Auto merge of rust-lang#9451 - kraktus:manual_filter2, r=dswij

Add `manual_filter` lint for `Option`

Share much of its implementation with `manual_map` and should greatly benefit from its previous feedback.
I'm sure it's possible to even more refactor both and would gladly take input on that as well as any clippy idiomatic usage, since this is my first lint addition.

I've added the lint to the complexity section for now, I don't know if every new lint needs to go in nursery first.

The matching could be expanded to more than `Some(<value>)` to lint on arbitrary struct matching inside the `Some` but I've left it like it was for `manual_map` for now. `needless_match::pat_same_as_expr` provides a more generic match example.

close rust-lang/rust-clippy#8822

changelog: Add lint [`manual_filter`] for `Option`

Author: bors

CHANGELOG.md

@@ -3988,6 +3988,7 @@ Released 2018-09-13
 [`manual_async_fn`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_async_fn
 [`manual_bits`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_bits
 [`manual_clamp`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_clamp
+[`manual_filter`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_filter
 [`manual_filter_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_filter_map
 [`manual_find`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_find
 [`manual_find_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#manual_find_map

clippy_lints/src/lib.register_all.rs

@@ -137,6 +137,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(match_result_ok::MATCH_RESULT_OK),
     LintId::of(matches::COLLAPSIBLE_MATCH),
     LintId::of(matches::INFALLIBLE_DESTRUCTURING_MATCH),
+    LintId::of(matches::MANUAL_FILTER),
     LintId::of(matches::MANUAL_MAP),
     LintId::of(matches::MANUAL_UNWRAP_OR),
     LintId::of(matches::MATCH_AS_REF),

clippy_lints/src/lib.register_complexity.rs

@@ -27,6 +27,7 @@ store.register_group(true, "clippy::complexity", Some("clippy_complexity"), vec!
     LintId::of(manual_strip::MANUAL_STRIP),
     LintId::of(map_unit_fn::OPTION_MAP_UNIT_FN),
     LintId::of(map_unit_fn::RESULT_MAP_UNIT_FN),
+    LintId::of(matches::MANUAL_FILTER),
     LintId::of(matches::MANUAL_UNWRAP_OR),
     LintId::of(matches::MATCH_AS_REF),
     LintId::of(matches::MATCH_SINGLE_BINDING),

clippy_lints/src/lib.register_lints.rs

@@ -259,6 +259,7 @@ store.register_lints(&[
     match_result_ok::MATCH_RESULT_OK,
     matches::COLLAPSIBLE_MATCH,
     matches::INFALLIBLE_DESTRUCTURING_MATCH,
+    matches::MANUAL_FILTER,
     matches::MANUAL_MAP,
     matches::MANUAL_UNWRAP_OR,
     matches::MATCH_AS_REF,

clippy_lints/src/matches/manual_filter.rs

@@ -0,0 +1,153 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::visitors::contains_unsafe_block;
+use clippy_utils::{is_res_lang_ctor, path_res, path_to_local_id};
+
+use rustc_hir::LangItem::OptionSome;
+use rustc_hir::{Arm, Expr, ExprKind, HirId, Pat, PatKind};
+use rustc_lint::LateContext;
+use rustc_span::{sym, SyntaxContext};
+
+use super::manual_utils::{check_with, SomeExpr};
+use super::MANUAL_FILTER;
+
+// Function called on the <expr> of `[&+]Some((ref | ref mut) x) => <expr>`
+// Need to check if it's of the form `<expr>=if <cond> {<then_expr>} else {<else_expr>}`
+// AND that only one `then/else_expr` resolves to `Some(x)` while the other resolves to `None`
+// return the `cond` expression if so.
+fn get_cond_expr<'tcx>(
+    cx: &LateContext<'tcx>,
+    pat: &Pat<'_>,
+    expr: &'tcx Expr<'_>,
+    ctxt: SyntaxContext,
+) -> Option<SomeExpr<'tcx>> {
+    if_chain! {
+        if let Some(block_expr) = peels_blocks_incl_unsafe_opt(expr);
+        if let ExprKind::If(cond, then_expr, Some(else_expr)) = block_expr.kind;
+        if let PatKind::Binding(_,target, ..) = pat.kind;
+        if let (then_visitor, else_visitor)
+            = (is_some_expr(cx, target, ctxt, then_expr),
+                is_some_expr(cx, target, ctxt, else_expr));
+        if then_visitor != else_visitor; // check that one expr resolves to `Some(x)`, the other to `None`
+        then {
+            return Some(SomeExpr {
+                    expr: peels_blocks_incl_unsafe(cond.peel_drop_temps()),
+                    needs_unsafe_block: contains_unsafe_block(cx, expr),
+                    needs_negated: !then_visitor // if the `then_expr` resolves to `None`, need to negate the cond
+                })
+            }
+    };
+    None
+}
+
+fn peels_blocks_incl_unsafe_opt<'a>(expr: &'a Expr<'a>) -> Option<&'a Expr<'a>> {
+    // we don't want to use `peel_blocks` here because we don't care if the block is unsafe, it's
+    // checked by `contains_unsafe_block`
+    if let ExprKind::Block(block, None) = expr.kind {
+        if block.stmts.is_empty() {
+            return block.expr;
+        }
+    };
+    None
+}
+
+fn peels_blocks_incl_unsafe<'a>(expr: &'a Expr<'a>) -> &'a Expr<'a> {
+    peels_blocks_incl_unsafe_opt(expr).unwrap_or(expr)
+}
+
+// function called for each <expr> expression:
+// Some(x) => if <cond> {
+//    <expr>
+// } else {
+//    <expr>
+// }
+// Returns true if <expr> resolves to `Some(x)`, `false` otherwise
+fn is_some_expr<'tcx>(cx: &LateContext<'_>, target: HirId, ctxt: SyntaxContext, expr: &'tcx Expr<'_>) -> bool {
+    if let Some(inner_expr) = peels_blocks_incl_unsafe_opt(expr) {
+        // there can be not statements in the block as they would be removed when switching to `.filter`
+        if let ExprKind::Call(callee, [arg]) = inner_expr.kind {
+            return ctxt == expr.span.ctxt()
+                && is_res_lang_ctor(cx, path_res(cx, callee), OptionSome)
+                && path_to_local_id(arg, target);
+        }
+    };
+    false
+}
+
+// given the closure: `|<pattern>| <expr>`
+// returns `|&<pattern>| <expr>`
+fn add_ampersand_if_copy(body_str: String, has_copy_trait: bool) -> String {
+    if has_copy_trait {
+        let mut with_ampersand = body_str;
+        with_ampersand.insert(1, '&');
+        with_ampersand
+    } else {
+        body_str
+    }
+}
+
+pub(super) fn check_match<'tcx>(
+    cx: &LateContext<'tcx>,
+    scrutinee: &'tcx Expr<'_>,
+    arms: &'tcx [Arm<'_>],
+    expr: &'tcx Expr<'_>,
+) {
+    let ty = cx.typeck_results().expr_ty(expr);
+    if is_type_diagnostic_item(cx, ty, sym::Option)
+    && let [first_arm, second_arm] = arms
+    && first_arm.guard.is_none()
+    && second_arm.guard.is_none()
+         {
+            check(cx, expr, scrutinee, first_arm.pat, first_arm.body, Some(second_arm.pat), second_arm.body);
+        }
+}
+
+pub(super) fn check_if_let<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx Expr<'_>,
+    let_pat: &'tcx Pat<'_>,
+    let_expr: &'tcx Expr<'_>,
+    then_expr: &'tcx Expr<'_>,
+    else_expr: &'tcx Expr<'_>,
+) {
+    check(cx, expr, let_expr, let_pat, then_expr, None, else_expr);
+}
+
+fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx Expr<'_>,
+    scrutinee: &'tcx Expr<'_>,
+    then_pat: &'tcx Pat<'_>,
+    then_body: &'tcx Expr<'_>,
+    else_pat: Option<&'tcx Pat<'_>>,
+    else_body: &'tcx Expr<'_>,
+) {
+    if let Some(sugg_info) = check_with(
+        cx,
+        expr,
+        scrutinee,
+        then_pat,
+        then_body,
+        else_pat,
+        else_body,
+        get_cond_expr,
+    ) {
+        let body_str = add_ampersand_if_copy(sugg_info.body_str, sugg_info.scrutinee_impl_copy);
+        span_lint_and_sugg(
+            cx,
+            MANUAL_FILTER,
+            expr.span,
+            "manual implementation of `Option::filter`",
+            "try this",
+            if sugg_info.needs_brackets {
+                format!(
+                    "{{ {}{}.filter({body_str}) }}",
+                    sugg_info.scrutinee_str, sugg_info.as_ref_str
+                )
+            } else {
+                format!("{}{}.filter({body_str})", sugg_info.scrutinee_str, sugg_info.as_ref_str)
+            },
+            sugg_info.app,
+        );
+    }
+}