clippy_lints/src/let_if_seq.rs

   1 use rustc::lint::*;
   2 use rustc::hir;
   3 use syntax_pos::{Span, NO_EXPANSION};
   4 use utils::{snippet, span_lint_and_then};
   5
   6 /// **What it does:** 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     "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<'a, 'tcx> LateLintPass<'a, 'tcx> for LetIfSeq {
  61     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx hir::Block) {
  62         let mut it = block.stmts.iter().peekable();
  63         while let Some(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, def_id, ref name, None) = decl.pat.node,
  69                 let hir::StmtExpr(ref if_, _) = expr.node,
  70                 let hir::ExprIf(ref cond, ref then, ref else_) = if_.node,
  71                 !used_in_expr(cx, def_id, cond),
  72                 let hir::ExprBlock(ref then) = then.node,
  73                 let Some(value) = check_assign(cx, def_id, &*then),
  74                 !used_in_expr(cx, def_id, value),
  75             ], {
  76                 let span = Span { lo: stmt.span.lo, hi: if_.span.hi, ctxt: NO_EXPANSION };
  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_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> hir::intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
 138     fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
 139         if_let_chain! {[
 140             let hir::ExprPath(ref qpath) = expr.node,
 141             self.id == self.cx.tables.qpath_def(qpath, expr.id).def_id(),
 142         ], {
 143             self.used = true;
 144             return;
 145         }}
 146         hir::intravisit::walk_expr(self, expr);
 147     }
 148     fn nested_visit_map<'this>(&'this mut self) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
 149         hir::intravisit::NestedVisitorMap::All(&self.cx.tcx.hir)
 150     }
 151 }
 152
 153 fn check_assign<'a, 'tcx>(
 154     cx: &LateContext<'a, 'tcx>,
 155     decl: hir::def_id::DefId,
 156     block: &'tcx hir::Block
 157 ) -> Option<&'tcx hir::Expr> {
 158     if_let_chain! {[
 159         block.expr.is_none(),
 160         let Some(expr) = block.stmts.iter().last(),
 161         let hir::StmtSemi(ref expr, _) = expr.node,
 162         let hir::ExprAssign(ref var, ref value) = expr.node,
 163         let hir::ExprPath(ref qpath) = var.node,
 164         decl == cx.tables.qpath_def(qpath, var.id).def_id(),
 165     ], {
 166         let mut v = UsedVisitor {
 167             cx: cx,
 168             id: decl,
 169             used: false,
 170         };
 171
 172         for s in block.stmts.iter().take(block.stmts.len()-1) {
 173             hir::intravisit::walk_stmt(&mut v, s);
 174
 175             if v.used {
 176                 return None;
 177             }
 178         }
 179
 180         return Some(value);
 181     }}
 182
 183     None
 184 }
 185
 186 fn used_in_expr<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, id: hir::def_id::DefId, expr: &'tcx hir::Expr) -> bool {
 187     let mut v = UsedVisitor {
 188         cx: cx,
 189         id: id,
 190         used: false,
 191     };
 192     hir::intravisit::walk_expr(&mut v, expr);
 193     v.used
 194 }