Auto merge of rust-lang#12432 - Ethiraric:fix-12411, r=y21

[`unused_enumerate_index`]: trigger on method calls

The lint used to check for patterns looking like:
```rs
for (_, x) in some_iter.enumerate() {
    // Index is ignored
}
```

This commit further checks for chained method calls constructs where we
can detect that the index is unused. Currently, this checks only for the
following patterns:
```rs
some_iter.enumerate().map_function(|(_, x)| ..)
let x = some_iter.enumerate();
x.map_function(|(_, x)| ..)
```
where `map_function` is one of `all`, `any`, `filter_map`, `find_map`,
`flat_map`, `for_each` or `map`.

Fixes rust-lang#12411.

*Please write a short comment explaining your change (or "none" for internal only changes)*

changelog: [`unused_enumerate_index`]: add detection for method chains such as `iter.enumerate().map(|(_, x)| x)`

Author: bors

clippy_lints/src/loops/unused_enumerate_index.rs

@@ -1,62 +1,41 @@
 use super::UNUSED_ENUMERATE_INDEX;
 use clippy_utils::diagnostics::{multispan_sugg, span_lint_and_then};
 use clippy_utils::source::snippet;
-use clippy_utils::{pat_is_wild, sugg};
+use clippy_utils::{match_def_path, pat_is_wild, sugg};
 use rustc_hir::def::DefKind;
 use rustc_hir::{Expr, ExprKind, Pat, PatKind};
 use rustc_lint::LateContext;
 use rustc_middle::ty;
 
 /// Checks for the `UNUSED_ENUMERATE_INDEX` lint.
-pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>, arg: &'tcx Expr<'_>, body: &'tcx Expr<'_>) {
-    let PatKind::Tuple([index, elem], _) = pat.kind else {
-        return;
-    };
-
-    let ExprKind::MethodCall(_method, self_arg, [], _) = arg.kind else {
-        return;
-    };
-
-    let ty = cx.typeck_results().expr_ty(arg);
-
-    if !pat_is_wild(cx, &index.kind, body) {
-        return;
-    }
-
-    let name = match *ty.kind() {
-        ty::Adt(base, _substs) => cx.tcx.def_path_str(base.did()),
-        _ => return,
-    };
-
-    if name != "std::iter::Enumerate" && name != "core::iter::Enumerate" {
-        return;
+///
+/// The lint is also partially implemented in `clippy_lints/src/methods/unused_enumerate_index.rs`.
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, pat: &Pat<'tcx>, arg: &Expr<'_>, body: &'tcx Expr<'tcx>) {
+    if let PatKind::Tuple([index, elem], _) = pat.kind
+        && let ExprKind::MethodCall(_method, self_arg, [], _) = arg.kind
+        && let ty = cx.typeck_results().expr_ty(arg)
+        && pat_is_wild(cx, &index.kind, body)
+        && let ty::Adt(base, _) = *ty.kind()
+        && match_def_path(cx, base.did(), &clippy_utils::paths::CORE_ITER_ENUMERATE_STRUCT)
+        && let Some((DefKind::AssocFn, call_id)) = cx.typeck_results().type_dependent_def(arg.hir_id)
+        && match_def_path(cx, call_id, &clippy_utils::paths::CORE_ITER_ENUMERATE_METHOD)
+    {
+        span_lint_and_then(
+            cx,
+            UNUSED_ENUMERATE_INDEX,
+            arg.span,
+            "you seem to use `.enumerate()` and immediately discard the index",
+            |diag| {
+                let base_iter = sugg::Sugg::hir(cx, self_arg, "base iter");
+                multispan_sugg(
+                    diag,
+                    "remove the `.enumerate()` call",
+                    vec![
+                        (pat.span, snippet(cx, elem.span, "..").into_owned()),
+                        (arg.span, base_iter.to_string()),
+                    ],
+                );
+            },
+        );
     }
-
-    let Some((DefKind::AssocFn, call_id)) = cx.typeck_results().type_dependent_def(arg.hir_id) else {
-        return;
-    };
-
-    let call_name = cx.tcx.def_path_str(call_id);
-
-    if call_name != "std::iter::Iterator::enumerate" && call_name != "core::iter::Iterator::enumerate" {
-        return;
-    }
-
-    span_lint_and_then(
-        cx,
-        UNUSED_ENUMERATE_INDEX,
-        arg.span,
-        "you seem to use `.enumerate()` and immediately discard the index",
-        |diag| {
-            let base_iter = sugg::Sugg::hir(cx, self_arg, "base iter");
-            multispan_sugg(
-                diag,
-                "remove the `.enumerate()` call",
-                vec![
-                    (pat.span, snippet(cx, elem.span, "..").into_owned()),
-                    (arg.span, base_iter.to_string()),
-                ],
-            );
-        },
-    );
 }

clippy_lints/src/methods/mod.rs

@@ -119,6 +119,7 @@ mod unnecessary_literal_unwrap;
 mod unnecessary_result_map_or_else;
 mod unnecessary_sort_by;
 mod unnecessary_to_owned;
+mod unused_enumerate_index;
 mod unwrap_expect_used;
 mod useless_asref;
 mod utils;
@@ -4429,6 +4430,7 @@ impl Methods {
                     zst_offset::check(cx, expr, recv);
                 },
                 ("all", [arg]) => {
+                    unused_enumerate_index::check(cx, expr, recv, arg);
                     if let Some(("cloned", recv2, [], _, _)) = method_call(recv) {
                         iter_overeager_cloned::check(
                             cx,
@@ -4447,23 +4449,26 @@ impl Methods {
                         unnecessary_lazy_eval::check(cx, expr, recv, arg, "and");
                     }
                 },
-                ("any", [arg]) => match method_call(recv) {
-                    Some(("cloned", recv2, [], _, _)) => iter_overeager_cloned::check(
-                        cx,
-                        expr,
-                        recv,
-                        recv2,
-                        iter_overeager_cloned::Op::NeedlessMove(arg),
-                        false,
-                    ),
-                    Some(("chars", recv, _, _, _))
-                        if let ExprKind::Closure(arg) = arg.kind
-                            && let body = cx.tcx.hir().body(arg.body)
-                            && let [param] = body.params =>
-                    {
-                        string_lit_chars_any::check(cx, expr, recv, param, peel_blocks(body.value), &self.msrv);
-                    },
-                    _ => {},
+                ("any", [arg]) => {
+                    unused_enumerate_index::check(cx, expr, recv, arg);
+                    match method_call(recv) {
+                        Some(("cloned", recv2, [], _, _)) => iter_overeager_cloned::check(
+                            cx,
+                            expr,
+                            recv,
+                            recv2,
+                            iter_overeager_cloned::Op::NeedlessMove(arg),
+                            false,
+                        ),
+                        Some(("chars", recv, _, _, _))
+                            if let ExprKind::Closure(arg) = arg.kind
+                                && let body = cx.tcx.hir().body(arg.body)
+                                && let [param] = body.params =>
+                        {
+                            string_lit_chars_any::check(cx, expr, recv, param, peel_blocks(body.value), &self.msrv);
+                        },
+                        _ => {},
+                    }
                 },
                 ("arg", [arg]) => {
                     suspicious_command_arg_space::check(cx, recv, arg, span);
@@ -4596,14 +4601,17 @@ impl Methods {
                     }
                 },
                 ("filter_map", [arg]) => {
+                    unused_enumerate_index::check(cx, expr, recv, arg);
                     unnecessary_filter_map::check(cx, expr, arg, name);
                     filter_map_bool_then::check(cx, expr, arg, call_span);
                     filter_map_identity::check(cx, expr, arg, span);
                 },
                 ("find_map", [arg]) => {
+                    unused_enumerate_index::check(cx, expr, recv, arg);
                     unnecessary_filter_map::check(cx, expr, arg, name);
                 },
                 ("flat_map", [arg]) => {
+                    unused_enumerate_index::check(cx, expr, recv, arg);
                     flat_map_identity::check(cx, expr, arg, span);
                     flat_map_option::check(cx, expr, arg, span);
                 },
@@ -4625,17 +4633,20 @@ impl Methods {
                     manual_try_fold::check(cx, expr, init, acc, call_span, &self.msrv);
                     unnecessary_fold::check(cx, expr, init, acc, span);
                 },
-                ("for_each", [arg]) => match method_call(recv) {
-                    Some(("inspect", _, [_], span2, _)) => inspect_for_each::check(cx, expr, span2),
-                    Some(("cloned", recv2, [], _, _)) => iter_overeager_cloned::check(
-                        cx,
-                        expr,
-                        recv,
-                        recv2,
-                        iter_overeager_cloned::Op::NeedlessMove(arg),
-                        false,
-                    ),
-                    _ => {},
+                ("for_each", [arg]) => {
+                    unused_enumerate_index::check(cx, expr, recv, arg);
+                    match method_call(recv) {
+                        Some(("inspect", _, [_], span2, _)) => inspect_for_each::check(cx, expr, span2),
+                        Some(("cloned", recv2, [], _, _)) => iter_overeager_cloned::check(
+                            cx,
+                            expr,
+                            recv,
+                            recv2,
+                            iter_overeager_cloned::Op::NeedlessMove(arg),
+                            false,
+                        ),
+                        _ => {},
+                    }
                 },
                 ("get", [arg]) => {
                     get_first::check(cx, expr, recv, arg);
@@ -4682,6 +4693,7 @@ impl Methods {
                 },
                 (name @ ("map" | "map_err"), [m_arg]) => {
                     if name == "map" {
+                        unused_enumerate_index::check(cx, expr, recv, m_arg);
                         map_clone::check(cx, expr, recv, m_arg, &self.msrv);
                         match method_call(recv) {
                             Some((map_name @ ("iter" | "into_iter"), recv2, _, _, _)) => {

clippy_lints/src/methods/unused_enumerate_index.rs

@@ -0,0 +1,135 @@
+use clippy_utils::diagnostics::{multispan_sugg_with_applicability, span_lint_hir_and_then};
+use clippy_utils::paths::{CORE_ITER_ENUMERATE_METHOD, CORE_ITER_ENUMERATE_STRUCT};
+use clippy_utils::source::{snippet, snippet_opt};
+use clippy_utils::{expr_or_init, is_trait_method, match_def_path, pat_is_wild};
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, FnDecl, PatKind, TyKind};
+use rustc_lint::LateContext;
+use rustc_middle::ty::AdtDef;
+use rustc_span::{sym, Span};
+
+use crate::loops::UNUSED_ENUMERATE_INDEX;
+
+/// Check for the `UNUSED_ENUMERATE_INDEX` lint outside of loops.
+///
+/// The lint is declared in `clippy_lints/src/loops/mod.rs`. There, the following pattern is
+/// checked:
+/// ```ignore
+/// for (_, x) in some_iter.enumerate() {
+///     // Index is ignored
+/// }
+/// ```
+///
+/// This `check` function checks for chained method calls constructs where we can detect that the
+/// index is unused. Currently, this checks only for the following patterns:
+/// ```ignore
+/// some_iter.enumerate().map_function(|(_, x)| ..)
+/// let x = some_iter.enumerate();
+/// x.map_function(|(_, x)| ..)
+/// ```
+/// where `map_function` is one of `all`, `any`, `filter_map`, `find_map`, `flat_map`, `for_each` or
+/// `map`.
+///
+/// # Preconditions
+/// This function must be called not on the `enumerate` call expression itself, but on any of the
+/// map functions listed above. It will ensure that `recv` is a `std::iter::Enumerate` instance and
+/// that the method call is one of the `std::iter::Iterator` trait.
+///
+/// * `call_expr`: The map function call expression
+/// * `recv`: The receiver of the call
+/// * `closure_arg`: The argument to the map function call containing the closure/function to apply
+pub(super) fn check(cx: &LateContext<'_>, call_expr: &Expr<'_>, recv: &Expr<'_>, closure_arg: &Expr<'_>) {
+    let recv_ty = cx.typeck_results().expr_ty(recv);
+    if let Some(recv_ty_defid) = recv_ty.ty_adt_def().map(AdtDef::did)
+        // If we call a method on a `std::iter::Enumerate` instance
+        && match_def_path(cx, recv_ty_defid, &CORE_ITER_ENUMERATE_STRUCT)
+        // If we are calling a method of the `Iterator` trait
+        && is_trait_method(cx, call_expr, sym::Iterator)
+        // And the map argument is a closure
+        && let ExprKind::Closure(closure) = closure_arg.kind
+        && let closure_body = cx.tcx.hir().body(closure.body)
+        // And that closure has one argument ...
+        && let [closure_param] = closure_body.params
+        // .. which is a tuple of 2 elements
+        && let PatKind::Tuple([index, elem], ..) = closure_param.pat.kind
+        // And that the first element (the index) is either `_` or unused in the body
+        && pat_is_wild(cx, &index.kind, closure_body)
+        // Try to find the initializer for `recv`. This is needed in case `recv` is a local_binding. In the
+        // first example below, `expr_or_init` would return `recv`.
+        // ```
+        // iter.enumerate().map(|(_, x)| x)
+        // ^^^^^^^^^^^^^^^^ `recv`, a call to `std::iter::Iterator::enumerate`
+        //
+        // let binding = iter.enumerate();
+        //               ^^^^^^^^^^^^^^^^ `recv_init_expr`
+        // binding.map(|(_, x)| x)
+        // ^^^^^^^ `recv`, not a call to `std::iter::Iterator::enumerate`
+        // ```
+        && let recv_init_expr = expr_or_init(cx, recv)
+        // Make sure the initializer is a method call. It may be that the `Enumerate` comes from something
+        // that we cannot control.
+        // This would for instance happen with:
+        // ```
+        // external_lib::some_function_returning_enumerate().map(|(_, x)| x)
+        // ```
+        && let ExprKind::MethodCall(_, enumerate_recv, _, enumerate_span) = recv_init_expr.kind
+        && let Some(enumerate_defid) = cx.typeck_results().type_dependent_def_id(recv_init_expr.hir_id)
+        // Make sure the method call is `std::iter::Iterator::enumerate`.
+        && match_def_path(cx, enumerate_defid, &CORE_ITER_ENUMERATE_METHOD)
+    {
+        // Check if the tuple type was explicit. It may be the type system _needs_ the type of the element
+        // that would be explicited in the closure.
+        let new_closure_param = match find_elem_explicit_type_span(closure.fn_decl) {
+            // We have an explicit type. Get its snippet, that of the binding name, and do `binding: ty`.
+            // Fallback to `..` if we fail getting either snippet.
+            Some(ty_span) => snippet_opt(cx, elem.span)
+                .and_then(|binding_name| snippet_opt(cx, ty_span).map(|ty_name| format!("{binding_name}: {ty_name}")))
+                .unwrap_or_else(|| "..".to_string()),
+            // Otherwise, we have no explicit type. We can replace with the binding name of the element.
+            None => snippet(cx, elem.span, "..").into_owned(),
+        };
+
+        // Suggest removing the tuple from the closure and the preceding call to `enumerate`, whose span we
+        // can get from the `MethodCall`.
+        span_lint_hir_and_then(
+            cx,
+            UNUSED_ENUMERATE_INDEX,
+            recv_init_expr.hir_id,
+            enumerate_span,
+            "you seem to use `.enumerate()` and immediately discard the index",
+            |diag| {
+                multispan_sugg_with_applicability(
+                    diag,
+                    "remove the `.enumerate()` call",
+                    Applicability::MachineApplicable,
+                    vec![
+                        (closure_param.span, new_closure_param),
+                        (
+                            enumerate_span.with_lo(enumerate_recv.span.source_callsite().hi()),
+                            String::new(),
+                        ),
+                    ],
+                );
+            },
+        );
+    }
+}
+
+/// Find the span of the explicit type of the element.
+///
+/// # Returns
+/// If the tuple argument:
+/// * Has no explicit type, returns `None`
+/// * Has an explicit tuple type with an implicit element type (`(usize, _)`), returns `None`
+/// * Has an explicit tuple type with an explicit element type (`(_, i32)`), returns the span for
+///   the element type.
+fn find_elem_explicit_type_span(fn_decl: &FnDecl<'_>) -> Option<Span> {
+    if let [tuple_ty] = fn_decl.inputs
+        && let TyKind::Tup([_idx_ty, elem_ty]) = tuple_ty.kind
+        && !matches!(elem_ty.kind, TyKind::Err(..) | TyKind::Infer)
+    {
+        Some(elem_ty.span)
+    } else {
+        None
+    }
+}

clippy_utils/src/paths.rs

@@ -19,6 +19,8 @@ pub const BTREESET_ITER: [&str; 6] = ["alloc", "collections", "btree", "set", "B
 pub const CLONE_TRAIT_METHOD: [&str; 4] = ["core", "clone", "Clone", "clone"];
 pub const CORE_ITER_CLONED: [&str; 6] = ["core", "iter", "traits", "iterator", "Iterator", "cloned"];
 pub const CORE_ITER_COPIED: [&str; 6] = ["core", "iter", "traits", "iterator", "Iterator", "copied"];
+pub const CORE_ITER_ENUMERATE_METHOD: [&str; 6] = ["core", "iter", "traits", "iterator", "Iterator", "enumerate"];
+pub const CORE_ITER_ENUMERATE_STRUCT: [&str; 5] = ["core", "iter", "adapters", "enumerate", "Enumerate"];
 pub const CORE_ITER_FILTER: [&str; 6] = ["core", "iter", "traits", "iterator", "Iterator", "filter"];
 pub const CORE_RESULT_OK_METHOD: [&str; 4] = ["core", "result", "Result", "ok"];
 pub const CSTRING_AS_C_STR: [&str; 5] = ["alloc", "ffi", "c_str", "CString", "as_c_str"];