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 crate::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_clippy_lint! {
  48     pub USELESS_LET_IF_SEQ,
  49     style,
  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_chain! {
  67                 if let Some(expr) = it.peek();
  68                 if let hir::StmtDecl(ref decl, _) = stmt.node;
  69                 if let hir::DeclLocal(ref decl) = decl.node;
  70                 if let hir::PatKind::Binding(mode, canonical_id, ref name, None) = decl.pat.node;
  71                 if let hir::StmtExpr(ref if_, _) = expr.node;
  72                 if let hir::ExprIf(ref cond, ref then, ref else_) = if_.node;
  73                 if !used_in_expr(cx, canonical_id, cond);
  74                 if let hir::ExprBlock(ref then, _) = then.node;
  75                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  76                 if !used_in_expr(cx, canonical_id, value);
  77                 then {
  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
 134 struct UsedVisitor<'a, 'tcx: 'a> {
 135     cx: &'a LateContext<'a, 'tcx>,
 136     id: ast::NodeId,
 137     used: bool,
 138 }
 139
 140 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
 141     fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
 142         if_chain! {
 143             if let hir::ExprPath(ref qpath) = expr.node;
 144             if let Def::Local(local_id) = self.cx.tables.qpath_def(qpath, expr.hir_id);
 145             if self.id == local_id;
 146             then {
 147                 self.used = true;
 148                 return;
 149             }
 150         }
 151         hir::intravisit::walk_expr(self, expr);
 152     }
 153     fn nested_visit_map<'this>(&'this mut self) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
 154         hir::intravisit::NestedVisitorMap::None
 155     }
 156 }
 157
 158 fn check_assign<'a, 'tcx>(
 159     cx: &LateContext<'a, 'tcx>,
 160     decl: ast::NodeId,
 161     block: &'tcx hir::Block,
 162 ) -> Option<&'tcx hir::Expr> {
 163     if_chain! {
 164         if block.expr.is_none();
 165         if let Some(expr) = block.stmts.iter().last();
 166         if let hir::StmtSemi(ref expr, _) = expr.node;
 167         if let hir::ExprAssign(ref var, ref value) = expr.node;
 168         if let hir::ExprPath(ref qpath) = var.node;
 169         if let Def::Local(local_id) = cx.tables.qpath_def(qpath, var.hir_id);
 170         if decl == local_id;
 171         then {
 172             let mut v = UsedVisitor {
 173                 cx,
 174                 id: decl,
 175                 used: false,
 176             };
 177
 178             for s in block.stmts.iter().take(block.stmts.len()-1) {
 179                 hir::intravisit::walk_stmt(&mut v, s);
 180
 181                 if v.used {
 182                     return None;
 183                 }
 184             }
 185
 186             return Some(value);
 187         }
 188     }
 189
 190     None
 191 }
 192
 193 fn used_in_expr<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, id: ast::NodeId, expr: &'tcx hir::Expr) -> bool {
 194     let mut v = UsedVisitor {
 195         cx,
 196         id,
 197         used: false,
 198     };
 199     hir::intravisit::walk_expr(&mut v, expr);
 200     v.used
 201 }