clippy_lints/src/let_if_seq.rs

   1 use crate::utils::{higher, qpath_res, snippet, span_lint_and_then};
   2 use if_chain::if_chain;
   3 use rustc::declare_lint_pass;
   4 use rustc::hir::map::Map;
   5 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   6 use rustc_errors::Applicability;
   7 use rustc_hir as hir;
   8 use rustc_hir::def::Res;
   9 use rustc_hir::intravisit;
  10 use rustc_hir::BindingAnnotation;
  11 use rustc_session::declare_tool_lint;
  12
  13 declare_clippy_lint! {
  14     /// **What it does:** Checks for variable declarations immediately followed by a
  15     /// conditional affectation.
  16     ///
  17     /// **Why is this bad?** This is not idiomatic Rust.
  18     ///
  19     /// **Known problems:** None.
  20     ///
  21     /// **Example:**
  22     /// ```rust,ignore
  23     /// let foo;
  24     ///
  25     /// if bar() {
  26     ///     foo = 42;
  27     /// } else {
  28     ///     foo = 0;
  29     /// }
  30     ///
  31     /// let mut baz = None;
  32     ///
  33     /// if bar() {
  34     ///     baz = Some(42);
  35     /// }
  36     /// ```
  37     ///
  38     /// should be written
  39     ///
  40     /// ```rust,ignore
  41     /// let foo = if bar() {
  42     ///     42
  43     /// } else {
  44     ///     0
  45     /// };
  46     ///
  47     /// let baz = if bar() {
  48     ///     Some(42)
  49     /// } else {
  50     ///     None
  51     /// };
  52     /// ```
  53     pub USELESS_LET_IF_SEQ,
  54     style,
  55     "unidiomatic `let mut` declaration followed by initialization in `if`"
  56 }
  57
  58 declare_lint_pass!(LetIfSeq => [USELESS_LET_IF_SEQ]);
  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_chain! {
  65                 if let Some(expr) = it.peek();
  66                 if let hir::StmtKind::Local(ref local) = stmt.kind;
  67                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind;
  68                 if let hir::StmtKind::Expr(ref if_) = expr.kind;
  69                 if let Some((ref cond, ref then, ref else_)) = higher::if_block(&if_);
  70                 if !used_in_expr(cx, canonical_id, cond);
  71                 if let hir::ExprKind::Block(ref then, _) = then.kind;
  72                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  73                 if !used_in_expr(cx, canonical_id, value);
  74                 then {
  75                     let span = stmt.span.to(if_.span);
  76
  77                     let has_interior_mutability = !cx.tables.node_type(canonical_id).is_freeze(
  78                         cx.tcx,
  79                         cx.param_env,
  80                         span
  81                     );
  82                     if has_interior_mutability { return; }
  83
  84                     let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
  85                         if let hir::ExprKind::Block(ref else_, _) = else_.kind {
  86                             if let Some(default) = check_assign(cx, canonical_id, else_) {
  87                                 (else_.stmts.len() > 1, default)
  88                             } else if let Some(ref default) = local.init {
  89                                 (true, &**default)
  90                             } else {
  91                                 continue;
  92                             }
  93                         } else {
  94                             continue;
  95                         }
  96                     } else if let Some(ref default) = local.init {
  97                         (false, &**default)
  98                     } else {
  99                         continue;
 100                     };
 101
 102                     let mutability = match mode {
 103                         BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
 104                         _ => "",
 105                     };
 106
 107                     // FIXME: this should not suggest `mut` if we can detect that the variable is not
 108                     // use mutably after the `if`
 109
 110                     let sug = format!(
 111                         "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
 112                         mut=mutability,
 113                         name=ident.name,
 114                         cond=snippet(cx, cond.span, "_"),
 115                         then=if then.stmts.len() > 1 { " ..;" } else { "" },
 116                         else=if default_multi_stmts { " ..;" } else { "" },
 117                         value=snippet(cx, value.span, "<value>"),
 118                         default=snippet(cx, default.span, "<default>"),
 119                     );
 120                     span_lint_and_then(cx,
 121                                        USELESS_LET_IF_SEQ,
 122                                        span,
 123                                        "`if _ { .. } else { .. }` is an expression",
 124                                        |db| {
 125                                            db.span_suggestion(
 126                                                 span,
 127                                                 "it is more idiomatic to write",
 128                                                 sug,
 129                                                 Applicability::HasPlaceholders,
 130                                             );
 131                                            if !mutability.is_empty() {
 132                                                db.note("you might not need `mut` at all");
 133                                            }
 134                                        });
 135                 }
 136             }
 137         }
 138     }
 139 }
 140
 141 struct UsedVisitor<'a, 'tcx> {
 142     cx: &'a LateContext<'a, 'tcx>,
 143     id: hir::HirId,
 144     used: bool,
 145 }
 146
 147 impl<'a, 'tcx> intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
 148     type Map = Map<'tcx>;
 149
 150     fn visit_expr(&mut self, expr: &'tcx hir::Expr<'_>) {
 151         if_chain! {
 152             if let hir::ExprKind::Path(ref qpath) = expr.kind;
 153             if let Res::Local(local_id) = qpath_res(self.cx, qpath, expr.hir_id);
 154             if self.id == local_id;
 155             then {
 156                 self.used = true;
 157                 return;
 158             }
 159         }
 160         intravisit::walk_expr(self, expr);
 161     }
 162     fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<'_, Self::Map> {
 163         intravisit::NestedVisitorMap::None
 164     }
 165 }
 166
 167 fn check_assign<'a, 'tcx>(
 168     cx: &LateContext<'a, 'tcx>,
 169     decl: hir::HirId,
 170     block: &'tcx hir::Block<'_>,
 171 ) -> Option<&'tcx hir::Expr<'tcx>> {
 172     if_chain! {
 173         if block.expr.is_none();
 174         if let Some(expr) = block.stmts.iter().last();
 175         if let hir::StmtKind::Semi(ref expr) = expr.kind;
 176         if let hir::ExprKind::Assign(ref var, ref value, _) = expr.kind;
 177         if let hir::ExprKind::Path(ref qpath) = var.kind;
 178         if let Res::Local(local_id) = qpath_res(cx, qpath, var.hir_id);
 179         if decl == local_id;
 180         then {
 181             let mut v = UsedVisitor {
 182                 cx,
 183                 id: decl,
 184                 used: false,
 185             };
 186
 187             for s in block.stmts.iter().take(block.stmts.len()-1) {
 188                 intravisit::walk_stmt(&mut v, s);
 189
 190                 if v.used {
 191                     return None;
 192                 }
 193             }
 194
 195             return Some(value);
 196         }
 197     }
 198
 199     None
 200 }
 201
 202 fn used_in_expr<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, id: hir::HirId, expr: &'tcx hir::Expr<'_>) -> bool {
 203     let mut v = UsedVisitor { cx, id, used: false };
 204     intravisit::walk_expr(&mut v, expr);
 205     v.used
 206 }