clippy_lints/src/let_if_seq.rs

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