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
  11 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  12 use crate::rustc::{declare_tool_lint, lint_array};
  13 use if_chain::if_chain;
  14 use crate::rustc::hir;
  15 use crate::rustc::hir::BindingAnnotation;
  16 use crate::rustc::hir::def::Def;
  17 use crate::syntax::ast;
  18 use crate::utils::{snippet, span_lint_and_then};
  19 use crate::rustc_errors::Applicability;
  20
  21 /// **What it does:** Checks for variable declarations immediately followed by a
  22 /// conditional affectation.
  23 ///
  24 /// **Why is this bad?** This is not idiomatic Rust.
  25 ///
  26 /// **Known problems:** None.
  27 ///
  28 /// **Example:**
  29 /// ```rust,ignore
  30 /// let foo;
  31 ///
  32 /// if bar() {
  33 ///     foo = 42;
  34 /// } else {
  35 ///     foo = 0;
  36 /// }
  37 ///
  38 /// let mut baz = None;
  39 ///
  40 /// if bar() {
  41 ///     baz = Some(42);
  42 /// }
  43 /// ```
  44 ///
  45 /// should be written
  46 ///
  47 /// ```rust,ignore
  48 /// let foo = if bar() {
  49 ///     42
  50 /// } else {
  51 ///     0
  52 /// };
  53 ///
  54 /// let baz = if bar() {
  55 ///     Some(42)
  56 /// } else {
  57 ///     None
  58 /// };
  59 /// ```
  60 declare_clippy_lint! {
  61     pub USELESS_LET_IF_SEQ,
  62     style,
  63     "unidiomatic `let mut` declaration followed by initialization in `if`"
  64 }
  65
  66 #[derive(Copy, Clone)]
  67 pub struct LetIfSeq;
  68
  69 impl LintPass for LetIfSeq {
  70     fn get_lints(&self) -> LintArray {
  71         lint_array!(USELESS_LET_IF_SEQ)
  72     }
  73 }
  74
  75 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LetIfSeq {
  76     fn check_block(&mut self, cx: &LateContext<'a, 'tcx>, block: &'tcx hir::Block) {
  77         let mut it = block.stmts.iter().peekable();
  78         while let Some(stmt) = it.next() {
  79             if_chain! {
  80                 if let Some(expr) = it.peek();
  81                 if let hir::StmtKind::Decl(ref decl, _) = stmt.node;
  82                 if let hir::DeclKind::Local(ref decl) = decl.node;
  83                 if let hir::PatKind::Binding(mode, canonical_id, ident, None) = decl.pat.node;
  84                 if let hir::StmtKind::Expr(ref if_, _) = expr.node;
  85                 if let hir::ExprKind::If(ref cond, ref then, ref else_) = if_.node;
  86                 if !used_in_expr(cx, canonical_id, cond);
  87                 if let hir::ExprKind::Block(ref then, _) = then.node;
  88                 if let Some(value) = check_assign(cx, canonical_id, &*then);
  89                 if !used_in_expr(cx, canonical_id, value);
  90                 then {
  91                     let span = stmt.span.to(if_.span);
  92
  93                     let (default_multi_stmts, default) = if let Some(ref else_) = *else_ {
  94                         if let hir::ExprKind::Block(ref else_, _) = else_.node {
  95                             if let Some(default) = check_assign(cx, canonical_id, else_) {
  96                                 (else_.stmts.len() > 1, default)
  97                             } else if let Some(ref default) = decl.init {
  98                                 (true, &**default)
  99                             } else {
 100                                 continue;
 101                             }
 102                         } else {
 103                             continue;
 104                         }
 105                     } else if let Some(ref default) = decl.init {
 106                         (false, &**default)
 107                     } else {
 108                         continue;
 109                     };
 110
 111                     let mutability = match mode {
 112                         BindingAnnotation::RefMut | BindingAnnotation::Mutable => "<mut> ",
 113                         _ => "",
 114                     };
 115
 116                     // FIXME: this should not suggest `mut` if we can detect that the variable is not
 117                     // use mutably after the `if`
 118
 119                     let sug = format!(
 120                         "let {mut}{name} = if {cond} {{{then} {value} }} else {{{else} {default} }};",
 121                         mut=mutability,
 122                         name=ident.name,
 123                         cond=snippet(cx, cond.span, "_"),
 124                         then=if then.stmts.len() > 1 { " ..;" } else { "" },
 125                         else=if default_multi_stmts { " ..;" } else { "" },
 126                         value=snippet(cx, value.span, "<value>"),
 127                         default=snippet(cx, default.span, "<default>"),
 128                     );
 129                     span_lint_and_then(cx,
 130                                        USELESS_LET_IF_SEQ,
 131                                        span,
 132                                        "`if _ { .. } else { .. }` is an expression",
 133                                        |db| {
 134                                            db.span_suggestion_with_applicability(
 135                                                 span,
 136                                                 "it is more idiomatic to write",
 137                                                 sug,
 138                                                 Applicability::HasPlaceholders,
 139                                             );
 140                                            if !mutability.is_empty() {
 141                                                db.note("you might not need `mut` at all");
 142                                            }
 143                                        });
 144                 }
 145             }
 146         }
 147     }
 148 }
 149
 150 struct UsedVisitor<'a, 'tcx: 'a> {
 151     cx: &'a LateContext<'a, 'tcx>,
 152     id: ast::NodeId,
 153     used: bool,
 154 }
 155
 156 impl<'a, 'tcx> hir::intravisit::Visitor<'tcx> for UsedVisitor<'a, 'tcx> {
 157     fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
 158         if_chain! {
 159             if let hir::ExprKind::Path(ref qpath) = expr.node;
 160             if let Def::Local(local_id) = self.cx.tables.qpath_def(qpath, expr.hir_id);
 161             if self.id == local_id;
 162             then {
 163                 self.used = true;
 164                 return;
 165             }
 166         }
 167         hir::intravisit::walk_expr(self, expr);
 168     }
 169     fn nested_visit_map<'this>(&'this mut self) -> hir::intravisit::NestedVisitorMap<'this, 'tcx> {
 170         hir::intravisit::NestedVisitorMap::None
 171     }
 172 }
 173
 174 fn check_assign<'a, 'tcx>(
 175     cx: &LateContext<'a, 'tcx>,
 176     decl: ast::NodeId,
 177     block: &'tcx hir::Block,
 178 ) -> Option<&'tcx hir::Expr> {
 179     if_chain! {
 180         if block.expr.is_none();
 181         if let Some(expr) = block.stmts.iter().last();
 182         if let hir::StmtKind::Semi(ref expr, _) = expr.node;
 183         if let hir::ExprKind::Assign(ref var, ref value) = expr.node;
 184         if let hir::ExprKind::Path(ref qpath) = var.node;
 185         if let Def::Local(local_id) = cx.tables.qpath_def(qpath, var.hir_id);
 186         if decl == local_id;
 187         then {
 188             let mut v = UsedVisitor {
 189                 cx,
 190                 id: decl,
 191                 used: false,
 192             };
 193
 194             for s in block.stmts.iter().take(block.stmts.len()-1) {
 195                 hir::intravisit::walk_stmt(&mut v, s);
 196
 197                 if v.used {
 198                     return None;
 199                 }
 200             }
 201
 202             return Some(value);
 203         }
 204     }
 205
 206     None
 207 }
 208
 209 fn used_in_expr<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, id: ast::NodeId, expr: &'tcx hir::Expr) -> bool {
 210     let mut v = UsedVisitor {
 211         cx,
 212         id,
 213         used: false,
 214     };
 215     hir::intravisit::walk_expr(&mut v, expr);
 216     v.used
 217 }