clippy_lints/src/let_if_seq.rs

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