clippy_lints/src/let_if_seq.rs

   1 use crate::utils::{path_to_local_id, 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::BindingAnnotation;
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_session::{declare_lint_pass, declare_tool_lint};
   8
   9 declare_clippy_lint! {
  10     /// **What it does:** Checks for variable declarations immediately followed by a
  11     /// conditional affectation.
  12     ///
  13     /// **Why is this bad?** This is not idiomatic Rust.
  14     ///
  15     /// **Known problems:** None.
  16     ///
  17     /// **Example:**
  18     /// ```rust,ignore
  19     /// let foo;
  20     ///
  21     /// if bar() {
  22     ///     foo = 42;
  23     /// } else {
  24     ///     foo = 0;
  25     /// }
  26     ///
  27     /// let mut baz = None;
  28     ///
  29     /// if bar() {
  30     ///     baz = Some(42);
  31     /// }
  32     /// ```
  33     ///
  34     /// should be written
  35     ///
  36     /// ```rust,ignore
  37     /// let foo = if bar() {
  38     ///     42
  39     /// } else {
  40     ///     0
  41     /// };
  42     ///
  43     /// let baz = if bar() {
  44     ///     Some(42)
  45     /// } else {
  46     ///     None
  47     /// };
  48     /// ```
  49     pub USELESS_LET_IF_SEQ,
  50     nursery,
  51     "unidiomatic `let mut` declaration followed by initialization in `if`"
  52 }
  53
  54 declare_lint_pass!(LetIfSeq => [USELESS_LET_IF_SEQ]);
  55
  56 impl<'tcx> LateLintPass<'tcx> for LetIfSeq {
  57     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx hir::Block<'_>) {
  58         let mut it = block.stmts.iter().peekable();
  59         while let Some(stmt) = it.next() {
  60             if_chain! {
  61                 if let Some(expr) = it.peek();
  62                 if let hir::StmtKind::Local(ref local) = stmt.kind;
  63                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind;
  64                 if let hir::StmtKind::Expr(ref if_) = expr.kind;
  65                 if let hir::ExprKind::If(ref cond, ref then, ref else_) = if_.kind;
  66                 if !LocalUsedVisitor::new(canonical_id).check_expr(cond);
  67                 if let hir::ExprKind::Block(ref then, _) = then.kind;
  68                 if let Some(value) = check_assign(canonical_id, &*then);
  69                 if !LocalUsedVisitor::new(canonical_id).check_expr(value);
  70                 then {
  71                     let span = stmt.span.to(if_.span);
  72
  73                     let has_interior_mutability = !cx.typeck_results().node_type(canonical_id).is_freeze(
  74                         cx.tcx.at(span),
  75                         cx.param_env,
  76                     );
  77                     if has_interior_mutability { return; }
  78
  79                     let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
  80                         if let hir::ExprKind::Block(ref else_, _) = else_.kind {
  81                             if let Some(default) = check_assign(canonical_id, else_) {
  82                                 (else_.stmts.len() > 1, default)
  83                             } else if let Some(ref default) = local.init {
  84                                 (true, &**default)
  85                             } else {
  86                                 continue;
  87                             }
  88                         } else {
  89                             continue;
  90                         }
  91                     } else if let Some(ref default) = local.init {
  92                         (false, &**default)
  93                     } else {
  94                         continue;
  95                     };
  96
  97                     let mutability = match mode {
  98                         BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
  99                         _ => "",
 100                     };
 101
 102                     // FIXME: this should not suggest `mut` if we can detect that the variable is not
 103                     // use mutably after the `if`
 104
 105                     let sug = format!(
 106                         "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
 107                         mut=mutability,
 108                         name=ident.name,
 109                         cond=snippet(cx, cond.span, "_"),
 110                         then=if then.stmts.len() > 1 { " ..;" } else { "" },
 111                         else=if default_multi_stmts { " ..;" } else { "" },
 112                         value=snippet(cx, value.span, "<value>"),
 113                         default=snippet(cx, default.span, "<default>"),
 114                     );
 115                     span_lint_and_then(cx,
 116                                        USELESS_LET_IF_SEQ,
 117                                        span,
 118                                        "`if _ { .. } else { .. }` is an expression",
 119                                        |diag| {
 120                                            diag.span_suggestion(
 121                                                 span,
 122                                                 "it is more idiomatic to write",
 123                                                 sug,
 124                                                 Applicability::HasPlaceholders,
 125                                             );
 126                                            if !mutability.is_empty() {
 127                                                diag.note("you might not need `mut` at all");
 128                                            }
 129                                        });
 130                 }
 131             }
 132         }
 133     }
 134 }
 135
 136 fn check_assign<'tcx>(decl: hir::HirId, block: &'tcx hir::Block<'_>) -> Option<&'tcx hir::Expr<'tcx>> {
 137     if_chain! {
 138         if block.expr.is_none();
 139         if let Some(expr) = block.stmts.iter().last();
 140         if let hir::StmtKind::Semi(ref expr) = expr.kind;
 141         if let hir::ExprKind::Assign(ref var, ref value, _) = expr.kind;
 142         if path_to_local_id(var, decl);
 143         then {
 144             let mut v = LocalUsedVisitor::new(decl);
 145
 146             if block.stmts.iter().take(block.stmts.len()-1).any(|stmt| v.check_stmt(stmt)) {
 147                 return None;
 148             }
 149
 150             return Some(value);
 151         }
 152     }
 153
 154     None
 155 }