Auto merge of #11829 - J-ZhengLi:issue11438, r=matthiaskrgr

new lint to detect infinite loop

closes: #11438

changelog: add new lint to detect infinite loop

~*I'll change the lint name*~. Should I name it  `infinite_loop` or `infinite_loops` is fine? Ahhhh, English is hard...

clippy_lints/src/declared_lints.rs

@@ -266,6 +266,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::loops::EXPLICIT_INTO_ITER_LOOP_INFO,
     crate::loops::EXPLICIT_ITER_LOOP_INFO,
     crate::loops::FOR_KV_MAP_INFO,
+    crate::loops::INFINITE_LOOP_INFO,
     crate::loops::ITER_NEXT_LOOP_INFO,
     crate::loops::MANUAL_FIND_INFO,
     crate::loops::MANUAL_FLATTEN_INFO,

clippy_lints/src/loops/infinite_loop.rs

@@ -0,0 +1,125 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::{fn_def_id, is_lint_allowed};
+use hir::intravisit::{walk_expr, Visitor};
+use hir::{Expr, ExprKind, FnRetTy, FnSig, Node};
+use rustc_ast::Label;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+
+use super::INFINITE_LOOP;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &Expr<'_>,
+    loop_block: &'tcx hir::Block<'_>,
+    label: Option<Label>,
+) {
+    if is_lint_allowed(cx, INFINITE_LOOP, expr.hir_id) {
+        return;
+    }
+
+    // Skip check if this loop is not in a function/method/closure. (In some weird case)
+    let Some(parent_fn_ret) = get_parent_fn_ret_ty(cx, expr) else {
+        return;
+    };
+    // Or, its parent function is already returning `Never`
+    if matches!(
+        parent_fn_ret,
+        FnRetTy::Return(hir::Ty {
+            kind: hir::TyKind::Never,
+            ..
+        })
+    ) {
+        return;
+    }
+
+    let mut loop_visitor = LoopVisitor {
+        cx,
+        label,
+        is_finite: false,
+        loop_depth: 0,
+    };
+    loop_visitor.visit_block(loop_block);
+
+    let is_finite_loop = loop_visitor.is_finite;
+
+    if !is_finite_loop {
+        span_lint_and_then(cx, INFINITE_LOOP, expr.span, "infinite loop detected", |diag| {
+            if let FnRetTy::DefaultReturn(ret_span) = parent_fn_ret {
+                diag.span_suggestion(
+                    ret_span,
+                    "if this is intentional, consider specifing `!` as function return",
+                    " -> !",
+                    Applicability::MaybeIncorrect,
+                );
+            } else {
+                diag.help("if this is not intended, try adding a `break` or `return` condition in the loop");
+            }
+        });
+    }
+}
+
+fn get_parent_fn_ret_ty<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>) -> Option<FnRetTy<'tcx>> {
+    for (_, parent_node) in cx.tcx.hir().parent_iter(expr.hir_id) {
+        match parent_node {
+            Node::Item(hir::Item {
+                kind: hir::ItemKind::Fn(FnSig { decl, .. }, _, _),
+                ..
+            })
+            | Node::TraitItem(hir::TraitItem {
+                kind: hir::TraitItemKind::Fn(FnSig { decl, .. }, _),
+                ..
+            })
+            | Node::ImplItem(hir::ImplItem {
+                kind: hir::ImplItemKind::Fn(FnSig { decl, .. }, _),
+                ..
+            })
+            | Node::Expr(Expr {
+                kind: ExprKind::Closure(hir::Closure { fn_decl: decl, .. }),
+                ..
+            }) => return Some(decl.output),
+            _ => (),
+        }
+    }
+    None
+}
+
+struct LoopVisitor<'hir, 'tcx> {
+    cx: &'hir LateContext<'tcx>,
+    label: Option<Label>,
+    loop_depth: usize,
+    is_finite: bool,
+}
+
+impl<'hir> Visitor<'hir> for LoopVisitor<'hir, '_> {
+    fn visit_expr(&mut self, ex: &'hir Expr<'_>) {
+        match &ex.kind {
+            ExprKind::Break(hir::Destination { label, .. }, ..) => {
+                // Assuming breaks the loop when `loop_depth` is 0,
+                // as it could only means this `break` breaks current loop or any of its upper loop.
+                // Or, the depth is not zero but the label is matched.
+                if self.loop_depth == 0 || (label.is_some() && *label == self.label) {
+                    self.is_finite = true;
+                }
+            },
+            ExprKind::Ret(..) => self.is_finite = true,
+            ExprKind::Loop(..) => {
+                self.loop_depth += 1;
+                walk_expr(self, ex);
+                self.loop_depth = self.loop_depth.saturating_sub(1);
+            },
+            _ => {
+                // Calls to a function that never return
+                if let Some(did) = fn_def_id(self.cx, ex) {
+                    let fn_ret_ty = self.cx.tcx.fn_sig(did).skip_binder().output().skip_binder();
+                    if fn_ret_ty.is_never() {
+                        self.is_finite = true;
+                        return;
+                    }
+                }
+                walk_expr(self, ex);
+            },
+        }
+    }
+}

clippy_lints/src/loops/mod.rs

@@ -3,6 +3,7 @@ mod explicit_counter_loop;
 mod explicit_into_iter_loop;
 mod explicit_iter_loop;
 mod for_kv_map;
+mod infinite_loop;
 mod iter_next_loop;
 mod manual_find;
 mod manual_flatten;
@@ -635,6 +636,48 @@ declare_clippy_lint! {
     "checking for emptiness of a `Vec` in the loop condition and popping an element in the body"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for infinite loops in a function where the return type is not `!`
+    /// and lint accordingly.
+    ///
+    /// ### Why is this bad?
+    /// A loop should be gently exited somewhere, or at least mark its parent function as
+    /// never return (`!`).
+    ///
+    /// ### Example
+    /// ```no_run,ignore
+    /// fn run_forever() {
+    ///     loop {
+    ///         // do something
+    ///     }
+    /// }
+    /// ```
+    /// If infinite loops are as intended:
+    /// ```no_run,ignore
+    /// fn run_forever() -> ! {
+    ///     loop {
+    ///         // do something
+    ///     }
+    /// }
+    /// ```
+    /// Otherwise add a `break` or `return` condition:
+    /// ```no_run,ignore
+    /// fn run_forever() {
+    ///     loop {
+    ///         // do something
+    ///         if condition {
+    ///             break;
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    #[clippy::version = "1.75.0"]
+    pub INFINITE_LOOP,
+    restriction,
+    "possibly unintended infinite loop"
+}
+
 pub struct Loops {
     msrv: Msrv,
     enforce_iter_loop_reborrow: bool,
@@ -669,6 +712,7 @@ impl_lint_pass!(Loops => [
     MANUAL_FIND,
     MANUAL_WHILE_LET_SOME,
     UNUSED_ENUMERATE_INDEX,
+    INFINITE_LOOP,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Loops {
@@ -707,10 +751,11 @@ impl<'tcx> LateLintPass<'tcx> for Loops {
         // check for `loop { if let {} else break }` that could be `while let`
         // (also matches an explicit "match" instead of "if let")
         // (even if the "match" or "if let" is used for declaration)
-        if let ExprKind::Loop(block, _, LoopSource::Loop, _) = expr.kind {
+        if let ExprKind::Loop(block, label, LoopSource::Loop, _) = expr.kind {
             // also check for empty `loop {}` statements, skipping those in #[panic_handler]
             empty_loop::check(cx, expr, block);
             while_let_loop::check(cx, expr, block);
+            infinite_loop::check(cx, expr, block, label);
         }
 
         while_let_on_iterator::check(cx, expr);