make never_loop applicability more flexible

Author: lapla-cogito

clippy_lints/src/loops/never_loop.rs

@@ -4,11 +4,13 @@ use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::higher::ForLoop;
 use clippy_utils::macros::root_macro_call_first_node;
 use clippy_utils::source::snippet;
+use clippy_utils::visitors::for_each_expr_without_closures;
 use rustc_errors::Applicability;
 use rustc_hir::{Block, Destination, Expr, ExprKind, HirId, InlineAsmOperand, Pat, Stmt, StmtKind, StructTailExpr};
 use rustc_lint::LateContext;
 use rustc_span::{Span, sym};
 use std::iter::once;
+use std::ops::ControlFlow;
 
 pub(super) fn check<'tcx>(
     cx: &LateContext<'tcx>,
@@ -27,14 +29,19 @@ pub(super) fn check<'tcx>(
                     ..
                 }) = for_loop
                 {
-                    // Suggests using an `if let` instead. This is `Unspecified` because the
-                    // loop may (probably) contain `break` statements which would be invalid
-                    // in an `if let`.
+                    let app = if let Some(block_expr) = block.expr
+                        && !is_loop_contains_break_continue(block_expr)
+                    {
+                        Applicability::MachineApplicable
+                    } else {
+                        Applicability::Unspecified
+                    };
+
                     diag.span_suggestion_verbose(
                         for_span.with_hi(iterator.span.hi()),
                         "if you need the first element of the iterator, try writing",
                         for_to_if_let_sugg(cx, iterator, pat),
-                        Applicability::Unspecified,
+                        app,
                     );
                 }
             });
@@ -43,6 +50,14 @@ pub(super) fn check<'tcx>(
     }
 }
 
+fn is_loop_contains_break_continue(expr: &Expr<'_>) -> bool {
+    for_each_expr_without_closures(expr, |e| match e.kind {
+        ExprKind::Break(..) | ExprKind::Continue(..) => ControlFlow::Break(()),
+        _ => ControlFlow::Continue(()),
+    })
+    .is_some()
+}
+
 /// The `never_loop` analysis keeps track of three things:
 ///
 /// * Has any (reachable) code path hit a `continue` of the main loop?

tests/ui/never_loop.rs

@@ -409,6 +409,12 @@ pub fn issue12205() -> Option<()> {
     }
 }
 
+fn no_break_or_continue_loop() {
+    for i in [1, 2, 3].iter().skip(1) {
+        return;
+    }
+}
+
 fn main() {
     test1();
     test2();

tests/ui/never_loop.stderr

@@ -175,5 +175,18 @@ LL | |         unimplemented!("not yet");
 LL | |     }
    | |_____^
 
-error: aborting due to 16 previous errors
+error: this loop never actually loops
+  --> tests/ui/never_loop.rs:413:5
+   |
+LL | /     for i in [1, 2, 3].iter().skip(1) {
+LL | |         return;
+LL | |     }
+   | |_____^
+   |
+help: if you need the first element of the iterator, try writing
+   |
+LL |     if let Some(i) = [1, 2, 3].iter().skip(1).next() {
+   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 17 previous errors