Prevent some false positives

clippy_lints/src/methods/iter_once_empty.rs

@@ -1,11 +1,10 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::is_lang_ctor;
-use clippy_utils::is_no_std_crate;
 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};
+use rustc_hir::{Expr, ExprKind, Node};
 use rustc_lint::LateContext;
 
 use super::{ITER_EMPTY, ITER_ONCE};
@@ -56,6 +55,24 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, method_name:
         _ => 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({}{})",

clippy_lints/src/types/mod.rs

@@ -490,7 +490,6 @@ fn check_ty(&mut self, cx: &LateContext<'_>, hir_ty: &hir::Ty<'_>, mut context:
                         }
                     }
                 }
-                #[allow(clippy::iter_empty)]
                 match *qpath {
                     QPath::Resolved(Some(ty), p) => {
                         context.is_nested_call = true;

clippy_utils/src/lib.rs

@@ -484,7 +484,6 @@ fn item_child_by_name(tcx: TyCtxt<'_>, def_id: DefId, name: &str) -> Option<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),

tests/ui/iter_empty.fixed

@@ -13,6 +13,14 @@ fn array() {
     // Don't trigger on non-iter methods
     let _: Option<String> = None.clone();
     let _: [String; 0] = [].clone();
+
+    // Don't trigger on match or if branches
+    let _ = match 123 {
+        123 => [].iter(),
+        _ => ["test"].iter(),
+    };
+
+    let _ = if false { ["test"].iter() } else { [].iter() };
 }
 
 macro_rules! in_macros {

tests/ui/iter_empty.rs

@@ -13,6 +13,14 @@ fn array() {
     // Don't trigger on non-iter methods
     let _: Option<String> = None.clone();
     let _: [String; 0] = [].clone();
+
+    // Don't trigger on match or if branches
+    let _ = match 123 {
+        123 => [].iter(),
+        _ => ["test"].iter(),
+    };
+
+    let _ = if false { ["test"].iter() } else { [].iter() };
 }
 
 macro_rules! in_macros {

tests/ui/iter_once.fixed

@@ -13,6 +13,14 @@ fn array() {
     // Don't trigger on non-iter methods
     let _: Option<String> = Some("test".to_string()).clone();
     let _: [String; 1] = ["test".to_string()].clone();
+
+    // Don't trigger on match or if branches
+    let _ = match 123 {
+        123 => [].iter(),
+        _ => ["test"].iter(),
+    };
+
+    let _ = if false { ["test"].iter() } else { [].iter() };
 }
 
 macro_rules! in_macros {

tests/ui/iter_once.rs

@@ -13,6 +13,14 @@ fn array() {
     // Don't trigger on non-iter methods
     let _: Option<String> = Some("test".to_string()).clone();
     let _: [String; 1] = ["test".to_string()].clone();
+
+    // Don't trigger on match or if branches
+    let _ = match 123 {
+        123 => [].iter(),
+        _ => ["test"].iter(),
+    };
+
+    let _ = if false { ["test"].iter() } else { [].iter() };
 }
 
 macro_rules! in_macros {