clippy_lints/src/let_if_seq.rs

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