clippy_lints/src/let_if_seq.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::source::snippet;
   3 use clippy_utils::{path_to_local_id, visitors::is_local_used};
   4 use if_chain::if_chain;
   5 use rustc_errors::Applicability;
   6 use rustc_hir as hir;
   7 use rustc_hir::BindingAnnotation;
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10
  11 declare_clippy_lint! {
  12     /// ### What it does
  13     /// Checks for variable declarations immediately followed by a
  14     /// conditional affectation.
  15     ///
  16     /// ### Why is this bad?
  17     /// This is not idiomatic Rust.
  18     ///
  19     /// ### Example
  20     /// ```rust,ignore
  21     /// let foo;
  22     ///
  23     /// if bar() {
  24     ///     foo = 42;
  25     /// } else {
  26     ///     foo = 0;
  27     /// }
  28     ///
  29     /// let mut baz = None;
  30     ///
  31     /// if bar() {
  32     ///     baz = Some(42);
  33     /// }
  34     /// ```
  35     ///
  36     /// should be written
  37     ///
  38     /// ```rust,ignore
  39     /// let foo = if bar() {
  40     ///     42
  41     /// } else {
  42     ///     0
  43     /// };
  44     ///
  45     /// let baz = if bar() {
  46     ///     Some(42)
  47     /// } else {
  48     ///     None
  49     /// };
  50     /// ```
  51     pub USELESS_LET_IF_SEQ,
  52     nursery,
  53     "unidiomatic `let mut` declaration followed by initialization in `if`"
  54 }
  55
  56 declare_lint_pass!(LetIfSeq => [USELESS_LET_IF_SEQ]);
  57
  58 impl<'tcx> LateLintPass<'tcx> for LetIfSeq {
  59     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx hir::Block<'_>) {
  60         let mut it = block.stmts.iter().peekable();
  61         while let Some(stmt) = it.next() {
  62             if_chain! {
  63                 if let Some(expr) = it.peek();
  64                 if let hir::StmtKind::Local(local) = stmt.kind;
  65                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind;
  66                 if let hir::StmtKind::Expr(if_) = expr.kind;
  67                 if let hir::ExprKind::If(hir::Expr { kind: hir::ExprKind::DropTemps(cond), ..}, then, else_) = if_.kind;
  68                 if !is_local_used(cx, *cond, canonical_id);
  69                 if let hir::ExprKind::Block(then, _) = then.kind;
  70                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  71                 if !is_local_used(cx, value, canonical_id);
  72                 then {
  73                     let span = stmt.span.to(if_.span);
  74
  75                     let has_interior_mutability = !cx.typeck_results().node_type(canonical_id).is_freeze(
  76                         cx.tcx.at(span),
  77                         cx.param_env,
  78                     );
  79                     if has_interior_mutability { return; }
  80
  81                     let (default_multi_stmts, default) = if let Some(else_) = else_ {
  82                         if let hir::ExprKind::Block(else_, _) = else_.kind {
  83                             if let Some(default) = check_assign(cx, canonical_id, else_) {
  84                                 (else_.stmts.len() > 1, default)
  85                             } else if let Some(default) = local.init {
  86                                 (true, default)
  87                             } else {
  88                                 continue;
  89                             }
  90                         } else {
  91                             continue;
  92                         }
  93                     } else if let Some(default) = local.init {
  94                         (false, default)
  95                     } else {
  96                         continue;
  97                     };
  98
  99                     let mutability = match mode {
 100                         BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
 101                         _ => "",
 102                     };
 103
 104                     // FIXME: this should not suggest `mut` if we can detect that the variable is not
 105                     // use mutably after the `if`
 106
 107                     let sug = format!(
 108                         "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
 109                         mut=mutability,
 110                         name=ident.name,
 111                         cond=snippet(cx, cond.span, "_"),
 112                         then=if then.stmts.len() > 1 { " ..;" } else { "" },
 113                         else=if default_multi_stmts { " ..;" } else { "" },
 114                         value=snippet(cx, value.span, "<value>"),
 115                         default=snippet(cx, default.span, "<default>"),
 116                     );
 117                     span_lint_and_then(cx,
 118                                        USELESS_LET_IF_SEQ,
 119                                        span,
 120                                        "`if _ { .. } else { .. }` is an expression",
 121                                        |diag| {
 122                                            diag.span_suggestion(
 123                                                 span,
 124                                                 "it is more idiomatic to write",
 125                                                 sug,
 126                                                 Applicability::HasPlaceholders,
 127                                             );
 128                                            if !mutability.is_empty() {
 129                                                diag.note("you might not need `mut` at all");
 130                                            }
 131                                        });
 132                 }
 133             }
 134         }
 135     }
 136 }
 137
 138 fn check_assign<'tcx>(
 139     cx: &LateContext<'tcx>,
 140     decl: hir::HirId,
 141     block: &'tcx hir::Block<'_>,
 142 ) -> Option<&'tcx hir::Expr<'tcx>> {
 143     if_chain! {
 144         if block.expr.is_none();
 145         if let Some(expr) = block.stmts.iter().last();
 146         if let hir::StmtKind::Semi(expr) = expr.kind;
 147         if let hir::ExprKind::Assign(var, value, _) = expr.kind;
 148         if path_to_local_id(var, decl);
 149         then {
 150             if block.stmts.iter().take(block.stmts.len()-1).any(|stmt| is_local_used(cx, stmt, decl)) {
 151                 None
 152             } else {
 153                 Some(value)
 154             }
 155         } else {
 156             None
 157         }
 158     }
 159 }