - `None.iter_mut()`
+    /// - `None.into_iter()`
+    ///
+    /// ### Why is this bad?
+    ///
+    /// It is simpler to use the empty function from the standard library:
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// use std::{slice, option};
+    /// let a: slice::Iter<i32> = [].iter();
+    /// let f: option::IntoIter<i32> = None.into_iter();
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// use std::iter;
+    /// let a: iter::Empty<i32> = iter::empty();
+    /// let b: iter::Empty<i32> = iter::empty();
+    /// ```
+    ///
+    /// ### Known problems
+    ///
+    /// The type of the resulting iterator might become incompatible with its usage
+    #[clippy::version = "1.64.0"]
+    pub ITER_EMPTY,
+    nursery,
+    "Iterator for empty array"
+}
+
+declare_lint_pass!(IterOnceEmpty => [ITER_ONCE, ITER_EMPTY]);
+
+impl EarlyLintPass for IterOnceEmpty {
+    fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &Expr) {
+        if expr.span.from_expansion() {
+            // Don't lint match expressions present in
+            // macro_rules! block
+            return;
+        }
+
+        let (method_name, args) = if let ExprKind::MethodCall(seg, args, _) = &expr.kind {
+            (seg.ident.as_str(), args)
+        } else {
+            return;
+        };
+        let arg = if args.len() == 1 {
+            &args[0]
+        } else {
+            return;
+        };
+
+        let item = match &arg.kind {
+            ExprKind::Array(v) if v.len() <= 1 => v.first(),
+            ExprKind::Path(None, p) => {
+                if p.segments.len() == 1 && p.segments[0].ident.name == rustc_span::sym::None {
+                    None
+                } else {
+                    return;
+                }
+            },
+            ExprKind::Call(f, some_args) if some_args.len() == 1 => {
+                if let ExprKind::Path(None, p) = &f.kind {
+                    if p.segments.len() == 1 && p.segments[0].ident.name == rustc_span::sym::Some {
+                        Some(&some_args[0])
+                    } else {
+                        return;
+                    }
+                } else {
+                    return;
+                }
+            },
+            _ => return,
+        };
+
+        if let Some(i) = item {
+            let (sugg, msg) = match method_name {
+                "iter" => (
+                    format!("std::iter::once(&{})", snippet(cx, i.span, "...")),
+                    "this `iter` call can be replaced with std::iter::once",
+                ),
+                "iter_mut" => (
+                    format!("std::iter::once(&mut {})", snippet(cx, i.span, "...")),
+                    "this `iter_mut` call can be replaced with std::iter::once",
+                ),
+                "into_iter" => (
+                    format!("std::iter::once({})", snippet(cx, i.span, "...")),
+                    "this `into_iter` call can be replaced with std::iter::once",
+                ),
+                _ => return,
+            };
+            span_lint_and_sugg(cx, ITER_ONCE, expr.span, msg, "try", sugg, Applicability::Unspecified);
+        } else {
+            let msg = match method_name {
+                "iter" => "this `iter call` can be replaced with std::iter::empty",
+                "iter_mut" => "this `iter_mut` call can be replaced with std::iter::empty",
+                "into_iter" => "this `into_iter` call can be replaced with std::iter::empty",
+                _ => return,
+            };
+            span_lint_and_sugg(
+                cx,
+                ITER_EMPTY,
+                expr.span,
+                msg,
+                "try",
+                "std::iter::empty()".to_string(),
+                Applicability::Unspecified,
+            );
+        }
+    }
+}
diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index d83508a2d6c..49fc3a89527 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -200,6 +200,8 @@ store.register_lints(&[
     invalid_utf8_in_unchecked::INVALID_UTF8_IN_UNCHECKED,
     items_after_statements::ITEMS_AFTER_STATEMENTS,
     iter_not_returning_iterator::ITER_NOT_RETURNING_ITERATOR,
+    iter_once_empty::ITER_EMPTY,
+    iter_once_empty::ITER_ONCE,
     large_const_arrays::LARGE_CONST_ARRAYS,
     large_enum_variant::LARGE_ENUM_VARIANT,
     large_include_file::LARGE_INCLUDE_FILE,
diff --git a/clippy_lints/src/lib.register_nursery.rs b/clippy_lints/src/lib.register_nursery.rs
index 7d731d52dbb..9953aca43ee 100644
--- a/clippy_lints/src/lib.register_nursery.rs
+++ b/clippy_lints/src/lib.register_nursery.rs
@@ -14,6 +14,8 @@ store.register_group(true, "clippy::nursery", Some("clippy_nursery"), vec![
     LintId::of(index_refutable_slice::INDEX_REFUTABLE_SLICE),
     LintId::of(let_if_seq::USELESS_LET_IF_SEQ),
     LintId::of(matches::SIGNIFICANT_DROP_IN_SCRUTINEE),
+    LintId::of(methods::ITER_EMPTY),
+    LintId::of(methods::ITER_ONCE),
     LintId::of(methods::ITER_WITH_DRAIN),
     LintId::of(missing_const_for_fn::MISSING_CONST_FOR_FN),
     LintId::of(mutable_debug_assertion::DEBUG_ASSERT_WITH_MUT_CALL),
diff --git a/clippy_lints/src/lib.register_pedantic.rs b/clippy_lints/src/lib.register_pedantic.rs
index 4250ee055e6..6063fb00a7b 100644
--- a/clippy_lints/src/lib.register_pedantic.rs
+++ b/clippy_lints/src/lib.register_pedantic.rs
@@ -41,6 +41,8 @@ store.register_group(true, "clippy::pedantic", Some("clippy_pedantic"), vec![
     LintId::of(invalid_upcast_comparisons::INVALID_UPCAST_COMPARISONS),
     LintId::of(items_after_statements::ITEMS_AFTER_STATEMENTS),
     LintId::of(iter_not_returning_iterator::ITER_NOT_RETURNING_ITERATOR),
+    LintId::of(iter_once_empty::ITER_EMPTY),
+    LintId::of(iter_once_empty::ITER_ONCE),
     LintId::of(large_stack_arrays::LARGE_STACK_ARRAYS),
     LintId::of(let_underscore::LET_UNDERSCORE_DROP),
     LintId::of(literal_representation::LARGE_DIGIT_GROUPS),
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 88c1a727f8d..78279db9ad5 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -258,6 +258,7 @@ mod invalid_upcast_comparisons;
 mod invalid_utf8_in_unchecked;
 mod items_after_statements;
 mod iter_not_returning_iterator;
+mod iter_once_empty;
 mod large_const_arrays;
 mod large_enum_variant;
 mod large_include_file;
@@ -931,6 +932,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     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));
+    store.register_early_pass(|| Box::new(iter_once_empty::IterOnceEmpty));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 
diff --git a/clippy_utils/src/lib.rs b/clippy_utils/src/lib.rs
index dcfc03475b4..9c58978a070 100644
--- a/clippy_utils/src/lib.rs
+++ b/clippy_utils/src/lib.rs
@@ -484,6 +484,7 @@ pub fn def_path_res(cx: &LateContext<'_>, path: &[&str]) -> Res {
     }
     fn find_primitive<'tcx>(tcx: TyCtxt<'tcx>, name: &str) -> impl Iterator<Item = DefId> + 'tcx {
         let single = |ty| tcx.incoherent_impls(ty).iter().copied();
+        #[allow(clippy::iter_empty)]
         let empty = || [].iter().copied();
         match name {
             "bool" => single(BoolSimplifiedType),
diff --git a/tests/ui/iter_empty.fixed b/tests/ui/iter_empty.fixed
new file mode 100644
index 00000000000..fb7118f0d4a
--- /dev/null
+++ b/tests/ui/iter_empty.fixed
@@ -0,0 +1,32 @@
+// run-rustfix
+#![warn(clippy::iter_empty)]
+#![allow(clippy::iter_next_slice, clippy::redundant_clone)]
+
+fn array() {
+    assert_eq!(std::iter::empty().next(), Option::<i32>::None);
+    assert_eq!(std::iter::empty().next(), Option::<&mut i32>::None);
+    assert_eq!(std::iter::empty().next(), Option::<&i32>::None);
+    assert_eq!(std::iter::empty().next(), Option::<i32>::None);
+    assert_eq!(std::iter::empty().next(), Option::<&mut i32>::None);
+    assert_eq!(std::iter::empty().next(), Option::<&i32>::None);
+
+    // Don't trigger on non-iter methods
+    let _: Option<String> = None.clone();
+    let _: [String; 0] = [].clone();
+}
+
+macro_rules! in_macros {
+    () => {
+        assert_eq!([].into_iter().next(), Option::<i32>::None);
+        assert_eq!([].iter_mut().next(), Option::<&mut i32>::None);
+        assert_eq!([].iter().next(), Option::<&i32>::None);
+        assert_eq!(None.into_iter().next(), Option::<i32>::None);
+        assert_eq!(None.iter_mut().next(), Option::<&mut i32>::None);
+        assert_eq!(None.iter().next(), Option::<&i32>::None);
+    };
+}
+
+fn main() {
+    array();
+    in_macros!();
+}
diff --git a/tests/ui/iter_empty.rs b/tests/ui/iter_empty.rs
new file mode 100644
index 00000000000..bb192fe16d1
--- /dev/null
+++ b/tests/ui/iter_empty.rs
@@ -0,0 +1,32 @@
+// run-rustfix
+#![warn(clippy::iter_empty)]
+#![allow(clippy::iter_next_slice, clippy::redundant_clone)]
+
+fn array() {
+    assert_eq!([].into_iter().next(), Option::<i32>::None);
+    assert_eq!([].iter_mut().next(), Option::<&mut i32>::None);
+    assert_eq!([].iter().next(), Option::<&i32>::None);
+    assert_eq!(None.into_iter().next(), Option::<i32>::None);
+    assert_eq!(None.iter_mut().next(), Option::<&mut i32>::None);
+    assert_eq!(None.iter().next(), Option::<&i32>::None);
+
+    // Don't trigger on non-iter methods
+    let _: Option<String> = None.clone();
+    let _: [String; 0] = [].clone();
+}
+
+macro_rules! in_macros {
+    () => {
+        assert_eq!([].into_iter().next(), Option::<i32>::None);
+        assert_eq!([].iter_mut().next(), Option::<&mut i32>::None);
+        assert_eq!([].iter().next(), Option::<&i32>::None);
+        assert_eq!(None.into_iter().next(), Option::<i32>::None);
+        assert_eq!(None.iter_mut().next(), Option::<&mut i32>::None);
+        assert_eq!(None.iter().next(), Option::<&i32>::None);
+    };
+}
+
+fn main() {
+    array();
+    in_macros!();
+}
diff --git a/tests/ui/iter_empty.stderr b/tests/ui/iter_empty.stderr
new file mode 100644
index 00000000000..f4f06e93b23
--- /dev/null
+++ b/tests/ui/iter_empty.stderr
@@ -0,0 +1,40 @@
+error: this `into_iter` call can be replaced with std::iter::empty
+  --> $DIR/iter_empty.rs:6:16
+   |
+LL |     assert_eq!([].into_iter().next(), Option::<i32>::None);
+   |                ^^^^^^^^^^^^^^ help: try: `std::iter::empty()`
+   |
+   = note: `-D clippy::iter-empty` implied by `-D warnings`
+
+error: this `iter_mut` call can be replaced with std::iter::empty
+  --> $DIR/iter_empty.rs:7:16
+   |
+LL |     assert_eq!([].iter_mut().next(), Option::<&mut i32>::None);
+   |                ^^^^^^^^^^^^^ help: try: `std::iter::empty()`
+
+error: this `iter call` can be replaced with std::iter::empty
+  --> $DIR/iter_empty.rs:8:16
+   |
+LL |     assert_eq!([].iter().next(), Option::<&i32>::None);
+   |                ^^^^^^^^^ help: try: `std::iter::empty()`
+
+error: this `into_iter` call can be replaced with std::iter::empty
+  --> $DIR/iter_empty.rs:9:16
+   |
+LL |     assert_eq!(None.into_iter().next(), Option::<i32>::None);
+   |                ^^^^^^^^^^^^^^^^ help: try: `std::iter::empty()`
+
+error: this `iter_mut` call can be replaced with std::iter::empty
+  --> $DIR/iter_empty.rs:10:16
+   |
+LL |     assert_eq!(None.iter_mut().next(), Option::<&mut i32>::None);
+   |                ^^^^^^^^^^^^^^^ help: try: `std::iter::empty()`
+
+error: this `iter call` can be replaced with std::iter::empty
+  --> $DIR/iter_empty.rs:11:16
+   |
+LL |     assert_eq!(None.iter().next(), Option::<&i32>::None);
+   |                ^^^^^^^^^^^ help: try: `std::iter::empty()`
+
+error: aborting due to 6 previous errors
+
diff --git a/tests/ui/iter_once.fixed b/tests/ui/iter_once.fixed
new file mode 100644
index 00000000000..7247e34d7e8
--- /dev/null
+++ b/tests/ui/iter_once.fixed
@@ -0,0 +1,32 @@
+// run-rustfix
+#![warn(clippy::iter_once)]
+#![allow(clippy::iter_next_slice, clippy::redundant_clone)]
+
+fn array() {
+    assert_eq!(std::iter::once(123).next(), Some(123));
+    assert_eq!(std::iter::once(&mut 123).next(), Some(&mut 123));
+    assert_eq!(std::iter::once(&123).next(), Some(&123));
+    assert_eq!(std::iter::once(123).next(), Some(123));
+    assert_eq!(std::iter::once(&mut 123).next(), Some(&mut 123));
+    assert_eq!(std::iter::once(&123).next(), Some(&123));
+
+    // Don't trigger on non-iter methods
+    let _: Option<String> = Some("test".to_string()).clone();
+    let _: [String; 1] = ["test".to_string()].clone();
+}
+
+macro_rules! in_macros {
+    () => {
+        assert_eq!([123].into_iter().next(), Some(123));
+        assert_eq!([123].iter_mut().next(), Some(&mut 123));
+        assert_eq!([123].iter().next(), Some(&123));
+        assert_eq!(Some(123).into_iter().next(), Some(123));
+        assert_eq!(Some(123).iter_mut().next(), Some(&mut 123));
+        assert_eq!(Some(123).iter().next(), Some(&123));
+    };
+}
+
+fn main() {
+    array();
+    in_macros!();
+}
diff --git a/tests/ui/iter_once.rs b/tests/ui/iter_once.rs
new file mode 100644
index 00000000000..3a2b9c95cc5
--- /dev/null
+++ b/tests/ui/iter_once.rs
@@ -0,0 +1,32 @@
+// run-rustfix
+#![warn(clippy::iter_once)]
+#![allow(clippy::iter_next_slice, clippy::redundant_clone)]
+
+fn array() {
+    assert_eq!([123].into_iter().next(), Some(123));
+    assert_eq!([123].iter_mut().next(), Some(&mut 123));
+    assert_eq!([123].iter().next(), Some(&123));
+    assert_eq!(Some(123).into_iter().next(), Some(123));
+    assert_eq!(Some(123).iter_mut().next(), Some(&mut 123));
+    assert_eq!(Some(123).iter().next(), Some(&123));
+
+    // Don't trigger on non-iter methods
+    let _: Option<String> = Some("test".to_string()).clone();
+    let _: [String; 1] = ["test".to_string()].clone();
+}
+
+macro_rules! in_macros {
+    () => {
+        assert_eq!([123].into_iter().next(), Some(123));
+        assert_eq!([123].iter_mut().next(), Some(&mut 123));
+        assert_eq!([123].iter().next(), Some(&123));
+        assert_eq!(Some(123).into_iter().next(), Some(123));
+        assert_eq!(Some(123).iter_mut().next(), Some(&mut 123));
+        assert_eq!(Some(123).iter().next(), Some(&123));
+    };
+}
+
+fn main() {
+    array();
+    in_macros!();
+}
diff --git a/tests/ui/iter_once.stderr b/tests/ui/iter_once.stderr
new file mode 100644
index 00000000000..d9e8f96f763
--- /dev/null
+++ b/tests/ui/iter_once.stderr
@@ -0,0 +1,40 @@
+error: this `into_iter` call can be replaced with std::iter::once
+  --> $DIR/iter_once.rs:6:16
+   |
+LL |     assert_eq!([123].into_iter().next(), Some(123));
+   |                ^^^^^^^^^^^^^^^^^ help: try: `std::iter::once(123)`
+   |
+   = note: `-D clippy::iter-once` implied by `-D warnings`
+
+error: this `iter_mut` call can be replaced with std::iter::once
+  --> $DIR/iter_once.rs:7:16
+   |
+LL |     assert_eq!([123].iter_mut().next(), Some(&mut 123));
+   |                ^^^^^^^^^^^^^^^^ help: try: `std::iter::once(&mut 123)`
+
+error: this `iter` call can be replaced with std::iter::once
+  --> $DIR/iter_once.rs:8:16
+   |
+LL |     assert_eq!([123].iter().next(), Some(&123));
+   |                ^^^^^^^^^^^^ help: try: `std::iter::once(&123)`
+
+error: this `into_iter` call can be replaced with std::iter::once
+  --> $DIR/iter_once.rs:9:16
+   |
+LL |     assert_eq!(Some(123).into_iter().next(), Some(123));
+   |                ^^^^^^^^^^^^^^^^^^^^^ help: try: `std::iter::once(123)`
+
+error: this `iter_mut` call can be replaced with std::iter::once
+  --> $DIR/iter_once.rs:10:16
+   |
+LL |     assert_eq!(Some(123).iter_mut().next(), Some(&mut 123));
+   |                ^^^^^^^^^^^^^^^^^^^^ help: try: `std::iter::once(&mut 123)`
+
+error: this `iter` call can be replaced with std::iter::once
+  --> $DIR/iter_once.rs:11:16
+   |
+LL |     assert_eq!(Some(123).iter().next(), Some(&123));
+   |                ^^^^^^^^^^^^^^^^ help: try: `std::iter::once(&123)`
+
+error: aborting due to 6 previous errors
+