clippy_lints/src/let_if_seq.rs

   1 use crate::utils::{snippet, span_lint_and_then};
   2 use if_chain::if_chain;
   3 use rustc::hir;
   4 use rustc::hir::def::Def;
   5 use rustc::hir::BindingAnnotation;
   6 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   7 use rustc::{declare_tool_lint, lint_array};
   8 use rustc_errors::Applicability;
   9 use syntax::ast;
  10
  11 /// **What it does:** Checks for variable declarations immediately followed by a
  12 /// conditional affectation.
  13 ///
  14 /// **Why is this bad?** This is not idiomatic Rust.
  15 ///
  16 /// **Known problems:** None.
  17 ///
  18 /// **Example:**
  19 /// ```rust,ignore
  20 /// let foo;
  21 ///
  22 /// if bar() {
  23 ///     foo = 42;
  24 /// } else {
  25 ///     foo = 0;
  26 /// }
  27 ///
  28 /// let mut baz = None;
  29 ///
  30 /// if bar() {
  31 ///     baz = Some(42);
  32 /// }
  33 /// ```
  34 ///
  35 /// should be written
  36 ///
  37 /// ```rust,ignore
  38 /// let foo = if bar() {
  39 ///     42
  40 /// } else {
  41 ///     0
  42 /// };
  43 ///
  44 /// let baz = if bar() {
  45 ///     Some(42)
  46 /// } else {
  47 ///     None
  48 /// };
  49 /// ```
  50 declare_clippy_lint! {
  51     pub USELESS_LET_IF_SEQ,
  52     style,
  53     "unidiomatic `let mut` declaration followed by initialization in `if`"
  54 }
  55
  56 #[derive(Copy, Clone)]
  57 pub struct LetIfSeq;
  58
  59 impl LintPass for LetIfSeq {
  60     fn get_lints(&self) -> LintArray {
  61         lint_array!(USELESS_LET_IF_SEQ)
  62     }
  63 }
  64
  65 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetIfSeq {
  66     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx hir::Block) {
  67         let mut it = block.stmts.iter().peekable();
  68         while let Some(stmt) = it.next() {
  69             if_chain! {
  70                 if let Some(expr) = it.peek();
  71                 if let hir::StmtKind::Local(ref local) = stmt.node;
  72                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.node;
  73                 if let hir::StmtKind::Expr(ref if_) = expr.node;
  74                 if let hir::ExprKind::If(ref cond, ref then, ref else_) = if_.node;
  75                 if !used_in_expr(cx, canonical_id, cond);
  76                 if let hir::ExprKind::Block(ref then, _) = then.node;
  77                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  78                 if !used_in_expr(cx, canonical_id, value);
  79                 then {
  80                     let span = stmt.span.to(if_.span);
  81
  82                     let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
  83                         if let hir::ExprKind::Block(ref else_, _) = else_.node {
  84                             if let Some(default) = check_assign(cx, canonical_id, else_) {
  85                                 (else_.stmts.len() > 1, default)
  86                             } else if let Some(ref default) = local.init {
  87                                 (true, &**default)
  88                             } else {
  89                                 continue;
  90                             }
  91                         } else {
  92                             continue;
  93                         }
  94                     } else if let Some(ref default) = local.init {
  95                         (false, &**default)
  96                     } else {
  97                         continue;
  98                     };
  99
 100                     let mutability = match mode {
 101                         BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
 102                         _ => "",
 103                     };
 104
 105                     // FIXME: this should not suggest `mut` if we can detect that the variable is not
 106                     // use mutably after the `if`
 107
 108                     let sug = format!(
 109                         "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
 110                         mut=mutability,
 111                         name=ident.name,
 112                         cond=snippet(cx, cond.span, "_"),
 113                         then=if then.stmts.len() > 1 { " ..;" } else { "" },
 114                         else=if default_multi_stmts { " ..;" } else { "" },
 115                         value=snippet(cx, value.span, "<value>"),
 116                         default=snippet(cx, default.span, "<default>"),
 117                     );
 118                     span_lint_and_then(cx,
 119                                        USELESS_LET_IF_SEQ,
 120                                        span,
 121                                        "`if _ { .. } else { .. }` is an expression",
 122                                        |db| {
 123                                            db.span_suggestion_with_applicability(
 124                                                 span,
 125                                                 "it is more idiomatic to write",
 126                                                 sug,
 127                                                 Applicability::HasPlaceholders,
 128                                             );
 129                                            if !mutability.is_empty() {
 130                                                db.note("you might not need `mut` at all");
 131                                            }
 132                                        });
 133                 }
 134             }
 135         }
 136     }
 137 }
 138
 139 struct UsedVisitor<'a, 'tcx: 'a> {
 140     cx: &'a LateContext<'a, 'tcx>,
 141     id: ast::NodeId,
 142     used: bool,
 143 }
 144
 145 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
 146     fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
 147         if_chain! {
 148             if let hir::ExprKind::Path(ref qpath) = expr.node;
 149             if let Def::Local(local_id) = self.cx.tables.qpath_def(qpath, expr.hir_id);
 150             if self.id == local_id;
 151             then {
 152                 self.used = true;
 153                 return;
 154             }
 155         }
 156         hir::intravisit::walk_expr(self, expr);
 157     }
 158     fn nested_visit_map<'this>(&'this mut self) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
 159         hir::intravisit::NestedVisitorMap::None
 160     }
 161 }
 162
 163 fn check_assign<'a, 'tcx>(
 164     cx: &LateContext<'a, 'tcx>,
 165     decl: ast::NodeId,
 166     block: &'tcx hir::Block,
 167 ) -> Option<&'tcx hir::Expr> {
 168     if_chain! {
 169         if block.expr.is_none();
 170         if let Some(expr) = block.stmts.iter().last();
 171         if let hir::StmtKind::Semi(ref expr) = expr.node;
 172         if let hir::ExprKind::Assign(ref var, ref value) = expr.node;
 173         if let hir::ExprKind::Path(ref qpath) = var.node;
 174         if let Def::Local(local_id) = cx.tables.qpath_def(qpath, var.hir_id);
 175         if decl == local_id;
 176         then {
 177             let mut v = UsedVisitor {
 178                 cx,
 179                 id: decl,
 180                 used: false,
 181             };
 182
 183             for s in block.stmts.iter().take(block.stmts.len()-1) {
 184                 hir::intravisit::walk_stmt(&mut v, s);
 185
 186                 if v.used {
 187                     return None;
 188                 }
 189             }
 190
 191             return Some(value);
 192         }
 193     }
 194
 195     None
 196 }
 197
 198 fn used_in_expr<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, id: ast::NodeId, expr: &'tcx hir::Expr) -> bool {
 199     let mut v = UsedVisitor { cx, id, used: false };
 200     hir::intravisit::walk_expr(&mut v, expr);
 201     v.used
 202 }