clippy_lints/src/let_if_seq.rs

   1 use crate::utils::{snippet, span_lint_and_then};
   2 use if_chain::if_chain;
   3 use rustc::hir;
   4 use rustc::hir::def::Def;
   5 use rustc::hir::BindingAnnotation;
   6 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   7 use rustc::{declare_tool_lint, lint_array};
   8 use rustc_errors::Applicability;
   9
  10 declare_clippy_lint! {
  11     /// **What it does:** Checks for variable declarations immediately followed by a
  12     /// conditional affectation.
  13     ///
  14     /// **Why is this bad?** This is not idiomatic Rust.
  15     ///
  16     /// **Known problems:** None.
  17     ///
  18     /// **Example:**
  19     /// ```rust,ignore
  20     /// let foo;
  21     ///
  22     /// if bar() {
  23     ///     foo = 42;
  24     /// } else {
  25     ///     foo = 0;
  26     /// }
  27     ///
  28     /// let mut baz = None;
  29     ///
  30     /// if bar() {
  31     ///     baz = Some(42);
  32     /// }
  33     /// ```
  34     ///
  35     /// should be written
  36     ///
  37     /// ```rust,ignore
  38     /// let foo = if bar() {
  39     ///     42
  40     /// } else {
  41     ///     0
  42     /// };
  43     ///
  44     /// let baz = if bar() {
  45     ///     Some(42)
  46     /// } else {
  47     ///     None
  48     /// };
  49     /// ```
  50     pub USELESS_LET_IF_SEQ,
  51     style,
  52     "unidiomatic `let mut` declaration followed by initialization in `if`"
  53 }
  54
  55 #[derive(Copy, Clone)]
  56 pub struct LetIfSeq;
  57
  58 impl LintPass for LetIfSeq {
  59     fn get_lints(&self) -> LintArray {
  60         lint_array!(USELESS_LET_IF_SEQ)
  61     }
  62
  63     fn name(&self) -> &'static str {
  64         "LetIfSeq"
  65     }
  66 }
  67
  68 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetIfSeq {
  69     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx hir::Block) {
  70         let mut it = block.stmts.iter().peekable();
  71         while let Some(stmt) = it.next() {
  72             if_chain! {
  73                 if let Some(expr) = it.peek();
  74                 if let hir::StmtKind::Local(ref local) = stmt.node;
  75                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.node;
  76                 if let hir::StmtKind::Expr(ref if_) = expr.node;
  77                 if let hir::ExprKind::If(ref cond, ref then, ref else_) = if_.node;
  78                 if !used_in_expr(cx, canonical_id, cond);
  79                 if let hir::ExprKind::Block(ref then, _) = then.node;
  80                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  81                 if !used_in_expr(cx, canonical_id, value);
  82                 then {
  83                     let span = stmt.span.to(if_.span);
  84
  85                     let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
  86                         if let hir::ExprKind::Block(ref else_, _) = else_.node {
  87                             if let Some(default) = check_assign(cx, canonical_id, else_) {
  88                                 (else_.stmts.len() > 1, default)
  89                             } else if let Some(ref default) = local.init {
  90                                 (true, &**default)
  91                             } else {
  92                                 continue;
  93                             }
  94                         } else {
  95                             continue;
  96                         }
  97                     } else if let Some(ref default) = local.init {
  98                         (false, &**default)
  99                     } else {
 100                         continue;
 101                     };
 102
 103                     let mutability = match mode {
 104                         BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
 105                         _ => "",
 106                     };
 107
 108                     // FIXME: this should not suggest `mut` if we can detect that the variable is not
 109                     // use mutably after the `if`
 110
 111                     let sug = format!(
 112                         "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
 113                         mut=mutability,
 114                         name=ident.name,
 115                         cond=snippet(cx, cond.span, "_"),
 116                         then=if then.stmts.len() > 1 { " ..;" } else { "" },
 117                         else=if default_multi_stmts { " ..;" } else { "" },
 118                         value=snippet(cx, value.span, "<value>"),
 119                         default=snippet(cx, default.span, "<default>"),
 120                     );
 121                     span_lint_and_then(cx,
 122                                        USELESS_LET_IF_SEQ,
 123                                        span,
 124                                        "`if _ { .. } else { .. }` is an expression",
 125                                        |db| {
 126                                            db.span_suggestion(
 127                                                 span,
 128                                                 "it is more idiomatic to write",
 129                                                 sug,
 130                                                 Applicability::HasPlaceholders,
 131                                             );
 132                                            if !mutability.is_empty() {
 133                                                db.note("you might not need `mut` at all");
 134                                            }
 135                                        });
 136                 }
 137             }
 138         }
 139     }
 140 }
 141
 142 struct UsedVisitor<'a, 'tcx: 'a> {
 143     cx: &'a LateContext<'a, 'tcx>,
 144     id: hir::HirId,
 145     used: bool,
 146 }
 147
 148 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
 149     fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
 150         if_chain! {
 151             if let hir::ExprKind::Path(ref qpath) = expr.node;
 152             if let Def::Local(local_id) = self.cx.tables.qpath_def(qpath, expr.hir_id);
 153             if self.id == self.cx.tcx.hir().node_to_hir_id(local_id);
 154             then {
 155                 self.used = true;
 156                 return;
 157             }
 158         }
 159         hir::intravisit::walk_expr(self, expr);
 160     }
 161     fn nested_visit_map<'this>(&'this mut self) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
 162         hir::intravisit::NestedVisitorMap::None
 163     }
 164 }
 165
 166 fn check_assign<'a, 'tcx>(
 167     cx: &LateContext<'a, 'tcx>,
 168     decl: hir::HirId,
 169     block: &'tcx hir::Block,
 170 ) -> Option<&'tcx hir::Expr> {
 171     if_chain! {
 172         if block.expr.is_none();
 173         if let Some(expr) = block.stmts.iter().last();
 174         if let hir::StmtKind::Semi(ref expr) = expr.node;
 175         if let hir::ExprKind::Assign(ref var, ref value) = expr.node;
 176         if let hir::ExprKind::Path(ref qpath) = var.node;
 177         if let Def::Local(local_id) = cx.tables.qpath_def(qpath, var.hir_id);
 178         if decl == cx.tcx.hir().node_to_hir_id(local_id);
 179         then {
 180             let mut v = UsedVisitor {
 181                 cx,
 182                 id: decl,
 183                 used: false,
 184             };
 185
 186             for s in block.stmts.iter().take(block.stmts.len()-1) {
 187                 hir::intravisit::walk_stmt(&mut v, s);
 188
 189                 if v.used {
 190                     return None;
 191                 }
 192             }
 193
 194             return Some(value);
 195         }
 196     }
 197
 198     None
 199 }
 200
 201 fn used_in_expr<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, id: hir::HirId, expr: &'tcx hir::Expr) -> bool {
 202     let mut v = UsedVisitor { cx, id, used: false };
 203     hir::intravisit::walk_expr(&mut v, expr);
 204     v.used
 205 }