clippy_lints/src/let_if_seq.rs

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