Auto merge of #12488 - Jacherr:issue-11525, r=llogiq

Disable `indexing_slicing` for custom Index impls

Fixes https://github.com/rust-lang/rust-clippy/issues/11525

Disables `indexing_slicing` for custom Index impls, specifically any implementations that also do not have a `get` method anywhere along the deref chain (so, for example, it still lints on Vec, which has its `get` method as part of the deref chain).

Thanks `@y21` for pointing me in the right direction with a couple of handy util functions for deref chain and inherent methods, saved a headache there!

changelog: FP: Disable `indexing_slicing` for custom Index impls

clippy_lints/src/indexing_slicing.rs

@@ -3,11 +3,13 @@
 use clippy_utils::consts::{constant, Constant};
 use clippy_utils::diagnostics::{span_lint, span_lint_and_then};
 use clippy_utils::higher;
+use clippy_utils::ty::{deref_chain, get_adt_inherent_method};
 use rustc_ast::ast::RangeLimits;
 use rustc_hir::{Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::ty;
+use rustc_middle::ty::{self, Ty};
 use rustc_session::impl_lint_pass;
+use rustc_span::sym;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -104,7 +106,15 @@ impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
             return;
         }
 
-        if let ExprKind::Index(array, index, _) = &expr.kind {
+        if let ExprKind::Index(array, index, _) = &expr.kind
+            && let expr_ty = cx.typeck_results().expr_ty(array)
+            && let mut deref = deref_chain(cx, expr_ty)
+            && deref.any(|l| {
+                l.peel_refs().is_slice()
+                    || l.peel_refs().is_array()
+                    || ty_has_applicable_get_function(cx, l.peel_refs(), expr_ty, expr)
+            })
+        {
             let note = "the suggestion might not be applicable in constant blocks";
             let ty = cx.typeck_results().expr_ty(array).peel_refs();
             if let Some(range) = higher::Range::hir(index) {
@@ -231,3 +241,33 @@ fn to_const_range(cx: &LateContext<'_>, range: higher::Range<'_>, array_size: u1
 
     (start, end)
 }
+
+/// Checks if the output Ty of the `get` method on this Ty (if any) matches the Ty returned by the
+/// indexing operation (if any).
+fn ty_has_applicable_get_function<'tcx>(
+    cx: &LateContext<'tcx>,
+    ty: Ty<'tcx>,
+    array_ty: Ty<'tcx>,
+    index_expr: &Expr<'_>,
+) -> bool {
+    if let ty::Adt(_, _) = array_ty.kind()
+        && let Some(get_output_ty) = get_adt_inherent_method(cx, ty, sym!(get)).map(|m| {
+            cx.tcx
+                .fn_sig(m.def_id)
+                .skip_binder()
+                .output()
+                .skip_binder()
+        })
+        && let ty::Adt(def, args) = get_output_ty.kind()
+        && cx.tcx.is_diagnostic_item(sym::Option, def.0.did)
+        && let Some(option_generic_param) = args.first()
+        && let generic_ty = option_generic_param.expect_ty().peel_refs()
+        // FIXME: ideally this would handle type params and projections properly, for now just assume it's the same type
+        && (cx.typeck_results().expr_ty(index_expr).peel_refs() == generic_ty.peel_refs()
+            || matches!(generic_ty.peel_refs().kind(), ty::Param(_) | ty::Alias(_, _)))
+    {
+        true
+    } else {
+        false
+    }
+}

clippy_lints/src/iter_without_into_iter.rs

@@ -1,7 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::get_parent_as_impl;
 use clippy_utils::source::snippet;
-use clippy_utils::ty::{implements_trait, make_normalized_projection};
+use clippy_utils::ty::{deref_chain, get_adt_inherent_method, implements_trait, make_normalized_projection};
 use rustc_ast::Mutability;
 use rustc_errors::Applicability;
 use rustc_hir::{FnRetTy, ImplItemKind, ImplicitSelfKind, ItemKind, TyKind};
@@ -9,8 +9,7 @@ use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::{self, Ty};
 use rustc_session::declare_lint_pass;
-use rustc_span::{sym, Symbol};
-use std::iter;
+use rustc_span::sym;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -124,33 +123,6 @@ fn is_ty_exported(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
         .is_some_and(|did| cx.effective_visibilities.is_exported(did))
 }
 
-/// Returns the deref chain of a type, starting with the type itself.
-fn deref_chain<'cx, 'tcx>(cx: &'cx LateContext<'tcx>, ty: Ty<'tcx>) -> impl Iterator<Item = Ty<'tcx>> + 'cx {
-    iter::successors(Some(ty), |&ty| {
-        if let Some(deref_did) = cx.tcx.lang_items().deref_trait()
-            && implements_trait(cx, ty, deref_did, &[])
-        {
-            make_normalized_projection(cx.tcx, cx.param_env, deref_did, sym::Target, [ty])
-        } else {
-            None
-        }
-    })
-}
-
-fn adt_has_inherent_method(cx: &LateContext<'_>, ty: Ty<'_>, method_name: Symbol) -> bool {
-    if let Some(ty_did) = ty.ty_adt_def().map(ty::AdtDef::did) {
-        cx.tcx.inherent_impls(ty_did).into_iter().flatten().any(|&did| {
-            cx.tcx
-                .associated_items(did)
-                .filter_by_name_unhygienic(method_name)
-                .next()
-                .is_some_and(|item| item.kind == ty::AssocKind::Fn)
-        })
-    } else {
-        false
-    }
-}
-
 impl LateLintPass<'_> for IterWithoutIntoIter {
     fn check_item(&mut self, cx: &LateContext<'_>, item: &rustc_hir::Item<'_>) {
         if !in_external_macro(cx.sess(), item.span)
@@ -167,7 +139,7 @@ impl LateLintPass<'_> for IterWithoutIntoIter {
             }
             && !deref_chain(cx, ty).any(|ty| {
                 // We can't check inherent impls for slices, but we know that they have an `iter(_mut)` method
-                ty.peel_refs().is_slice() || adt_has_inherent_method(cx, ty, expected_method_name)
+                ty.peel_refs().is_slice() || get_adt_inherent_method(cx, ty, expected_method_name).is_some()
             })
             && let Some(iter_assoc_span) = imp.items.iter().find_map(|item| {
                 if item.ident.name == sym!(IntoIter) {