Auto merge of #8422 - buttercrab:only_used_in_recursion, r=llogiq

new lint: `only_used_in_recursion`

changelog:
- added `only_used_in_recursion`.
- fixed code that variables are only used in recursion.
- this would not lint when `unused_variable`

Issue: #8390

Author: bors

CHANGELOG.md

@@ -3355,6 +3355,7 @@ Released 2018-09-13
 [`not_unsafe_ptr_arg_deref`]: https://rust-lang.github.io/rust-clippy/master/index.html#not_unsafe_ptr_arg_deref
 [`octal_escapes`]: https://rust-lang.github.io/rust-clippy/master/index.html#octal_escapes
 [`ok_expect`]: https://rust-lang.github.io/rust-clippy/master/index.html#ok_expect
+[`only_used_in_recursion`]: https://rust-lang.github.io/rust-clippy/master/index.html#only_used_in_recursion
 [`op_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#op_ref
 [`option_as_ref_deref`]: https://rust-lang.github.io/rust-clippy/master/index.html#option_as_ref_deref
 [`option_env_unwrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#option_env_unwrap

clippy_lints/src/lib.register_all.rs

@@ -235,6 +235,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(non_expressive_names::JUST_UNDERSCORES_AND_DIGITS),
     LintId::of(non_octal_unix_permissions::NON_OCTAL_UNIX_PERMISSIONS),
     LintId::of(octal_escapes::OCTAL_ESCAPES),
+    LintId::of(only_used_in_recursion::ONLY_USED_IN_RECURSION),
     LintId::of(open_options::NONSENSICAL_OPEN_OPTIONS),
     LintId::of(option_env_unwrap::OPTION_ENV_UNWRAP),
     LintId::of(overflow_check_conditional::OVERFLOW_CHECK_CONDITIONAL),

clippy_lints/src/lib.register_complexity.rs

@@ -64,6 +64,7 @@ store.register_group(true, "clippy::complexity", Some("clippy_complexity"), vec!
     LintId::of(neg_cmp_op_on_partial_ord::NEG_CMP_OP_ON_PARTIAL_ORD),
     LintId::of(no_effect::NO_EFFECT),
     LintId::of(no_effect::UNNECESSARY_OPERATION),
+    LintId::of(only_used_in_recursion::ONLY_USED_IN_RECURSION),
     LintId::of(overflow_check_conditional::OVERFLOW_CHECK_CONDITIONAL),
     LintId::of(partialeq_ne_impl::PARTIALEQ_NE_IMPL),
     LintId::of(precedence::PRECEDENCE),

clippy_lints/src/lib.register_lints.rs

@@ -399,6 +399,7 @@ store.register_lints(&[
     non_send_fields_in_send_ty::NON_SEND_FIELDS_IN_SEND_TY,
     nonstandard_macro_braces::NONSTANDARD_MACRO_BRACES,
     octal_escapes::OCTAL_ESCAPES,
+    only_used_in_recursion::ONLY_USED_IN_RECURSION,
     open_options::NONSENSICAL_OPEN_OPTIONS,
     option_env_unwrap::OPTION_ENV_UNWRAP,
     option_if_let_else::OPTION_IF_LET_ELSE,

clippy_lints/src/lib.rs

@@ -318,6 +318,7 @@ mod non_octal_unix_permissions;
 mod non_send_fields_in_send_ty;
 mod nonstandard_macro_braces;
 mod octal_escapes;
+mod only_used_in_recursion;
 mod open_options;
 mod option_env_unwrap;
 mod option_if_let_else;
@@ -857,6 +858,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move || Box::new(borrow_as_ptr::BorrowAsPtr::new(msrv)));
     store.register_late_pass(move || Box::new(manual_bits::ManualBits::new(msrv)));
     store.register_late_pass(|| Box::new(default_union_representation::DefaultUnionRepresentation));
+    store.register_late_pass(|| Box::new(only_used_in_recursion::OnlyUsedInRecursion));
     store.register_late_pass(|| Box::new(dbg_macro::DbgMacro));
     let cargo_ignore_publish = conf.cargo_ignore_publish;
     store.register_late_pass(move || {

clippy_lints/src/methods/option_map_or_none.rs

@@ -14,18 +14,15 @@ use super::RESULT_MAP_OR_INTO_OPTION;
 
 // The expression inside a closure may or may not have surrounding braces
 // which causes problems when generating a suggestion.
-fn reduce_unit_expression<'a>(
-    cx: &LateContext<'_>,
-    expr: &'a hir::Expr<'_>,
-) -> Option<(&'a hir::Expr<'a>, &'a [hir::Expr<'a>])> {
+fn reduce_unit_expression<'a>(expr: &'a hir::Expr<'_>) -> Option<(&'a hir::Expr<'a>, &'a [hir::Expr<'a>])> {
     match expr.kind {
         hir::ExprKind::Call(func, arg_char) => Some((func, arg_char)),
         hir::ExprKind::Block(block, _) => {
             match (block.stmts, block.expr) {
                 (&[], Some(inner_expr)) => {
                     // If block only contains an expression,
                     // reduce `|x| { x + 1 }` to `|x| x + 1`
-                    reduce_unit_expression(cx, inner_expr)
+                    reduce_unit_expression(inner_expr)
                 },
                 _ => None,
             }
@@ -77,7 +74,7 @@ pub(super) fn check<'tcx>(
         if let hir::ExprKind::Closure(_, _, id, span, _) = map_arg.kind;
             let arg_snippet = snippet(cx, span, "..");
             let body = cx.tcx.hir().body(id);
-                if let Some((func, [arg_char])) = reduce_unit_expression(cx, &body.value);
+                if let Some((func, [arg_char])) = reduce_unit_expression(&body.value);
                 if let Some(id) = path_def_id(cx, func).and_then(|ctor_id| cx.tcx.parent(ctor_id));
                 if Some(id) == cx.tcx.lang_items().option_some_variant();
                 then {