Auto merge of #9187 - sgued:iter-once, r=flip1995

Add lint recommending using `std::iter::once` and `std::iter::empty`

```
changelog: [`iter_once`]: add new lint
changelog: [`iter_empty`]: add new lint
```

fixes #9186

- \[ ] 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`

[lint_naming]: https://rust-lang.github.io/rfcs/0344-conventions-galore.html#lints

The lint doesn't really follow the naming conventions. I don't have any better idea so I'm open to suggestions.

clippy_lints/src/dereference.rs

@@ -760,7 +760,7 @@ fn walk_parents<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> (Position, &
                                     .and_then(|subs| subs.get(1..))
                                 {
                                     Some(subs) => cx.tcx.mk_substs(subs.iter().copied()),
-                                    None => cx.tcx.mk_substs([].iter()),
+                                    None => cx.tcx.mk_substs(std::iter::empty::<ty::subst::GenericArg<'_>>()),
                                 } && let impl_ty = if cx.tcx.fn_sig(id).skip_binder().inputs()[0].is_ref() {
                                     // Trait methods taking `&self`
                                     sub_ty

clippy_lints/src/derive.rs

@@ -516,7 +516,10 @@ fn param_env_for_derived_eq(tcx: TyCtxt<'_>, did: DefId, eq_trait_id: DefId) ->
         tcx.mk_predicates(ty_predicates.iter().map(|&(p, _)| p).chain(
             params.iter().filter(|&&(_, needs_eq)| needs_eq).map(|&(param, _)| {
                 tcx.mk_predicate(Binder::dummy(PredicateKind::Trait(TraitPredicate {
-                    trait_ref: TraitRef::new(eq_trait_id, tcx.mk_substs([tcx.mk_param_from_def(param)].into_iter())),
+                    trait_ref: TraitRef::new(
+                        eq_trait_id,
+                        tcx.mk_substs(std::iter::once(tcx.mk_param_from_def(param))),
+                    ),
                     constness: BoundConstness::NotConst,
                     polarity: ImplPolarity::Positive,
                 })))

clippy_lints/src/lib.register_lints.rs

@@ -315,6 +315,8 @@ store.register_lints(&[
     methods::ITER_NEXT_SLICE,
     methods::ITER_NTH,
     methods::ITER_NTH_ZERO,
+    methods::ITER_ON_EMPTY_COLLECTIONS,
+    methods::ITER_ON_SINGLE_ITEMS,
     methods::ITER_OVEREAGER_CLONED,
     methods::ITER_SKIP_NEXT,
     methods::ITER_WITH_DRAIN,

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_ON_EMPTY_COLLECTIONS),
+    LintId::of(methods::ITER_ON_SINGLE_ITEMS),
     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),

clippy_lints/src/methods/iter_on_single_or_empty_collections.rs

@@ -0,0 +1,107 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet;
+use clippy_utils::{get_expr_use_or_unification_node, is_lang_ctor, is_no_std_crate};
+
+use rustc_errors::Applicability;
+use rustc_hir::LangItem::{OptionNone, OptionSome};
+use rustc_hir::{Expr, ExprKind, Node};
+use rustc_lint::LateContext;
+
+use super::{ITER_ON_EMPTY_COLLECTIONS, ITER_ON_SINGLE_ITEMS};
+
+enum IterType {
+    Iter,
+    IterMut,
+    IntoIter,
+}
+
+impl IterType {
+    fn ref_prefix(&self) -> &'static str {
+        match self {
+            Self::Iter => "&",
+            Self::IterMut => "&mut ",
+            Self::IntoIter => "",
+        }
+    }
+}
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, method_name: &str, recv: &Expr<'_>) {
+    let item = match &recv.kind {
+        ExprKind::Array(v) if v.len() <= 1 => v.first(),
+        ExprKind::Path(p) => {
+            if is_lang_ctor(cx, p, OptionNone) {
+                None
+            } else {
+                return;
+            }
+        },
+        ExprKind::Call(f, some_args) if some_args.len() == 1 => {
+            if let ExprKind::Path(p) = &f.kind {
+                if is_lang_ctor(cx, p, OptionSome) {
+                    Some(&some_args[0])
+                } else {
+                    return;
+                }
+            } else {
+                return;
+            }
+        },
+        _ => return,
+    };
+    let iter_type = match method_name {
+        "iter" => IterType::Iter,
+        "iter_mut" => IterType::IterMut,
+        "into_iter" => IterType::IntoIter,
+        _ => return,
+    };
+
+    let is_unified = match get_expr_use_or_unification_node(cx.tcx, expr) {
+        Some((Node::Expr(parent), child_id)) => match parent.kind {
+            ExprKind::If(e, _, _) | ExprKind::Match(e, _, _) if e.hir_id == child_id => false,
+            ExprKind::If(_, _, _)
+            | ExprKind::Match(_, _, _)
+            | ExprKind::Closure(_)
+            | ExprKind::Ret(_)
+            | ExprKind::Break(_, _) => true,
+            _ => false,
+        },
+        Some((Node::Stmt(_) | Node::Local(_), _)) => false,
+        _ => true,
+    };
+
+    if is_unified {
+        return;
+    }
+
+    if let Some(i) = item {
+        let sugg = format!(
+            "{}::iter::once({}{})",
+            if is_no_std_crate(cx) { "core" } else { "std" },
+            iter_type.ref_prefix(),
+            snippet(cx, i.span, "...")
+        );
+        span_lint_and_sugg(
+            cx,
+            ITER_ON_SINGLE_ITEMS,
+            expr.span,
+            &format!("`{method_name}` call on a collection with only one item"),
+            "try",
+            sugg,
+            Applicability::MaybeIncorrect,
+        );
+    } else {
+        span_lint_and_sugg(
+            cx,
+            ITER_ON_EMPTY_COLLECTIONS,
+            expr.span,
+            &format!("`{method_name}` call on an empty collection"),
+            "try",
+            if is_no_std_crate(cx) {
+                "core::iter::empty()".to_string()
+            } else {
+                "std::iter::empty()".to_string()
+            },
+            Applicability::MaybeIncorrect,
+        );
+    }
+}

clippy_lints/src/methods/mod.rs

@@ -33,6 +33,7 @@ mod iter_count;
 mod iter_next_slice;
 mod iter_nth;
 mod iter_nth_zero;
+mod iter_on_single_or_empty_collections;
 mod iter_overeager_cloned;
 mod iter_skip_next;
 mod iter_with_drain;
@@ -2304,6 +2305,69 @@ declare_clippy_lint! {
     more clearly with `if .. else ..`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for calls to `iter`, `iter_mut` or `into_iter` on collections containing a single item
+    ///
+    /// ### 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_ON_SINGLE_ITEMS,
+    nursery,
+    "Iterator for array of length 1"
+}
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for calls to `iter`, `iter_mut` or `into_iter` on empty collections
+    ///
+    /// ### 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_ON_EMPTY_COLLECTIONS,
+    nursery,
+    "Iterator for empty array"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Option<RustcVersion>,
@@ -2406,6 +2470,8 @@ impl_lint_pass!(Methods => [
     NEEDLESS_OPTION_TAKE,
     NO_EFFECT_REPLACE,
     OBFUSCATED_IF_ELSE,
+    ITER_ON_SINGLE_ITEMS,
+    ITER_ON_EMPTY_COLLECTIONS
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -2708,6 +2774,9 @@ impl Methods {
                 ("is_digit", [radix]) => is_digit_ascii_radix::check(cx, expr, recv, radix, self.msrv),
                 ("is_none", []) => check_is_some_is_none(cx, expr, recv, false),
                 ("is_some", []) => check_is_some_is_none(cx, expr, recv, true),
+                ("iter" | "iter_mut" | "into_iter", []) => {
+                    iter_on_single_or_empty_collections::check(cx, expr, name, recv);
+                },
                 ("join", [join_arg]) => {
                     if let Some(("collect", _, span)) = method_call(recv) {
                         unnecessary_join::check(cx, expr, recv, join_arg, span);

clippy_lints/src/redundant_slicing.rs

@@ -11,6 +11,8 @@ use rustc_middle::ty::adjustment::{Adjust, AutoBorrow, AutoBorrowMutability};
 use rustc_middle::ty::subst::GenericArg;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
+use std::iter;
+
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for redundant slicing expressions which use the full range, and
@@ -134,7 +136,7 @@ impl<'tcx> LateLintPass<'tcx> for RedundantSlicing {
                 } else if let Some(target_id) = cx.tcx.lang_items().deref_target() {
                     if let Ok(deref_ty) = cx.tcx.try_normalize_erasing_regions(
                         cx.param_env,
-                        cx.tcx.mk_projection(target_id, cx.tcx.mk_substs([GenericArg::from(indexed_ty)].into_iter())),
+                        cx.tcx.mk_projection(target_id, cx.tcx.mk_substs(iter::once(GenericArg::from(indexed_ty)))),
                     ) {
                         if deref_ty == expr_ty {
                             let snip = snippet_with_context(cx, indexed.span, ctxt, "..", &mut app).0;

clippy_lints/src/utils/conf.rs

@@ -350,7 +350,7 @@ define_Conf! {
     /// Lint: DISALLOWED_SCRIPT_IDENTS.
     ///
     /// The list of unicode scripts allowed to be used in the scope.
-    (allowed_scripts: Vec<String> = ["Latin"].iter().map(ToString::to_string).collect()),
+    (allowed_scripts: Vec<String> = vec!["Latin".to_string()]),
     /// Lint: NON_SEND_FIELDS_IN_SEND_TY.
     ///
     /// Whether to apply the raw pointer heuristic to determine if a type is `Send`.

clippy_lints/src/utils/internal_lints/metadata_collector.rs

@@ -797,7 +797,7 @@ fn get_lint_group_and_level_or_lint(
     let result = cx.lint_store.check_lint_name(
         lint_name,
         Some(sym::clippy),
-        &[Ident::with_dummy_span(sym::clippy)].into_iter().collect(),
+        &std::iter::once(Ident::with_dummy_span(sym::clippy)).collect(),
     );
     if let CheckLintNameResult::Tool(Ok(lint_lst)) = result {
         if let Some(group) = get_lint_group(cx, lint_lst[0]) {