Add manual_filter lint for Option

Share much of its implementation with `manual_map` and should greatly benefit from its previous feedback.

Author: kraktus

CHANGELOG.md

@@ -3986,6 +3986,7 @@ 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

@@ -135,6 +135,7 @@ 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

@@ -257,6 +257,7 @@ 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 +1,181 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::{is_res_lang_ctor, path_res, path_to_local_id};
+use rustc_hir::intravisit::{walk_expr, Visitor};
+use rustc_hir::LangItem::OptionSome;
+use rustc_hir::{Arm, Block, BlockCheckMode, Expr, ExprKind, HirId, Pat, PatKind, UnsafeSource};
+use rustc_lint::LateContext;
+use rustc_span::{sym, SyntaxContext};
+
+use super::manual_map::{check_with, SomeExpr};
+use super::MANUAL_FILTER;
+
+#[derive(Default)]
+struct NeedsUnsafeBlock(pub bool);
+
+impl<'tcx> Visitor<'tcx> for NeedsUnsafeBlock {
+    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+        match expr.kind {
+            ExprKind::Block(
+                Block {
+                    rules: BlockCheckMode::UnsafeBlock(UnsafeSource::UserProvided),
+                    ..
+                },
+                _,
+            ) => {
+                self.0 = true;
+            },
+            _ => walk_expr(self, expr),
+        }
+    }
+}
+
+// Function called on the `expr` of `[&+]Some((ref | ref mut) x) => <expr>`
+// Need to check if it's of the `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 `cond` if
+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)
+            = (handle_if_or_else_expr(cx, target, ctxt, then_expr),
+                handle_if_or_else_expr(cx, target, ctxt, else_expr));
+        if then_visitor != else_visitor; // check that one expr resolves to `Some(x)`, the other to `None`
+        then {
+            let mut needs_unsafe_block = NeedsUnsafeBlock::default();
+            needs_unsafe_block.visit_expr(expr);
+            return Some(SomeExpr {
+                    expr: peels_blocks_incl_unsafe(cond.peel_drop_temps()),
+                    needs_unsafe_block: needs_unsafe_block.0,
+                    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 `NeedsUnsafeBlock`
+    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 <ifelse> expression:
+// Some(x) => if <cond> {
+//    <ifelse>
+// } else {
+//    <ifelse>
+// }
+// Returns true if <ifelse> resolves to `Some(x)`, `false` otherwise
+fn handle_if_or_else_expr<'tcx>(
+    cx: &LateContext<'_>,
+    target: HirId,
+    ctxt: SyntaxContext,
+    if_or_else_expr: &'tcx Expr<'_>,
+) -> bool {
+    if let Some(inner_expr) = peels_blocks_incl_unsafe_opt(if_or_else_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 == if_or_else_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_chain! {
+        if is_type_diagnostic_item(cx, ty, sym::Option);
+        if arms.len() == 2;
+        if arms[0].guard.is_none();
+        if arms[1].guard.is_none();
+        then {
+            check(cx, expr, scrutinee, arms[0].pat, arms[0].body, Some(arms[1].pat), arms[1].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,
+        );
+    }
+}