clippy_lints/src/let_if_seq.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10 use crate::utils::{snippet, span_lint_and_then};
  11 use if_chain::if_chain;
  12 use rustc::hir;
  13 use rustc::hir::def::Def;
  14 use rustc::hir::BindingAnnotation;
  15 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  16 use rustc::{declare_tool_lint, lint_array};
  17 use rustc_errors::Applicability;
  18 use syntax::ast;
  19
  20 /// **What it does:** Checks for variable declarations immediately followed by a
  21 /// conditional affectation.
  22 ///
  23 /// **Why is this bad?** This is not idiomatic Rust.
  24 ///
  25 /// **Known problems:** None.
  26 ///
  27 /// **Example:**
  28 /// ```rust,ignore
  29 /// let foo;
  30 ///
  31 /// if bar() {
  32 ///     foo = 42;
  33 /// } else {
  34 ///     foo = 0;
  35 /// }
  36 ///
  37 /// let mut baz = None;
  38 ///
  39 /// if bar() {
  40 ///     baz = Some(42);
  41 /// }
  42 /// ```
  43 ///
  44 /// should be written
  45 ///
  46 /// ```rust,ignore
  47 /// let foo = if bar() {
  48 ///     42
  49 /// } else {
  50 ///     0
  51 /// };
  52 ///
  53 /// let baz = if bar() {
  54 ///     Some(42)
  55 /// } else {
  56 ///     None
  57 /// };
  58 /// ```
  59 declare_clippy_lint! {
  60     pub USELESS_LET_IF_SEQ,
  61     style,
  62     "unidiomatic `let mut` declaration followed by initialization in `if`"
  63 }
  64
  65 #[derive(Copy, Clone)]
  66 pub struct LetIfSeq;
  67
  68 impl LintPass for LetIfSeq {
  69     fn get_lints(&self) -> LintArray {
  70         lint_array!(USELESS_LET_IF_SEQ)
  71     }
  72 }
  73
  74 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetIfSeq {
  75     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx hir::Block) {
  76         let mut it = block.stmts.iter().peekable();
  77         while let Some(stmt) = it.next() {
  78             if_chain! {
  79                 if let Some(expr) = it.peek();
  80                 if let hir::StmtKind::Decl(ref decl, _) = stmt.node;
  81                 if let hir::DeclKind::Local(ref decl) = decl.node;
  82                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = decl.pat.node;
  83                 if let hir::StmtKind::Expr(ref if_, _) = expr.node;
  84                 if let hir::ExprKind::If(ref cond, ref then, ref else_) = if_.node;
  85                 if !used_in_expr(cx, canonical_id, cond);
  86                 if let hir::ExprKind::Block(ref then, _) = then.node;
  87                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  88                 if !used_in_expr(cx, canonical_id, value);
  89                 then {
  90                     let span = stmt.span.to(if_.span);
  91
  92                     let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
  93                         if let hir::ExprKind::Block(ref else_, _) = else_.node {
  94                             if let Some(default) = check_assign(cx, canonical_id, else_) {
  95                                 (else_.stmts.len() > 1, default)
  96                             } else if let Some(ref default) = decl.init {
  97                                 (true, &**default)
  98                             } else {
  99                                 continue;
 100                             }
 101                         } else {
 102                             continue;
 103                         }
 104                     } else if let Some(ref default) = decl.init {
 105                         (false, &**default)
 106                     } else {
 107                         continue;
 108                     };
 109
 110                     let mutability = match mode {
 111                         BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
 112                         _ => "",
 113                     };
 114
 115                     // FIXME: this should not suggest `mut` if we can detect that the variable is not
 116                     // use mutably after the `if`
 117
 118                     let sug = format!(
 119                         "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
 120                         mut=mutability,
 121                         name=ident.name,
 122                         cond=snippet(cx, cond.span, "_"),
 123                         then=if then.stmts.len() > 1 { " ..;" } else { "" },
 124                         else=if default_multi_stmts { " ..;" } else { "" },
 125                         value=snippet(cx, value.span, "<value>"),
 126                         default=snippet(cx, default.span, "<default>"),
 127                     );
 128                     span_lint_and_then(cx,
 129                                        USELESS_LET_IF_SEQ,
 130                                        span,
 131                                        "`if _ { .. } else { .. }` is an expression",
 132                                        |db| {
 133                                            db.span_suggestion_with_applicability(
 134                                                 span,
 135                                                 "it is more idiomatic to write",
 136                                                 sug,
 137                                                 Applicability::HasPlaceholders,
 138                                             );
 139                                            if !mutability.is_empty() {
 140                                                db.note("you might not need `mut` at all");
 141                                            }
 142                                        });
 143                 }
 144             }
 145         }
 146     }
 147 }
 148
 149 struct UsedVisitor<'a, 'tcx: 'a> {
 150     cx: &'a LateContext<'a, 'tcx>,
 151     id: ast::NodeId,
 152     used: bool,
 153 }
 154
 155 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
 156     fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
 157         if_chain! {
 158             if let hir::ExprKind::Path(ref qpath) = expr.node;
 159             if let Def::Local(local_id) = self.cx.tables.qpath_def(qpath, expr.hir_id);
 160             if self.id == local_id;
 161             then {
 162                 self.used = true;
 163                 return;
 164             }
 165         }
 166         hir::intravisit::walk_expr(self, expr);
 167     }
 168     fn nested_visit_map<'this>(&'this mut self) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
 169         hir::intravisit::NestedVisitorMap::None
 170     }
 171 }
 172
 173 fn check_assign<'a, 'tcx>(
 174     cx: &LateContext<'a, 'tcx>,
 175     decl: ast::NodeId,
 176     block: &'tcx hir::Block,
 177 ) -> Option<&'tcx hir::Expr> {
 178     if_chain! {
 179         if block.expr.is_none();
 180         if let Some(expr) = block.stmts.iter().last();
 181         if let hir::StmtKind::Semi(ref expr, _) = expr.node;
 182         if let hir::ExprKind::Assign(ref var, ref value) = expr.node;
 183         if let hir::ExprKind::Path(ref qpath) = var.node;
 184         if let Def::Local(local_id) = cx.tables.qpath_def(qpath, var.hir_id);
 185         if decl == local_id;
 186         then {
 187             let mut v = UsedVisitor {
 188                 cx,
 189                 id: decl,
 190                 used: false,
 191             };
 192
 193             for s in block.stmts.iter().take(block.stmts.len()-1) {
 194                 hir::intravisit::walk_stmt(&mut v, s);
 195
 196                 if v.used {
 197                     return None;
 198                 }
 199             }
 200
 201             return Some(value);
 202         }
 203     }
 204
 205     None
 206 }
 207
 208 fn used_in_expr<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, id: ast::NodeId, expr: &'tcx hir::Expr) -> bool {
 209     let mut v = UsedVisitor { cx, id, used: false };
 210     hir::intravisit::walk_expr(&mut v, expr);
 211     v.used
 212 }