clippy_lints/src/let_if_seq.rs

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