[unconditional_recursion]: catch From -> Into -> From

Author: y21

clippy_lints/src/unconditional_recursion.rs

@@ -1,5 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::{expr_or_init, get_trait_def_id, path_def_id};
+use clippy_utils::{expr_or_init, fn_def_id, get_trait_def_id, path_def_id};
+use itertools::Itertools;
 use rustc_ast::BinOpKind;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir as hir;
@@ -19,11 +20,11 @@ use rustc_trait_selection::traits::error_reporting::suggestions::ReturnsVisitor;
 
 declare_clippy_lint! {
     /// ### What it does
-    /// Checks that there isn't an infinite recursion in `PartialEq` trait
-    /// implementation.
+    /// Checks that there isn't an infinite recursion in trait
+    /// implementations.
     ///
     /// ### Why is this bad?
-    /// This is a hard to find infinite recursion which will crashing any code
+    /// This is a hard to find infinite recursion which will crash any code
     /// using it.
     ///
     /// ### Example
@@ -201,7 +202,6 @@ fn check_partial_eq(cx: &LateContext<'_>, method_span: Span, method_def_id: Loca
     }
 }
 
-#[allow(clippy::unnecessary_def_path)]
 fn check_to_string(cx: &LateContext<'_>, method_span: Span, method_def_id: LocalDefId, name: Ident, expr: &Expr<'_>) {
     let args = cx
         .tcx
@@ -224,7 +224,7 @@ fn check_to_string(cx: &LateContext<'_>, method_span: Span, method_def_id: Local
         && let Some(trait_) = impl_.of_trait
         && let Some(trait_def_id) = trait_.trait_def_id()
         // The trait is `ToString`.
-        && Some(trait_def_id) == get_trait_def_id(cx, &["alloc", "string", "ToString"])
+        && cx.tcx.is_diagnostic_item(sym::ToString, trait_def_id)
     {
         let is_bad = match expr.kind {
             ExprKind::MethodCall(segment, _receiver, &[_arg], _) if segment.ident.name == name.name => {
@@ -291,7 +291,6 @@ where
         self.map
     }
 
-    #[allow(clippy::unnecessary_def_path)]
     fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
         if self.found_default_call {
             return;
@@ -303,7 +302,7 @@ where
             && is_default_method_on_current_ty(self.cx.tcx, qpath, self.implemented_ty_id)
             && let Some(method_def_id) = path_def_id(self.cx, f)
             && let Some(trait_def_id) = self.cx.tcx.trait_of_item(method_def_id)
-            && Some(trait_def_id) == get_trait_def_id(self.cx, &["core", "default", "Default"])
+            && self.cx.tcx.is_diagnostic_item(sym::Default, trait_def_id)
         {
             self.found_default_call = true;
             span_error(self.cx, self.method_span, expr);
@@ -394,6 +393,39 @@ impl UnconditionalRecursion {
     }
 }
 
+fn check_from(cx: &LateContext<'_>, method_span: Span, method_def_id: LocalDefId, expr: &Expr<'_>) {
+    let Some(sig) = cx
+        .typeck_results()
+        .liberated_fn_sigs()
+        .get(cx.tcx.local_def_id_to_hir_id(method_def_id))
+    else {
+        return;
+    };
+    let typeck = cx.typeck_results();
+
+    // Check if we are calling `Into::into` where the node args match with our `From::from` signature:
+    // From::from signature: fn(S1) -> S2
+    // <S1 as Into<S2>>::into(s1), node_args=[S1, S2]
+    // If they do match, then it must mean that it is the blanket impl,
+    // which calls back into our `From::from` again (`Into` is not specializable).
+    // rustc's unconditional_recursion already catches calling `From::from` directly
+    if let Some(fn_def_id) = fn_def_id(cx, expr)
+        && let node_args = match expr.kind {
+            ExprKind::MethodCall(..) => typeck.node_args(expr.hir_id),
+            ExprKind::Call(callee, _) => typeck.node_args(callee.hir_id),
+            _ => return,
+        }
+        && let Some((s1, s2)) = node_args.iter().filter_map(ty::GenericArg::as_type).collect_tuple()
+        && let Some(trait_def_id) = cx.tcx.trait_of_item(fn_def_id)
+        && cx.tcx.is_diagnostic_item(sym::Into, trait_def_id)
+        && get_impl_trait_def_id(cx, method_def_id) == cx.tcx.get_diagnostic_item(sym::From)
+        && s1 == sig.inputs()[0]
+        && s2 == sig.output()
+    {
+        span_error(cx, method_span, expr);
+    }
+}
+
 impl<'tcx> LateLintPass<'tcx> for UnconditionalRecursion {
     fn check_fn(
         &mut self,
@@ -410,10 +442,11 @@ impl<'tcx> LateLintPass<'tcx> for UnconditionalRecursion {
             // Doesn't have a conditional return.
             && !has_conditional_return(body, expr)
         {
-            if name.name == sym::eq || name.name == sym::ne {
-                check_partial_eq(cx, method_span, method_def_id, name, expr);
-            } else if name.name == sym::to_string {
-                check_to_string(cx, method_span, method_def_id, name, expr);
+            match name.name {
+                sym::eq | sym::ne => check_partial_eq(cx, method_span, method_def_id, name, expr),
+                sym::to_string => check_to_string(cx, method_span, method_def_id, name, expr),
+                sym::from => check_from(cx, method_span, method_def_id, expr),
+                _ => {},
             }
             self.check_default_new(cx, decl, body, method_span, method_def_id);
         }

tests/ui/unconditional_recursion.rs

@@ -1,7 +1,11 @@
 //@no-rustfix
 
 #![warn(clippy::unconditional_recursion)]
-#![allow(clippy::partialeq_ne_impl, clippy::default_constructed_unit_structs)]
+#![allow(
+    clippy::partialeq_ne_impl,
+    clippy::default_constructed_unit_structs,
+    clippy::only_used_in_recursion
+)]
 
 enum Foo {
     A,
@@ -350,4 +354,22 @@ mod issue12154 {
     }
 }
 
+// From::from -> Into::into -> From::from
+struct BadFromTy1<'a>(&'a ());
+struct BadIntoTy1<'b>(&'b ());
+impl<'a> From<BadFromTy1<'a>> for BadIntoTy1<'static> {
+    fn from(f: BadFromTy1<'a>) -> Self {
+        f.into()
+    }
+}
+
+// From::from -> From::from
+struct BadFromTy2<'a>(&'a ());
+struct BadIntoTy2<'b>(&'b ());
+impl<'a> From<BadFromTy2<'a>> for BadIntoTy2<'static> {
+    fn from(f: BadFromTy2<'a>) -> Self {
+        Into::into(f)
+    }
+}
+
 fn main() {}