Add Collapsible match lint

CHANGELOG.md

@@ -1770,6 +1770,7 @@ Released 2018-09-13
 [`cmp_owned`]: https://rust-lang.github.io/rust-clippy/master/index.html#cmp_owned
 [`cognitive_complexity`]: https://rust-lang.github.io/rust-clippy/master/index.html#cognitive_complexity
 [`collapsible_if`]: https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_if
+[`collapsible_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match
 [`comparison_chain`]: https://rust-lang.github.io/rust-clippy/master/index.html#comparison_chain
 [`comparison_to_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#comparison_to_empty
 [`copy_iterator`]: https://rust-lang.github.io/rust-clippy/master/index.html#copy_iterator

clippy_lints/src/collapsible_match.rs

@@ -0,0 +1,172 @@
+use crate::utils::visitors::LocalUsedVisitor;
+use crate::utils::{span_lint_and_then, SpanlessEq};
+use if_chain::if_chain;
+use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res};
+use rustc_hir::{Arm, Expr, ExprKind, Guard, HirId, Pat, PatKind, QPath, StmtKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::{DefIdTree, TyCtxt};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::{MultiSpan, Span};
+
+declare_clippy_lint! {
+    /// **What it does:** Finds nested `match` or `if let` expressions where the patterns may be "collapsed" together
+    /// without adding any branches.
+    ///
+    /// Note that this lint is not intended to find _all_ cases where nested match patterns can be merged, but only
+    /// cases where merging would most likely make the code more readable.
+    ///
+    /// **Why is this bad?** It is unnecessarily verbose and complex.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// fn func(opt: Option<Result<u64, String>>) {
+    ///     let n = match opt {
+    ///         Some(n) => match n {
+    ///             Ok(n) => n,
+    ///             _ => return,
+    ///         }
+    ///         None => return,
+    ///     };
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// fn func(opt: Option<Result<u64, String>>) {
+    ///     let n = match opt {
+    ///         Some(Ok(n)) => n,
+    ///         _ => return,
+    ///     };
+    /// }
+    /// ```
+    pub COLLAPSIBLE_MATCH,
+    style,
+    "Nested `match` or `if let` expressions where the patterns may be \"collapsed\" together."
+}
+
+declare_lint_pass!(CollapsibleMatch => [COLLAPSIBLE_MATCH]);
+
+impl<'tcx> LateLintPass<'tcx> for CollapsibleMatch {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
+        if let ExprKind::Match(_expr, arms, _source) = expr.kind {
+            if let Some(wild_arm) = arms.iter().rfind(|arm| arm_is_wild_like(arm, cx.tcx)) {
+                for arm in arms {
+                    check_arm(arm, wild_arm, cx);
+                }
+            }
+        }
+    }
+}
+
+fn check_arm(arm: &Arm<'_>, wild_outer_arm: &Arm<'_>, cx: &LateContext<'_>) {
+    if_chain! {
+        let expr = strip_singleton_blocks(arm.body);
+        if let ExprKind::Match(expr_in, arms_inner, _) = expr.kind;
+        // the outer arm pattern and the inner match
+        if expr_in.span.ctxt() == arm.pat.span.ctxt();
+        // there must be no more than two arms in the inner match for this lint
+        if arms_inner.len() == 2;
+        // no if guards on the inner match
+        if arms_inner.iter().all(|arm| arm.guard.is_none());
+        // match expression must be a local binding
+        // match <local> { .. }
+        if let ExprKind::Path(QPath::Resolved(None, path)) = expr_in.kind;
+        if let Res::Local(binding_id) = path.res;
+        // one of the branches must be "wild-like"
+        if let Some(wild_inner_arm_idx) = arms_inner.iter().rposition(|arm_inner| arm_is_wild_like(arm_inner, cx.tcx));
+        let (wild_inner_arm, non_wild_inner_arm) =
+            (&arms_inner[wild_inner_arm_idx], &arms_inner[1 - wild_inner_arm_idx]);
+        if !pat_contains_or(non_wild_inner_arm.pat);
+        // the binding must come from the pattern of the containing match arm
+        // ..<local>.. => match <local> { .. }
+        if let Some(binding_span) = find_pat_binding(arm.pat, binding_id);
+        // the "wild-like" branches must be equal
+        if SpanlessEq::new(cx).eq_expr(wild_inner_arm.body, wild_outer_arm.body);
+        // the binding must not be used in the if guard
+        if !matches!(arm.guard, Some(Guard::If(guard)) if LocalUsedVisitor::new(binding_id).check_expr(guard));
+        // ...or anywhere in the inner match
+        if !arms_inner.iter().any(|arm| LocalUsedVisitor::new(binding_id).check_arm(arm));
+        then {
+            span_lint_and_then(
+                cx,
+                COLLAPSIBLE_MATCH,
+                expr.span,
+                "Unnecessary nested match",
+                |diag| {
+                    let mut help_span = MultiSpan::from_spans(vec![binding_span, non_wild_inner_arm.pat.span]);
+                    help_span.push_span_label(binding_span, "Replace this binding".into());
+                    help_span.push_span_label(non_wild_inner_arm.pat.span, "with this pattern".into());
+                    diag.span_help(help_span, "The outer pattern can be modified to include the inner pattern.");
+                },
+            );
+        }
+    }
+}
+
+fn strip_singleton_blocks<'hir>(mut expr: &'hir Expr<'hir>) -> &'hir Expr<'hir> {
+    while let ExprKind::Block(block, _) = expr.kind {
+        match (block.stmts, block.expr) {
+            ([stmt], None) => match stmt.kind {
+                StmtKind::Expr(e) | StmtKind::Semi(e) => expr = e,
+                _ => break,
+            },
+            ([], Some(e)) => expr = e,
+            _ => break,
+        }
+    }
+    expr
+}
+
+/// A "wild-like" pattern is wild ("_") or `None`.
+/// For this lint to apply, both the outer and inner match expressions
+/// must have "wild-like" branches that can be combined.
+fn arm_is_wild_like(arm: &Arm<'_>, tcx: TyCtxt<'_>) -> bool {
+    if arm.guard.is_some() {
+        return false;
+    }
+    match arm.pat.kind {
+        PatKind::Binding(..) | PatKind::Wild => true,
+        PatKind::Path(QPath::Resolved(None, path)) if is_none_ctor(path.res, tcx) => true,
+        _ => false,
+    }
+}
+
+fn find_pat_binding(pat: &Pat<'_>, hir_id: HirId) -> Option<Span> {
+    let mut span = None;
+    pat.walk_short(|p| match &p.kind {
+        // ignore OR patterns
+        PatKind::Or(_) => false,
+        PatKind::Binding(_bm, _, _ident, _) => {
+            let found = p.hir_id == hir_id;
+            if found {
+                span = Some(p.span);
+            }
+            !found
+        },
+        _ => true,
+    });
+    span
+}
+
+fn pat_contains_or(pat: &Pat<'_>) -> bool {
+    let mut result = false;
+    pat.walk(|p| {
+        let is_or = matches!(p.kind, PatKind::Or(_));
+        result |= is_or;
+        !is_or
+    });
+    result
+}
+
+fn is_none_ctor(res: Res, tcx: TyCtxt<'_>) -> bool {
+    if let Some(none_id) = tcx.lang_items().option_none_variant() {
+        if let Res::Def(DefKind::Ctor(CtorOf::Variant, CtorKind::Const), id) = res {
+            if let Some(variant_id) = tcx.parent(id) {
+                return variant_id == none_id;
+            }
+        }
+    }
+    false
+}

clippy_lints/src/default.rs

@@ -280,8 +280,7 @@ fn field_reassigned_by_stmt<'tcx>(this: &Stmt<'tcx>, binding_name: Symbol) -> Op
         // only take assignments to fields where the left-hand side field is a field of
         // the same binding as the previous statement
         if let ExprKind::Field(ref binding, field_ident) = assign_lhs.kind;
-        if let ExprKind::Path(ref qpath) = binding.kind;
-        if let QPath::Resolved(_, path) = qpath;
+        if let ExprKind::Path(QPath::Resolved(_, path)) = binding.kind;
         if let Some(second_binding_name) = path.segments.last();
         if second_binding_name.ident.name == binding_name;
         then {

clippy_lints/src/if_let_some_result.rs

@@ -41,8 +41,7 @@ declare_lint_pass!(OkIfLet => [IF_LET_SOME_RESULT]);
 impl<'tcx> LateLintPass<'tcx> for OkIfLet {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
         if_chain! { //begin checking variables
-            if let ExprKind::Match(ref op, ref body, source) = expr.kind; //test if expr is a match
-            if let MatchSource::IfLetDesugar { .. } = source; //test if it is an If Let
+            if let ExprKind::Match(ref op, ref body, MatchSource::IfLetDesugar { .. }) = expr.kind; //test if expr is if let
             if let ExprKind::MethodCall(_, ok_span, ref result_types, _) = op.kind; //check is expr.ok() has type Result<T,E>.ok(, _)
             if let PatKind::TupleStruct(QPath::Resolved(_, ref x), ref y, _)  = body[0].pat.kind; //get operation
             if method_chain_args(op, &["ok"]).is_some(); //test to see if using ok() methoduse std::marker::Sized;

clippy_lints/src/implicit_return.rs

@@ -68,8 +68,7 @@ fn expr_match(cx: &LateContext<'_>, expr: &Expr<'_>) {
                 if_chain! {
                     if let StmtKind::Semi(expr, ..) = &stmt.kind;
                     // make sure it's a break, otherwise we want to skip
-                    if let ExprKind::Break(.., break_expr) = &expr.kind;
-                    if let Some(break_expr) = break_expr;
+                    if let ExprKind::Break(.., Some(break_expr)) = &expr.kind;
                     then {
                             lint(cx, expr.span, break_expr.span, LINT_BREAK);
                     }

clippy_lints/src/implicit_saturating_sub.rs

@@ -59,8 +59,7 @@ impl<'tcx> LateLintPass<'tcx> for ImplicitSaturatingSub {
             if let Some(target) = subtracts_one(cx, e);
 
             // Extracting out the variable name
-            if let ExprKind::Path(ref assign_path) = target.kind;
-            if let QPath::Resolved(_, ref ares_path) = assign_path;
+            if let ExprKind::Path(QPath::Resolved(_, ref ares_path)) = target.kind;
 
             then {
                 // Handle symmetric conditions in the if statement

clippy_lints/src/large_const_arrays.rs

@@ -52,8 +52,7 @@ impl<'tcx> LateLintPass<'tcx> for LargeConstArrays {
             if let ItemKind::Const(hir_ty, _) = &item.kind;
             let ty = hir_ty_to_ty(cx.tcx, hir_ty);
             if let ty::Array(element_type, cst) = ty.kind();
-            if let ConstKind::Value(val) = cst.val;
-            if let ConstValue::Scalar(element_count) = val;
+            if let ConstKind::Value(ConstValue::Scalar(element_count)) = cst.val;
             if let Ok(element_count) = element_count.to_machine_usize(&cx.tcx);
             if let Ok(element_size) = cx.layout_of(element_type).map(|l| l.size.bytes());
             if self.maximum_allowed_size < element_count * element_size;

clippy_lints/src/large_stack_arrays.rs

@@ -43,8 +43,7 @@ impl<'tcx> LateLintPass<'tcx> for LargeStackArrays {
         if_chain! {
             if let ExprKind::Repeat(_, _) = expr.kind;
             if let ty::Array(element_type, cst) = cx.typeck_results().expr_ty(expr).kind();
-            if let ConstKind::Value(val) = cst.val;
-            if let ConstValue::Scalar(element_count) = val;
+            if let ConstKind::Value(ConstValue::Scalar(element_count)) = cst.val;
             if let Ok(element_count) = element_count.to_machine_usize(&cx.tcx);
             if let Ok(element_size) = cx.layout_of(element_type).map(|l| l.size.bytes());
             if self.maximum_allowed_size < element_count * element_size;

clippy_lints/src/let_if_seq.rs

@@ -1,12 +1,11 @@
+use crate::utils::visitors::LocalUsedVisitor;
 use crate::utils::{higher, qpath_res, snippet, span_lint_and_then};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::def::Res;
-use rustc_hir::intravisit;
 use rustc_hir::BindingAnnotation;
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::hir::map::Map;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
 declare_clippy_lint! {
@@ -66,10 +65,10 @@ impl<'tcx> LateLintPass<'tcx> for LetIfSeq {
                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind;
                 if let hir::StmtKind::Expr(ref if_) = expr.kind;
                 if let Some((ref cond, ref then, ref else_)) = higher::if_block(&if_);
-                if !used_in_expr(cx, canonical_id, cond);
+                if !LocalUsedVisitor::new(canonical_id).check_expr(cond);
                 if let hir::ExprKind::Block(ref then, _) = then.kind;
                 if let Some(value) = check_assign(cx, canonical_id, &*then);
-                if !used_in_expr(cx, canonical_id, value);
+                if !LocalUsedVisitor::new(canonical_id).check_expr(value);
                 then {
                     let span = stmt.span.to(if_.span);
 
@@ -136,32 +135,6 @@ impl<'tcx> LateLintPass<'tcx> for LetIfSeq {
     }
 }
 
-struct UsedVisitor<'a, 'tcx> {
-    cx: &'a LateContext<'tcx>,
-    id: hir::HirId,
-    used: bool,
-}
-
-impl<'a, 'tcx> intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
-    type Map = Map<'tcx>;
-
-    fn visit_expr(&mut self, expr: &'tcx hir::Expr<'_>) {
-        if_chain! {
-            if let hir::ExprKind::Path(ref qpath) = expr.kind;
-            if let Res::Local(local_id) = qpath_res(self.cx, qpath, expr.hir_id);
-            if self.id == local_id;
-            then {
-                self.used = true;
-                return;
-            }
-        }
-        intravisit::walk_expr(self, expr);
-    }
-    fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
-        intravisit::NestedVisitorMap::None
-    }
-}
-
 fn check_assign<'tcx>(
     cx: &LateContext<'tcx>,
     decl: hir::HirId,
@@ -176,18 +149,10 @@ fn check_assign<'tcx>(
         if let Res::Local(local_id) = qpath_res(cx, qpath, var.hir_id);
         if decl == local_id;
         then {
-            let mut v = UsedVisitor {
-                cx,
-                id: decl,
-                used: false,
-            };
-
-            for s in block.stmts.iter().take(block.stmts.len()-1) {
-                intravisit::walk_stmt(&mut v, s);
+            let mut v = LocalUsedVisitor::new(decl);
 
-                if v.used {
-                    return None;
-                }
+            if block.stmts.iter().take(block.stmts.len()-1).any(|stmt| v.check_stmt(stmt)) {
+                return None;
             }
 
             return Some(value);
@@ -196,9 +161,3 @@ fn check_assign<'tcx>(
 
     None
 }
-
-fn used_in_expr<'tcx>(cx: &LateContext<'tcx>, id: hir::HirId, expr: &'tcx hir::Expr<'_>) -> bool {
-    let mut v = UsedVisitor { cx, id, used: false };
-    intravisit::walk_expr(&mut v, expr);
-    v.used
-}

clippy_lints/src/lib.rs

@@ -172,6 +172,7 @@ mod cargo_common_metadata;
 mod checked_conversions;
 mod cognitive_complexity;
 mod collapsible_if;
+mod collapsible_match;
 mod comparison_chain;
 mod copies;
 mod copy_iterator;
@@ -531,6 +532,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &checked_conversions::CHECKED_CONVERSIONS,
         &cognitive_complexity::COGNITIVE_COMPLEXITY,
         &collapsible_if::COLLAPSIBLE_IF,
+        &collapsible_match::COLLAPSIBLE_MATCH,
         &comparison_chain::COMPARISON_CHAIN,
         &copies::IFS_SAME_COND,
         &copies::IF_SAME_THEN_ELSE,
@@ -960,6 +962,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box len_zero::LenZero);
     store.register_late_pass(|| box attrs::Attributes);
     store.register_late_pass(|| box blocks_in_if_conditions::BlocksInIfConditions);
+    store.register_late_pass(|| box collapsible_match::CollapsibleMatch);
     store.register_late_pass(|| box unicode::Unicode);
     store.register_late_pass(|| box unit_return_expecting_ord::UnitReturnExpectingOrd);
     store.register_late_pass(|| box strings::StringAdd);
@@ -1351,6 +1354,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&booleans::NONMINIMAL_BOOL),
         LintId::of(&bytecount::NAIVE_BYTECOUNT),
         LintId::of(&collapsible_if::COLLAPSIBLE_IF),
+        LintId::of(&collapsible_match::COLLAPSIBLE_MATCH),
         LintId::of(&comparison_chain::COMPARISON_CHAIN),
         LintId::of(&copies::IFS_SAME_COND),
         LintId::of(&copies::IF_SAME_THEN_ELSE),
@@ -1617,6 +1621,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&blacklisted_name::BLACKLISTED_NAME),
         LintId::of(&blocks_in_if_conditions::BLOCKS_IN_IF_CONDITIONS),
         LintId::of(&collapsible_if::COLLAPSIBLE_IF),
+        LintId::of(&collapsible_match::COLLAPSIBLE_MATCH),
         LintId::of(&comparison_chain::COMPARISON_CHAIN),
         LintId::of(&default::FIELD_REASSIGN_WITH_DEFAULT),
         LintId::of(&doc::MISSING_SAFETY_DOC),