clippy_lints/src/let_if_seq.rs

   1 use crate::utils::{qpath_res, snippet, span_lint_and_then, visitors::LocalUsedVisitor};
   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::BindingAnnotation;
   7 use rustc_lint::{LateContext, LateLintPass};
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   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     nursery,
  52     "unidiomatic `let mut` declaration followed by initialization in `if`"
  53 }
  54
  55 declare_lint_pass!(LetIfSeq => [USELESS_LET_IF_SEQ]);
  56
  57 impl<'tcx> LateLintPass<'tcx> for LetIfSeq {
  58     fn check_block(&mut self, cx: &LateContext<'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.kind;
  64                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind;
  65                 if let hir::StmtKind::Expr(ref if_) = expr.kind;
  66                 if let hir::ExprKind::If(ref cond, ref then, ref else_) = if_.kind;
  67                 if !LocalUsedVisitor::new(canonical_id).check_expr(cond);
  68                 if let hir::ExprKind::Block(ref then, _) = then.kind;
  69                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  70                 if !LocalUsedVisitor::new(canonical_id).check_expr(value);
  71                 then {
  72                     let span = stmt.span.to(if_.span);
  73
  74                     let has_interior_mutability = !cx.typeck_results().node_type(canonical_id).is_freeze(
  75                         cx.tcx.at(span),
  76                         cx.param_env,
  77                     );
  78                     if has_interior_mutability { return; }
  79
  80                     let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
  81                         if let hir::ExprKind::Block(ref else_, _) = else_.kind {
  82                             if let Some(default) = check_assign(cx, canonical_id, else_) {
  83                                 (else_.stmts.len() > 1, default)
  84                             } else if let Some(ref default) = local.init {
  85                                 (true, &**default)
  86                             } else {
  87                                 continue;
  88                             }
  89                         } else {
  90                             continue;
  91                         }
  92                     } else if let Some(ref default) = local.init {
  93                         (false, &**default)
  94                     } else {
  95                         continue;
  96                     };
  97
  98                     let mutability = match mode {
  99                         BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
 100                         _ => "",
 101                     };
 102
 103                     // FIXME: this should not suggest `mut` if we can detect that the variable is not
 104                     // use mutably after the `if`
 105
 106                     let sug = format!(
 107                         "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
 108                         mut=mutability,
 109                         name=ident.name,
 110                         cond=snippet(cx, cond.span, "_"),
 111                         then=if then.stmts.len() > 1 { " ..;" } else { "" },
 112                         else=if default_multi_stmts { " ..;" } else { "" },
 113                         value=snippet(cx, value.span, "<value>"),
 114                         default=snippet(cx, default.span, "<default>"),
 115                     );
 116                     span_lint_and_then(cx,
 117                                        USELESS_LET_IF_SEQ,
 118                                        span,
 119                                        "`if _ { .. } else { .. }` is an expression",
 120                                        |diag| {
 121                                            diag.span_suggestion(
 122                                                 span,
 123                                                 "it is more idiomatic to write",
 124                                                 sug,
 125                                                 Applicability::HasPlaceholders,
 126                                             );
 127                                            if !mutability.is_empty() {
 128                                                diag.note("you might not need `mut` at all");
 129                                            }
 130                                        });
 131                 }
 132             }
 133         }
 134     }
 135 }
 136
 137 fn check_assign<'tcx>(
 138     cx: &LateContext<'tcx>,
 139     decl: hir::HirId,
 140     block: &'tcx hir::Block<'_>,
 141 ) -> Option<&'tcx hir::Expr<'tcx>> {
 142     if_chain! {
 143         if block.expr.is_none();
 144         if let Some(expr) = block.stmts.iter().last();
 145         if let hir::StmtKind::Semi(ref expr) = expr.kind;
 146         if let hir::ExprKind::Assign(ref var, ref value, _) = expr.kind;
 147         if let hir::ExprKind::Path(ref qpath) = var.kind;
 148         if let Res::Local(local_id) = qpath_res(cx, qpath, var.hir_id);
 149         if decl == local_id;
 150         then {
 151             let mut v = LocalUsedVisitor::new(decl);
 152
 153             if block.stmts.iter().take(block.stmts.len()-1).any(|stmt| v.check_stmt(stmt)) {
 154                 return None;
 155             }
 156
 157             return Some(value);
 158         }
 159     }
 160
 161     None
 162 }