new lint: `explicit_into_iter_fn_arg`

7 files changed, 235 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a2648d9faa6..7a127c08197 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4642,6 +4642,7 @@ Released 2018-09-13
 [`explicit_auto_deref`]: https://rust-lang.github.io/rust-clippy/master/index.html#explicit_auto_deref
 [`explicit_counter_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#explicit_counter_loop
 [`explicit_deref_methods`]: https://rust-lang.github.io/rust-clippy/master/index.html#explicit_deref_methods
+[`explicit_into_iter_fn_arg`]: https://rust-lang.github.io/rust-clippy/master/index.html#explicit_into_iter_fn_arg
 [`explicit_into_iter_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#explicit_into_iter_loop
 [`explicit_iter_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop
 [`explicit_write`]: https://rust-lang.github.io/rust-clippy/master/index.html#explicit_write
diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index 4ade25e1257..fceb6e4b2ae 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -157,6 +157,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::exhaustive_items::EXHAUSTIVE_ENUMS_INFO,
     crate::exhaustive_items::EXHAUSTIVE_STRUCTS_INFO,
     crate::exit::EXIT_INFO,
+    crate::explicit_into_iter_fn_arg::EXPLICIT_INTO_ITER_FN_ARG_INFO,
     crate::explicit_write::EXPLICIT_WRITE_INFO,
     crate::extra_unused_type_parameters::EXTRA_UNUSED_TYPE_PARAMETERS_INFO,
     crate::fallible_impl_from::FALLIBLE_IMPL_FROM_INFO,
diff --git a/clippy_lints/src/explicit_into_iter_fn_arg.rs b/clippy_lints/src/explicit_into_iter_fn_arg.rs
new file mode 100644
index 00000000000..007a3f43449
--- /dev/null
+++ b/clippy_lints/src/explicit_into_iter_fn_arg.rs
@@ -0,0 +1,144 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::source::snippet;
+use clippy_utils::{get_parent_expr, is_trait_method};
+use rustc_errors::Applicability;
+use rustc_hir::def_id::DefId;
+use rustc_hir::*;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+use rustc_span::Span;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for calls to [`IntoIterator::into_iter`](https://doc.rust-lang.org/stable/std/iter/trait.IntoIterator.html#tymethod.into_iter)
+    /// in a call argument that accepts `IntoIterator`.
+    ///
+    /// ### Why is this bad?
+    /// If a function has a generic parameter with an `IntoIterator` trait bound, it means that the function
+    /// will *have* to call `.into_iter()` to get an iterator out of it, thereby making the call to `.into_iter()`
+    /// at call site redundant.
+    ///
+    /// Consider this example:
+    /// ```rs,ignore
+    /// fn foo<T: IntoIterator<Item = u32>>(iter: T) {
+    ///   let it = iter.into_iter();
+    ///                ^^^^^^^^^^^^ the function has to call `.into_iter()` to get the iterator
+    /// }
+    ///
+    /// foo(vec![1, 2, 3].into_iter());
+    ///                  ^^^^^^^^^^^^ ... making this `.into_iter()` call redundant.
+    /// ```
+    ///
+    /// The reason for why calling `.into_iter()` twice (once at call site and again inside of the function) works in the first place
+    /// is because there is a blanket implementation of `IntoIterator` for all types that implement `Iterator` in the standard library,
+    /// in which it simply returns itself, effectively making the second call to `.into_iter()` a "no-op":
+    /// ```rust,ignore
+    /// impl<I: Iterator> IntoIterator for I {
+    ///     type Item = I::Item;
+    ///     type IntoIter = I;
+    ///
+    ///     fn into_iter(self) -> I {
+    ///        self
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// ### Example
+    /// ```rust
+    /// fn even_sum<I: IntoIterator<Item = u32>>(iter: I) -> u32 {
+    ///     iter.into_iter().filter(|&x| x % 2 == 0).sum()
+    /// }
+    ///
+    /// let _ = even_sum(vec![1, 2, 3].into_iter());
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// fn even_sum<I: IntoIterator<Item = u32>>(iter: I) -> u32 {
+    ///     iter.into_iter().filter(|&x| x % 2 == 0).sum()
+    /// }
+    ///
+    /// let _ = even_sum(vec![1, 2, 3]);
+    /// ```
+    #[clippy::version = "1.71.0"]
+    pub EXPLICIT_INTO_ITER_FN_ARG,
+    pedantic,
+    "explicit call to `.into_iter()` in function argument accepting `IntoIterator`"
+}
+declare_lint_pass!(ExplicitIntoIterFnArg => [EXPLICIT_INTO_ITER_FN_ARG]);
+
+enum MethodOrFunction {
+    Method,
+    Function,
+}
+
+/// Returns the span of the `IntoIterator` trait bound in the function pointed to by `fn_did`
+fn into_iter_bound(cx: &LateContext<'_>, fn_did: DefId, param_index: u32) -> Option<Span> {
+    if let Some(into_iter_did) = cx.tcx.get_diagnostic_item(sym::IntoIterator) {
+        cx.tcx
+            .predicates_of(fn_did)
+            .predicates
+            .iter()
+            .find_map(|&(ref pred, span)| {
+                if let ty::PredicateKind::Clause(ty::Clause::Trait(tr)) = pred.kind().skip_binder()
+                    && tr.def_id() == into_iter_did
+                    && tr.self_ty().is_param(param_index)
+                {
+                    Some(span)
+                } else {
+                    None
+                }
+            })
+    } else {
+        None
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for ExplicitIntoIterFnArg {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if expr.span.from_expansion() {
+            return;
+        }
+
+        if let ExprKind::MethodCall(name, recv, ..) = expr.kind
+            && is_trait_method(cx, expr, sym::IntoIterator)
+            && name.ident.name == sym::into_iter
+            && let Some(parent_expr) = get_parent_expr(cx, expr)
+        {
+            let parent_expr = match parent_expr.kind {
+                ExprKind::Call(recv, args) if let ExprKind::Path(ref qpath) = recv.kind => {
+                    cx.qpath_res(qpath, recv.hir_id).opt_def_id()
+                        .map(|did| (did, args, MethodOrFunction::Function))
+                }
+                ExprKind::MethodCall(.., args, _) => {
+                    cx.typeck_results().type_dependent_def_id(parent_expr.hir_id)
+                        .map(|did| (did, args, MethodOrFunction::Method))
+                }
+                _ => None,
+            };
+
+            if let Some((parent_fn_did, args, kind)) = parent_expr
+                && let sig = cx.tcx.fn_sig(parent_fn_did).skip_binder().skip_binder()
+                && let Some(arg_pos) = args.iter().position(|x| x.hir_id == expr.hir_id)
+                && let Some(&into_iter_param) = sig.inputs().get(match kind {
+                    MethodOrFunction::Function => arg_pos,
+                    MethodOrFunction::Method => arg_pos + 1, // skip self arg
+                })
+                && let ty::Param(param) = into_iter_param.kind()
+                && let Some(span) = into_iter_bound(cx, parent_fn_did, param.index)
+            {
+                let sugg = snippet(cx, recv.span.source_callsite(), "<expr>").into_owned();
+                span_lint_and_then(cx, EXPLICIT_INTO_ITER_FN_ARG, expr.span, "explicit call to `.into_iter()` in function argument accepting `IntoIterator`", |diag| {
+                    diag.span_suggestion(
+                        expr.span,
+                        "consider removing `.into_iter()`",
+                        sugg,
+                        Applicability::MachineApplicable,
+                    );
+                    diag.span_note(span, "this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`");
+                });
+            }
+        }
+    }
+}
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 10404d51ba5..bfe52e182b2 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -123,6 +123,7 @@ mod eta_reduction;
 mod excessive_bools;
 mod exhaustive_items;
 mod exit;
+mod explicit_into_iter_fn_arg;
 mod explicit_write;
 mod extra_unused_type_parameters;
 mod fallible_impl_from;
@@ -994,6 +995,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_early_pass(|| Box::new(ref_patterns::RefPatterns));
     store.register_late_pass(|_| Box::new(default_constructed_unit_structs::DefaultConstructedUnitStructs));
     store.register_early_pass(|| Box::new(needless_else::NeedlessElse));
+    store.register_late_pass(|_| Box::new(explicit_into_iter_fn_arg::ExplicitIntoIterFnArg));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 
diff --git a/tests/ui/explicit_into_iter_fn_arg.fixed b/tests/ui/explicit_into_iter_fn_arg.fixed
new file mode 100644
index 00000000000..219b60bd051
--- /dev/null
+++ b/tests/ui/explicit_into_iter_fn_arg.fixed
@@ -0,0 +1,24 @@
+//@run-rustfix
+
+#![allow(unused)]
+#![warn(clippy::explicit_into_iter_fn_arg)]
+
+fn a<T>(_: T) {}
+fn b<T: IntoIterator<Item = i32>>(_: T) {}
+fn c(_: impl IntoIterator<Item = i32>) {}
+fn d<T>(_: T)
+where
+    T: IntoIterator<Item = i32>,
+{
+}
+fn e<T: IntoIterator<Item = i32>>(_: T) {}
+fn f(_: std::vec::IntoIter<i32>) {}
+
+fn main() {
+    a(vec![1, 2].into_iter());
+    b(vec![1, 2]);
+    c(vec![1, 2]);
+    d(vec![1, 2]);
+    e([&1, &2, &3].into_iter().cloned());
+    f(vec![1, 2].into_iter());
+}
diff --git a/tests/ui/explicit_into_iter_fn_arg.rs b/tests/ui/explicit_into_iter_fn_arg.rs
new file mode 100644
index 00000000000..358f4bc661d
--- /dev/null
+++ b/tests/ui/explicit_into_iter_fn_arg.rs
@@ -0,0 +1,24 @@
+//@run-rustfix
+
+#![allow(unused)]
+#![warn(clippy::explicit_into_iter_fn_arg)]
+
+fn a<T>(_: T) {}
+fn b<T: IntoIterator<Item = i32>>(_: T) {}
+fn c(_: impl IntoIterator<Item = i32>) {}
+fn d<T>(_: T)
+where
+    T: IntoIterator<Item = i32>,
+{
+}
+fn e<T: IntoIterator<Item = i32>>(_: T) {}
+fn f(_: std::vec::IntoIter<i32>) {}
+
+fn main() {
+    a(vec![1, 2].into_iter());
+    b(vec![1, 2].into_iter());
+    c(vec![1, 2].into_iter());
+    d(vec![1, 2].into_iter());
+    e([&1, &2, &3].into_iter().cloned());
+    f(vec![1, 2].into_iter());
+}
diff --git a/tests/ui/explicit_into_iter_fn_arg.stderr b/tests/ui/explicit_into_iter_fn_arg.stderr
new file mode 100644
index 00000000000..a0bd81c21ea
--- /dev/null
+++ b/tests/ui/explicit_into_iter_fn_arg.stderr
@@ -0,0 +1,39 @@
+error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
+  --> $DIR/explicit_into_iter_fn_arg.rs:19:7
+   |
+LL |     b(vec![1, 2].into_iter());
+   |       ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `vec![1, 2]`
+   |
+note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
+  --> $DIR/explicit_into_iter_fn_arg.rs:7:9
+   |
+LL | fn b<T: IntoIterator<Item = i32>>(_: T) {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: `-D clippy::explicit-into-iter-fn-arg` implied by `-D warnings`
+
+error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
+  --> $DIR/explicit_into_iter_fn_arg.rs:20:7
+   |
+LL |     c(vec![1, 2].into_iter());
+   |       ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `vec![1, 2]`
+   |
+note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
+  --> $DIR/explicit_into_iter_fn_arg.rs:8:14
+   |
+LL | fn c(_: impl IntoIterator<Item = i32>) {}
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
+  --> $DIR/explicit_into_iter_fn_arg.rs:21:7
+   |
+LL |     d(vec![1, 2].into_iter());
+   |       ^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `.into_iter()`: `vec![1, 2]`
+   |
+note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
+  --> $DIR/explicit_into_iter_fn_arg.rs:11:8
+   |
+LL |     T: IntoIterator<Item = i32>,
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+