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_errors::Applicability;
   4 use rustc_hir as hir;
   5 use rustc_hir::def::Res;
   6 use rustc_hir::intravisit;
   7 use rustc_hir::BindingAnnotation;
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::hir::map::Map;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11
  12 declare_clippy_lint! {
  13     /// **What it does:** Checks for variable declarations immediately followed by a
  14     /// conditional affectation.
  15     ///
  16     /// **Why is this bad?** This is not idiomatic Rust.
  17     ///
  18     /// **Known problems:** None.
  19     ///
  20     /// **Example:**
  21     /// ```rust,ignore
  22     /// let foo;
  23     ///
  24     /// if bar() {
  25     ///     foo = 42;
  26     /// } else {
  27     ///     foo = 0;
  28     /// }
  29     ///
  30     /// let mut baz = None;
  31     ///
  32     /// if bar() {
  33     ///     baz = Some(42);
  34     /// }
  35     /// ```
  36     ///
  37     /// should be written
  38     ///
  39     /// ```rust,ignore
  40     /// let foo = if bar() {
  41     ///     42
  42     /// } else {
  43     ///     0
  44     /// };
  45     ///
  46     /// let baz = if bar() {
  47     ///     Some(42)
  48     /// } else {
  49     ///     None
  50     /// };
  51     /// ```
  52     pub USELESS_LET_IF_SEQ,
  53     nursery,
  54     "unidiomatic `let mut` declaration followed by initialization in `if`"
  55 }
  56
  57 declare_lint_pass!(LetIfSeq => [USELESS_LET_IF_SEQ]);
  58
  59 impl<'tcx> LateLintPass<'tcx> for LetIfSeq {
  60     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx hir::Block<'_>) {
  61         let mut it = block.stmts.iter().peekable();
  62         while let Some(stmt) = it.next() {
  63             if_chain! {
  64                 if let Some(expr) = it.peek();
  65                 if let hir::StmtKind::Local(ref local) = stmt.kind;
  66                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind;
  67                 if let hir::StmtKind::Expr(ref if_) = expr.kind;
  68                 if let Some((ref cond, ref then, ref else_)) = higher::if_block(&if_);
  69                 if !used_in_expr(cx, canonical_id, cond);
  70                 if let hir::ExprKind::Block(ref then, _) = then.kind;
  71                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  72                 if !used_in_expr(cx, canonical_id, value);
  73                 then {
  74                     let span = stmt.span.to(if_.span);
  75
  76                     let has_interior_mutability = !cx.typeck_results().node_type(canonical_id).is_freeze(
  77                         cx.tcx.at(span),
  78                         cx.param_env,
  79                     );
  80                     if has_interior_mutability { return; }
  81
  82                     let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
  83                         if let hir::ExprKind::Block(ref else_, _) = else_.kind {
  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                                        |diag| {
 123                                            diag.span_suggestion(
 124                                                 span,
 125                                                 "it is more idiomatic to write",
 126                                                 sug,
 127                                                 Applicability::HasPlaceholders,
 128                                             );
 129                                            if !mutability.is_empty() {
 130                                                diag.note("you might not need `mut` at all");
 131                                            }
 132                                        });
 133                 }
 134             }
 135         }
 136     }
 137 }
 138
 139 struct UsedVisitor<'a, 'tcx> {
 140     cx: &'a LateContext<'tcx>,
 141     id: hir::HirId,
 142     used: bool,
 143 }
 144
 145 impl<'a, 'tcx> intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
 146     type Map = Map<'tcx>;
 147
 148     fn visit_expr(&mut self, expr: &'tcx hir::Expr<'_>) {
 149         if_chain! {
 150             if let hir::ExprKind::Path(ref qpath) = expr.kind;
 151             if let Res::Local(local_id) = qpath_res(self.cx, qpath, expr.hir_id);
 152             if self.id == local_id;
 153             then {
 154                 self.used = true;
 155                 return;
 156             }
 157         }
 158         intravisit::walk_expr(self, expr);
 159     }
 160     fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
 161         intravisit::NestedVisitorMap::None
 162     }
 163 }
 164
 165 fn check_assign<'tcx>(
 166     cx: &LateContext<'tcx>,
 167     decl: hir::HirId,
 168     block: &'tcx hir::Block<'_>,
 169 ) -> Option<&'tcx hir::Expr<'tcx>> {
 170     if_chain! {
 171         if block.expr.is_none();
 172         if let Some(expr) = block.stmts.iter().last();
 173         if let hir::StmtKind::Semi(ref expr) = expr.kind;
 174         if let hir::ExprKind::Assign(ref var, ref value, _) = expr.kind;
 175         if let hir::ExprKind::Path(ref qpath) = var.kind;
 176         if let Res::Local(local_id) = qpath_res(cx, qpath, var.hir_id);
 177         if decl == local_id;
 178         then {
 179             let mut v = UsedVisitor {
 180                 cx,
 181                 id: decl,
 182                 used: false,
 183             };
 184
 185             for s in block.stmts.iter().take(block.stmts.len()-1) {
 186                 intravisit::walk_stmt(&mut v, s);
 187
 188                 if v.used {
 189                     return None;
 190                 }
 191             }
 192
 193             return Some(value);
 194         }
 195     }
 196
 197     None
 198 }
 199
 200 fn used_in_expr<'tcx>(cx: &LateContext<'tcx>, id: hir::HirId, expr: &'tcx hir::Expr<'_>) -> bool {
 201     let mut v = UsedVisitor { cx, id, used: false };
 202     intravisit::walk_expr(&mut v, expr);
 203     v.used
 204 }