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_lint_pass, declare_tool_lint};
   8 use rustc_errors::Applicability;
   9
  10 declare_clippy_lint! {
  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     pub USELESS_LET_IF_SEQ,
  51     style,
  52     "unidiomatic `let mut` declaration followed by initialization in `if`"
  53 }
  54
  55 declare_lint_pass!(LetIfSeq => [USELESS_LET_IF_SEQ]);
  56
  57 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetIfSeq {
  58     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx hir::Block) {
  59         let mut it = block.stmts.iter().peekable();
  60         while let Some(stmt) = it.next() {
  61             if_chain! {
  62                 if let Some(expr) = it.peek();
  63                 if let hir::StmtKind::Local(ref local) = stmt.node;
  64                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.node;
  65                 if let hir::StmtKind::Expr(ref if_) = expr.node;
  66                 if let hir::ExprKind::If(ref cond, ref then, ref else_) = if_.node;
  67                 if !used_in_expr(cx, canonical_id, cond);
  68                 if let hir::ExprKind::Block(ref then, _) = then.node;
  69                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  70                 if !used_in_expr(cx, canonical_id, value);
  71                 then {
  72                     let span = stmt.span.to(if_.span);
  73
  74                     let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
  75                         if let hir::ExprKind::Block(ref else_, _) = else_.node {
  76                             if let Some(default) = check_assign(cx, canonical_id, else_) {
  77                                 (else_.stmts.len() > 1, default)
  78                             } else if let Some(ref default) = local.init {
  79                                 (true, &**default)
  80                             } else {
  81                                 continue;
  82                             }
  83                         } else {
  84                             continue;
  85                         }
  86                     } else if let Some(ref default) = local.init {
  87                         (false, &**default)
  88                     } else {
  89                         continue;
  90                     };
  91
  92                     let mutability = match mode {
  93                         BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
  94                         _ => "",
  95                     };
  96
  97                     // FIXME: this should not suggest `mut` if we can detect that the variable is not
  98                     // use mutably after the `if`
  99
 100                     let sug = format!(
 101                         "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
 102                         mut=mutability,
 103                         name=ident.name,
 104                         cond=snippet(cx, cond.span, "_"),
 105                         then=if then.stmts.len() > 1 { " ..;" } else { "" },
 106                         else=if default_multi_stmts { " ..;" } else { "" },
 107                         value=snippet(cx, value.span, "<value>"),
 108                         default=snippet(cx, default.span, "<default>"),
 109                     );
 110                     span_lint_and_then(cx,
 111                                        USELESS_LET_IF_SEQ,
 112                                        span,
 113                                        "`if _ { .. } else { .. }` is an expression",
 114                                        |db| {
 115                                            db.span_suggestion(
 116                                                 span,
 117                                                 "it is more idiomatic to write",
 118                                                 sug,
 119                                                 Applicability::HasPlaceholders,
 120                                             );
 121                                            if !mutability.is_empty() {
 122                                                db.note("you might not need `mut` at all");
 123                                            }
 124                                        });
 125                 }
 126             }
 127         }
 128     }
 129 }
 130
 131 struct UsedVisitor<'a, 'tcx: 'a> {
 132     cx: &'a LateContext<'a, 'tcx>,
 133     id: hir::HirId,
 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_chain! {
 140             if let hir::ExprKind::Path(ref qpath) = expr.node;
 141             if let Def::Local(local_id) = self.cx.tables.qpath_def(qpath, expr.hir_id);
 142             if self.id == local_id;
 143             then {
 144                 self.used = true;
 145                 return;
 146             }
 147         }
 148         hir::intravisit::walk_expr(self, expr);
 149     }
 150     fn nested_visit_map<'this>(&'this mut self) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
 151         hir::intravisit::NestedVisitorMap::None
 152     }
 153 }
 154
 155 fn check_assign<'a, 'tcx>(
 156     cx: &LateContext<'a, 'tcx>,
 157     decl: hir::HirId,
 158     block: &'tcx hir::Block,
 159 ) -> Option<&'tcx hir::Expr> {
 160     if_chain! {
 161         if block.expr.is_none();
 162         if let Some(expr) = block.stmts.iter().last();
 163         if let hir::StmtKind::Semi(ref expr) = expr.node;
 164         if let hir::ExprKind::Assign(ref var, ref value) = expr.node;
 165         if let hir::ExprKind::Path(ref qpath) = var.node;
 166         if let Def::Local(local_id) = cx.tables.qpath_def(qpath, var.hir_id);
 167         if decl == local_id;
 168         then {
 169             let mut v = UsedVisitor {
 170                 cx,
 171                 id: decl,
 172                 used: false,
 173             };
 174
 175             for s in block.stmts.iter().take(block.stmts.len()-1) {
 176                 hir::intravisit::walk_stmt(&mut v, s);
 177
 178                 if v.used {
 179                     return None;
 180                 }
 181             }
 182
 183             return Some(value);
 184         }
 185     }
 186
 187     None
 188 }
 189
 190 fn used_in_expr<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, id: hir::HirId, expr: &'tcx hir::Expr) -> bool {
 191     let mut v = UsedVisitor { cx, id, used: false };
 192     hir::intravisit::walk_expr(&mut v, expr);
 193     v.used
 194 }