clippy_lints/src/matches/needless_match.rs

   1 use super::NEEDLESS_MATCH;
   2 use clippy_utils::diagnostics::span_lint_and_sugg;
   3 use clippy_utils::source::snippet_with_applicability;
   4 use clippy_utils::ty::{is_type_diagnostic_item, same_type_and_consts};
   5 use clippy_utils::{
   6     eq_expr_value, get_parent_expr_for_hir, get_parent_node, higher, is_else_clause, is_lang_ctor, over,
   7     peel_blocks_with_stmt,
   8 };
   9 use rustc_errors::Applicability;
  10 use rustc_hir::LangItem::OptionNone;
  11 use rustc_hir::{Arm, BindingAnnotation, ByRef, Expr, ExprKind, FnRetTy, Guard, Node, Pat, PatKind, Path, QPath};
  12 use rustc_lint::LateContext;
  13 use rustc_span::sym;
  14 use rustc_typeck::hir_ty_to_ty;
  15
  16 pub(crate) fn check_match(cx: &LateContext<'_>, ex: &Expr<'_>, arms: &[Arm<'_>], expr: &Expr<'_>) {
  17     if arms.len() > 1 && expr_ty_matches_p_ty(cx, ex, expr) && check_all_arms(cx, ex, arms) {
  18         let mut applicability = Applicability::MachineApplicable;
  19         span_lint_and_sugg(
  20             cx,
  21             NEEDLESS_MATCH,
  22             expr.span,
  23             "this match expression is unnecessary",
  24             "replace it with",
  25             snippet_with_applicability(cx, ex.span, "..", &mut applicability).to_string(),
  26             applicability,
  27         );
  28     }
  29 }
  30
  31 /// Check for nop `if let` expression that assembled as unnecessary match
  32 ///
  33 /// ```rust,ignore
  34 /// if let Some(a) = option {
  35 ///     Some(a)
  36 /// } else {
  37 ///     None
  38 /// }
  39 /// ```
  40 /// OR
  41 /// ```rust,ignore
  42 /// if let SomeEnum::A = some_enum {
  43 ///     SomeEnum::A
  44 /// } else if let SomeEnum::B = some_enum {
  45 ///     SomeEnum::B
  46 /// } else {
  47 ///     some_enum
  48 /// }
  49 /// ```
  50 pub(crate) fn check_if_let<'tcx>(cx: &LateContext<'tcx>, ex: &Expr<'_>, if_let: &higher::IfLet<'tcx>) {
  51     if !is_else_clause(cx.tcx, ex) && expr_ty_matches_p_ty(cx, if_let.let_expr, ex) && check_if_let_inner(cx, if_let) {
  52         let mut applicability = Applicability::MachineApplicable;
  53         span_lint_and_sugg(
  54             cx,
  55             NEEDLESS_MATCH,
  56             ex.span,
  57             "this if-let expression is unnecessary",
  58             "replace it with",
  59             snippet_with_applicability(cx, if_let.let_expr.span, "..", &mut applicability).to_string(),
  60             applicability,
  61         );
  62     }
  63 }
  64
  65 fn check_all_arms(cx: &LateContext<'_>, match_expr: &Expr<'_>, arms: &[Arm<'_>]) -> bool {
  66     for arm in arms {
  67         let arm_expr = peel_blocks_with_stmt(arm.body);
  68
  69         if let Some(guard_expr) = &arm.guard {
  70             match guard_expr {
  71                 // gives up if `pat if expr` can have side effects
  72                 Guard::If(if_cond) => {
  73                     if if_cond.can_have_side_effects() {
  74                         return false;
  75                     }
  76                 },
  77                 // gives up `pat if let ...` arm
  78                 Guard::IfLet(_) => {
  79                     return false;
  80                 },
  81             };
  82         }
  83
  84         if let PatKind::Wild = arm.pat.kind {
  85             if !eq_expr_value(cx, match_expr, strip_return(arm_expr)) {
  86                 return false;
  87             }
  88         } else if !pat_same_as_expr(arm.pat, arm_expr) {
  89             return false;
  90         }
  91     }
  92
  93     true
  94 }
  95
  96 fn check_if_let_inner(cx: &LateContext<'_>, if_let: &higher::IfLet<'_>) -> bool {
  97     if let Some(if_else) = if_let.if_else {
  98         if !pat_same_as_expr(if_let.let_pat, peel_blocks_with_stmt(if_let.if_then)) {
  99             return false;
 100         }
 101
 102         // Recursively check for each `else if let` phrase,
 103         if let Some(ref nested_if_let) = higher::IfLet::hir(cx, if_else) {
 104             return check_if_let_inner(cx, nested_if_let);
 105         }
 106
 107         if matches!(if_else.kind, ExprKind::Block(..)) {
 108             let else_expr = peel_blocks_with_stmt(if_else);
 109             if matches!(else_expr.kind, ExprKind::Block(..)) {
 110                 return false;
 111             }
 112             let ret = strip_return(else_expr);
 113             let let_expr_ty = cx.typeck_results().expr_ty(if_let.let_expr);
 114             if is_type_diagnostic_item(cx, let_expr_ty, sym::Option) {
 115                 if let ExprKind::Path(ref qpath) = ret.kind {
 116                     return is_lang_ctor(cx, qpath, OptionNone) || eq_expr_value(cx, if_let.let_expr, ret);
 117                 }
 118                 return false;
 119             }
 120             return eq_expr_value(cx, if_let.let_expr, ret);
 121         }
 122     }
 123
 124     false
 125 }
 126
 127 /// Strip `return` keyword if the expression type is `ExprKind::Ret`.
 128 fn strip_return<'hir>(expr: &'hir Expr<'hir>) -> &'hir Expr<'hir> {
 129     if let ExprKind::Ret(Some(ret)) = expr.kind {
 130         ret
 131     } else {
 132         expr
 133     }
 134 }
 135
 136 /// Manually check for coercion casting by checking if the type of the match operand or let expr
 137 /// differs with the assigned local variable or the function return type.
 138 fn expr_ty_matches_p_ty(cx: &LateContext<'_>, expr: &Expr<'_>, p_expr: &Expr<'_>) -> bool {
 139     if let Some(p_node) = get_parent_node(cx.tcx, p_expr.hir_id) {
 140         match p_node {
 141             // Compare match_expr ty with local in `let local = match match_expr {..}`
 142             Node::Local(local) => {
 143                 let results = cx.typeck_results();
 144                 return same_type_and_consts(results.node_type(local.hir_id), results.expr_ty(expr));
 145             },
 146             // compare match_expr ty with RetTy in `fn foo() -> RetTy`
 147             Node::Item(..) => {
 148                 if let Some(fn_decl) = p_node.fn_decl() {
 149                     if let FnRetTy::Return(ret_ty) = fn_decl.output {
 150                         return same_type_and_consts(hir_ty_to_ty(cx.tcx, ret_ty), cx.typeck_results().expr_ty(expr));
 151                     }
 152                 }
 153             },
 154             // check the parent expr for this whole block `{ match match_expr {..} }`
 155             Node::Block(block) => {
 156                 if let Some(block_parent_expr) = get_parent_expr_for_hir(cx, block.hir_id) {
 157                     return expr_ty_matches_p_ty(cx, expr, block_parent_expr);
 158                 }
 159             },
 160             // recursively call on `if xxx {..}` etc.
 161             Node::Expr(p_expr) => {
 162                 return expr_ty_matches_p_ty(cx, expr, p_expr);
 163             },
 164             _ => {},
 165         }
 166     }
 167     false
 168 }
 169
 170 fn pat_same_as_expr(pat: &Pat<'_>, expr: &Expr<'_>) -> bool {
 171     let expr = strip_return(expr);
 172     match (&pat.kind, &expr.kind) {
 173         // Example: `Some(val) => Some(val)`
 174         (PatKind::TupleStruct(QPath::Resolved(_, path), tuple_params, _), ExprKind::Call(call_expr, call_params)) => {
 175             if let ExprKind::Path(QPath::Resolved(_, call_path)) = call_expr.kind {
 176                 return over(path.segments, call_path.segments, |pat_seg, call_seg| {
 177                     pat_seg.ident.name == call_seg.ident.name
 178                 }) && same_non_ref_symbols(tuple_params, call_params);
 179             }
 180         },
 181         // Example: `val => val`
 182         (
 183             PatKind::Binding(annot, _, pat_ident, _),
 184             ExprKind::Path(QPath::Resolved(
 185                 _,
 186                 Path {
 187                     segments: [first_seg, ..],
 188                     ..
 189                 },
 190             )),
 191         ) => {
 192             return !matches!(annot, BindingAnnotation(ByRef::Yes, _)) && pat_ident.name == first_seg.ident.name;
 193         },
 194         // Example: `Custom::TypeA => Custom::TypeB`, or `None => None`
 195         (PatKind::Path(QPath::Resolved(_, p_path)), ExprKind::Path(QPath::Resolved(_, e_path))) => {
 196             return over(p_path.segments, e_path.segments, |p_seg, e_seg| {
 197                 p_seg.ident.name == e_seg.ident.name
 198             });
 199         },
 200         // Example: `5 => 5`
 201         (PatKind::Lit(pat_lit_expr), ExprKind::Lit(expr_spanned)) => {
 202             if let ExprKind::Lit(pat_spanned) = &pat_lit_expr.kind {
 203                 return pat_spanned.node == expr_spanned.node;
 204             }
 205         },
 206         _ => {},
 207     }
 208
 209     false
 210 }
 211
 212 fn same_non_ref_symbols(pats: &[Pat<'_>], exprs: &[Expr<'_>]) -> bool {
 213     if pats.len() != exprs.len() {
 214         return false;
 215     }
 216
 217     for i in 0..pats.len() {
 218         if !pat_same_as_expr(&pats[i], &exprs[i]) {
 219             return false;
 220         }
 221     }
 222
 223     true
 224 }