never_loop: suggest using an if let instead of a for loop

clippy_lints/src/loops/never_loop.rs

@@ -1,13 +1,38 @@
 use super::NEVER_LOOP;
-use clippy_utils::diagnostics::span_lint;
-use rustc_hir::{Block, Expr, ExprKind, HirId, InlineAsmOperand, Stmt, StmtKind};
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::source::{indent_of, reindent_multiline, snippet_opt};
+use rustc_ast::util::parser::PREC_POSTFIX;
+use rustc_errors::Applicability;
+use rustc_hir::{is_range_literal, Block, Expr, ExprKind, HirId, InlineAsmOperand, LoopSource, Stmt, StmtKind};
 use rustc_lint::LateContext;
 use std::iter::{once, Iterator};
 
 pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-    if let ExprKind::Loop(block, _, _, _) = expr.kind {
+    if let ExprKind::Loop(block, _, source, _) = expr.kind {
         match never_loop_block(block, expr.hir_id) {
-            NeverLoopResult::AlwaysBreak => span_lint(cx, NEVER_LOOP, expr.span, "this loop never actually loops"),
+            NeverLoopResult::AlwaysBreak => {
+                span_lint_and_then(cx, NEVER_LOOP, expr.span, "this loop never actually loops", |diag| {
+                    if_chain! {
+                        if let LoopSource::ForLoop = source;
+                        let hir = cx.tcx.hir();
+                        // Refer to the docs for `lower_expr_for` for a
+                        // specification of how `for` loops are desugared.
+                        let parent_arm = hir.get_parent_node(expr.hir_id);
+                        let parent_match = hir.get_parent_node(parent_arm);
+                        if let ExprKind::Match(scrutinee, _, _) = hir.expect_expr(parent_match).kind;
+                        if let ExprKind::Call(_, &[ref iterator]) = scrutinee.kind;
+                        if let Some(sugg) = for_to_if_let_sugg(cx, expr, iterator);
+                        then {
+                            diag.span_suggestion_verbose(
+                                expr.span,
+                                "if you need the first element of the iterator, try writing",
+                                sugg,
+                                Applicability::HasPlaceholders,
+                            );
+                        }
+                    };
+                })
+            },
             NeverLoopResult::MayContinueMainLoop | NeverLoopResult::Otherwise => (),
         }
     }
@@ -170,3 +195,21 @@ fn never_loop_expr_branch<'a, T: Iterator<Item = &'a Expr<'a>>>(e: &mut T, main_
     e.map(|e| never_loop_expr(e, main_loop_id))
         .fold(NeverLoopResult::AlwaysBreak, combine_branches)
 }
+
+fn for_to_if_let_sugg(cx: &LateContext<'_>, loop_expr: &Expr<'_>, iterator: &Expr<'_>) -> Option<String> {
+    // Check whether the iterator expr needs parens to parse as `(iterator).into_iter()`.
+    // Ranges need special casing because they are converted to a struct in HIR.
+    let (open_paren, close_paren) = if iterator.precedence().order() < PREC_POSTFIX || is_range_literal(iterator) {
+        ("(", ")")
+    } else {
+        ("", "")
+    };
+    let iterator_snippet = snippet_opt(cx, iterator.span)?;
+
+    #[rustfmt::skip]
+    let sugg = format!(
+"for {open_paren}{iterator}{close_paren}.into_iter().next() {{
+    /* ... */
+}}", open_paren = open_paren, iterator = iterator_snippet, close_paren = close_paren);
+    Some(reindent_multiline(sugg.into(), true, indent_of(cx, loop_expr.span)).into())
+}

tests/ui/never_loop.stderr

@@ -75,6 +75,13 @@ LL | |             _ => return,
 LL | |         }
 LL | |     }
    | |_____^
+   |
+help: if you need the first element of the iterator, try writing
+   |
+LL |     for (0..10).into_iter().next() {
+LL |         /* ... */
+LL |     }
+   |
 
 error: this loop never actually loops
   --> $DIR/never_loop.rs:157:5