Move check_for_loop_arg back to mod; split into 4 lint files

clippy_lints/src/loops/explicit_into_iter_loop.rs

@@ -0,0 +1,20 @@
+use super::EXPLICIT_INTO_ITER_LOOP;
+use crate::utils::{snippet_with_applicability, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+
+pub(super) fn check_explicit_into_iter_loop(cx: &LateContext<'_>, method_args: &'hir [Expr<'hir>], arg: &Expr<'_>) {
+    let mut applicability = Applicability::MachineApplicable;
+    let object = snippet_with_applicability(cx, method_args[0].span, "_", &mut applicability);
+    span_lint_and_sugg(
+        cx,
+        EXPLICIT_INTO_ITER_LOOP,
+        arg.span,
+        "it is more concise to loop over containers instead of using explicit \
+            iteration methods",
+        "to write this more concisely, try",
+        object.to_string(),
+        applicability,
+    );
+}

clippy_lints/src/loops/explicit_iter_loop.rs

@@ -0,0 +1,21 @@
+use super::EXPLICIT_ITER_LOOP;
+use crate::utils::{snippet_with_applicability, span_lint_and_sugg};
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+
+pub(super) fn lint_iter_method(cx: &LateContext<'_>, args: &[Expr<'_>], arg: &Expr<'_>, method_name: &str) {
+    let mut applicability = Applicability::MachineApplicable;
+    let object = snippet_with_applicability(cx, args[0].span, "_", &mut applicability);
+    let muta = if method_name == "iter_mut" { "mut " } else { "" };
+    span_lint_and_sugg(
+        cx,
+        EXPLICIT_ITER_LOOP,
+        arg.span,
+        "it is more concise to loop over references to containers instead of using explicit \
+         iteration methods",
+        "to write this more concisely, try",
+        format!("&{}{}", muta, object),
+        applicability,
+    )
+}

clippy_lints/src/loops/for_loops_over_fallibles.rs

@@ -0,0 +1,45 @@
+use super::FOR_LOOPS_OVER_FALLIBLES;
+use crate::utils::{is_type_diagnostic_item, snippet, span_lint_and_help};
+use rustc_hir::{Expr, Pat};
+use rustc_lint::LateContext;
+use rustc_span::symbol::sym;
+
+/// Checks for `for` loops over `Option`s and `Result`s.
+pub(super) fn check_arg_type(cx: &LateContext<'_>, pat: &Pat<'_>, arg: &Expr<'_>) {
+    let ty = cx.typeck_results().expr_ty(arg);
+    if is_type_diagnostic_item(cx, ty, sym::option_type) {
+        span_lint_and_help(
+            cx,
+            FOR_LOOPS_OVER_FALLIBLES,
+            arg.span,
+            &format!(
+                "for loop over `{0}`, which is an `Option`. This is more readably written as an \
+                `if let` statement",
+                snippet(cx, arg.span, "_")
+            ),
+            None,
+            &format!(
+                "consider replacing `for {0} in {1}` with `if let Some({0}) = {1}`",
+                snippet(cx, pat.span, "_"),
+                snippet(cx, arg.span, "_")
+            ),
+        );
+    } else if is_type_diagnostic_item(cx, ty, sym::result_type) {
+        span_lint_and_help(
+            cx,
+            FOR_LOOPS_OVER_FALLIBLES,
+            arg.span,
+            &format!(
+                "for loop over `{0}`, which is a `Result`. This is more readably written as an \
+                `if let` statement",
+                snippet(cx, arg.span, "_")
+            ),
+            None,
+            &format!(
+                "consider replacing `for {0} in {1}` with `if let Ok({0}) = {1}`",
+                snippet(cx, pat.span, "_"),
+                snippet(cx, arg.span, "_")
+            ),
+        );
+    }
+}

clippy_lints/src/loops/iter_next_loop.rs

@@ -0,0 +1,14 @@
+use super::ITER_NEXT_LOOP;
+use crate::utils::span_lint;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+
+pub(super) fn lint(cx: &LateContext<'_>, expr: &Expr<'_>) {
+    span_lint(
+        cx,
+        ITER_NEXT_LOOP,
+        expr.span,
+        "you are iterating over `Iterator::next()` which is an Option; this will compile but is \
+        probably not what you want",
+    );
+}

clippy_lints/src/loops/mod.rs

@@ -1,11 +1,14 @@
 mod empty_loop;
 mod explicit_counter_loop;
-mod for_loop_arg;
+mod explicit_into_iter_loop;
+mod explicit_iter_loop;
 mod for_loop_over_map_kv;
 mod for_loop_range;
+mod for_loops_over_fallibles;
 mod for_mut_range_bound;
 mod for_single_element_loop;
 mod infinite_loop;
+mod iter_next_loop;
 mod manual_flatten;
 mod manual_memcpy;
 mod needless_collect;
@@ -15,11 +18,13 @@ mod utils;
 mod while_let_loop;
 mod while_let_on_iterator;
 
-use crate::utils::higher;
+use crate::utils::{higher, is_type_diagnostic_item, match_trait_method, match_type, paths};
-use rustc_hir::{Expr, ExprKind, LoopSource, Pat};
+use rustc_hir::{Expr, ExprKind, LoopSource, Mutability, Pat};
 use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::{self, Ty, TyS};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::source_map::Span;
+use rustc_span::symbol::sym;
 use utils::{get_span_of_entire_for_loop, make_iterator_snippet, IncrementVisitor, InitializeVisitor};
 
 declare_clippy_lint! {
@@ -588,10 +593,77 @@ fn check_for_loop<'tcx>(
         for_loop_range::check_for_loop_range(cx, pat, arg, body, expr);
         explicit_counter_loop::check_for_loop_explicit_counter(cx, pat, arg, body, expr);
     }
-    for_loop_arg::check_for_loop_arg(cx, pat, arg, expr);
+    check_for_loop_arg(cx, pat, arg, expr);
     for_loop_over_map_kv::check_for_loop_over_map_kv(cx, pat, arg, body, expr);
     for_mut_range_bound::check_for_mut_range_bound(cx, arg, body);
     for_single_element_loop::check_for_single_element_loop(cx, pat, arg, body, expr);
     same_item_push::detect_same_item_push(cx, pat, arg, body, expr);
     manual_flatten::check_manual_flatten(cx, pat, arg, body, span);
 }
+
+fn check_for_loop_arg(cx: &LateContext<'_>, pat: &Pat<'_>, arg: &Expr<'_>, expr: &Expr<'_>) {
+    let mut next_loop_linted = false; // whether or not ITER_NEXT_LOOP lint was used
+    if let ExprKind::MethodCall(ref method, _, ref args, _) = arg.kind {
+        // just the receiver, no arguments
+        if args.len() == 1 {
+            let method_name = &*method.ident.as_str();
+            // check for looping over x.iter() or x.iter_mut(), could use &x or &mut x
+            if method_name == "iter" || method_name == "iter_mut" {
+                if is_ref_iterable_type(cx, &args[0]) {
+                    explicit_iter_loop::lint_iter_method(cx, args, arg, method_name);
+                }
+            } else if method_name == "into_iter" && match_trait_method(cx, arg, &paths::INTO_ITERATOR) {
+                let receiver_ty = cx.typeck_results().expr_ty(&args[0]);
+                let receiver_ty_adjusted = cx.typeck_results().expr_ty_adjusted(&args[0]);
+                if TyS::same_type(receiver_ty, receiver_ty_adjusted) {
+                    explicit_into_iter_loop::check_explicit_into_iter_loop(cx, args, arg);
+                } else {
+                    let ref_receiver_ty = cx.tcx.mk_ref(
+                        cx.tcx.lifetimes.re_erased,
+                        ty::TypeAndMut {
+                            ty: receiver_ty,
+                            mutbl: Mutability::Not,
+                        },
+                    );
+                    if TyS::same_type(receiver_ty_adjusted, ref_receiver_ty) {
+                        explicit_iter_loop::lint_iter_method(cx, args, arg, method_name)
+                    }
+                }
+            } else if method_name == "next" && match_trait_method(cx, arg, &paths::ITERATOR) {
+                iter_next_loop::lint(cx, expr);
+                next_loop_linted = true;
+            }
+        }
+    }
+    if !next_loop_linted {
+        for_loops_over_fallibles::check_arg_type(cx, pat, arg);
+    }
+}
+
+/// Returns `true` if the type of expr is one that provides `IntoIterator` impls
+/// for `&T` and `&mut T`, such as `Vec`.
+#[rustfmt::skip]
+fn is_ref_iterable_type(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    // no walk_ptrs_ty: calling iter() on a reference can make sense because it
+    // will allow further borrows afterwards
+    let ty = cx.typeck_results().expr_ty(e);
+    is_iterable_array(ty, cx) ||
+    is_type_diagnostic_item(cx, ty, sym::vec_type) ||
+    match_type(cx, ty, &paths::LINKED_LIST) ||
+    is_type_diagnostic_item(cx, ty, sym!(hashmap_type)) ||
+    is_type_diagnostic_item(cx, ty, sym!(hashset_type)) ||
+    is_type_diagnostic_item(cx, ty, sym!(vecdeque_type)) ||
+    match_type(cx, ty, &paths::BINARY_HEAP) ||
+    match_type(cx, ty, &paths::BTREEMAP) ||
+    match_type(cx, ty, &paths::BTREESET)
+}
+
+fn is_iterable_array<'tcx>(ty: Ty<'tcx>, cx: &LateContext<'tcx>) -> bool {
+    // IntoIterator is currently only implemented for array sizes <= 32 in rustc
+    match ty.kind() {
+        ty::Array(_, n) => n
+            .try_eval_usize(cx.tcx, cx.param_env)
+            .map_or(false, |val| (0..=32).contains(&val)),
+        _ => false,
+    }
+}