clippy_lints/src/collapsible_if.rs

   1 //! Checks for if expressions that contain only an if expression.
   2 //!
   3 //! For example, the lint would catch:
   4 //!
   5 //! ```rust,ignore
   6 //! if x {
   7 //!     if y {
   8 //!         println!("Hello world");
   9 //!     }
  10 //! }
  11 //! ```
  12 //!
  13 //! This lint is **warn** by default
  14
  15 use if_chain::if_chain;
  16 use rustc::lint::{EarlyContext, EarlyLintPass, LintArray, LintPass};
  17 use rustc::{declare_tool_lint, lint_array};
  18 use syntax::ast;
  19
  20 use crate::utils::sugg::Sugg;
  21 use crate::utils::{in_macro, snippet_block, snippet_block_with_applicability, span_lint_and_sugg, span_lint_and_then};
  22 use rustc_errors::Applicability;
  23
  24 /// **What it does:** Checks for nested `if` statements which can be collapsed
  25 /// by `&&`-combining their conditions and for `else { if ... }` expressions
  26 /// that
  27 /// can be collapsed to `else if ...`.
  28 ///
  29 /// **Why is this bad?** Each `if`-statement adds one level of nesting, which
  30 /// makes code look more complex than it really is.
  31 ///
  32 /// **Known problems:** None.
  33 ///
  34 /// **Example:**
  35 /// ```rust,ignore
  36 /// if x {
  37 ///     if y {
  38 ///         …
  39 ///     }
  40 /// }
  41 ///
  42 /// // or
  43 ///
  44 /// if x {
  45 ///     …
  46 /// } else {
  47 ///     if y {
  48 ///         …
  49 ///     }
  50 /// }
  51 /// ```
  52 ///
  53 /// Should be written:
  54 ///
  55 /// ```rust.ignore
  56 /// if x && y {
  57 ///     …
  58 /// }
  59 ///
  60 /// // or
  61 ///
  62 /// if x {
  63 ///     …
  64 /// } else if y {
  65 ///     …
  66 /// }
  67 /// ```
  68 declare_clippy_lint! {
  69     pub COLLAPSIBLE_IF,
  70     style,
  71     "`if`s that can be collapsed (e.g. `if x { if y { ... } }` and `else { if x { ... } }`)"
  72 }
  73
  74 #[derive(Copy, Clone)]
  75 pub struct CollapsibleIf;
  76
  77 impl LintPass for CollapsibleIf {
  78     fn get_lints(&self) -> LintArray {
  79         lint_array!(COLLAPSIBLE_IF)
  80     }
  81
  82     fn name(&self) -> &'static str {
  83         "CollapsibleIf"
  84     }
  85 }
  86
  87 impl EarlyLintPass for CollapsibleIf {
  88     fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr) {
  89         if !in_macro(expr.span) {
  90             check_if(cx, expr)
  91         }
  92     }
  93 }
  94
  95 fn check_if(cx: &EarlyContext<'_>, expr: &ast::Expr) {
  96     match expr.node {
  97         ast::ExprKind::If(ref check, ref then, ref else_) => {
  98             if let Some(ref else_) = *else_ {
  99                 check_collapsible_maybe_if_let(cx, else_);
 100             } else {
 101                 check_collapsible_no_if_let(cx, expr, check, then);
 102             }
 103         },
 104         ast::ExprKind::IfLet(_, _, _, Some(ref else_)) => {
 105             check_collapsible_maybe_if_let(cx, else_);
 106         },
 107         _ => (),
 108     }
 109 }
 110
 111 fn block_starts_with_comment(cx: &EarlyContext<'_>, expr: &ast::Block) -> bool {
 112     // We trim all opening braces and whitespaces and then check if the next string is a comment.
 113     let trimmed_block_text = snippet_block(cx, expr.span, "..")
 114         .trim_start_matches(|c: char| c.is_whitespace() || c == '{')
 115         .to_owned();
 116     trimmed_block_text.starts_with("//") || trimmed_block_text.starts_with("/*")
 117 }
 118
 119 fn check_collapsible_maybe_if_let(cx: &EarlyContext<'_>, else_: &ast::Expr) {
 120     if_chain! {
 121         if let ast::ExprKind::Block(ref block, _) = else_.node;
 122         if !block_starts_with_comment(cx, block);
 123         if let Some(else_) = expr_block(block);
 124         if !in_macro(else_.span);
 125         then {
 126             match else_.node {
 127                 ast::ExprKind::If(..) | ast::ExprKind::IfLet(..) => {
 128                     let mut applicability = Applicability::MachineApplicable;
 129                     span_lint_and_sugg(
 130                         cx,
 131                         COLLAPSIBLE_IF,
 132                         block.span,
 133                         "this `else { if .. }` block can be collapsed",
 134                         "try",
 135                         snippet_block_with_applicability(cx, else_.span, "..", &mut applicability).into_owned(),
 136                         applicability,
 137                     );
 138                 }
 139                 _ => (),
 140             }
 141         }
 142     }
 143 }
 144
 145 fn check_collapsible_no_if_let(cx: &EarlyContext<'_>, expr: &ast::Expr, check: &ast::Expr, then: &ast::Block) {
 146     if_chain! {
 147         if !block_starts_with_comment(cx, then);
 148         if let Some(inner) = expr_block(then);
 149         if let ast::ExprKind::If(ref check_inner, ref content, None) = inner.node;
 150         then {
 151             if expr.span.ctxt() != inner.span.ctxt() {
 152                 return;
 153             }
 154             span_lint_and_then(cx, COLLAPSIBLE_IF, expr.span, "this if statement can be collapsed", |db| {
 155                 let lhs = Sugg::ast(cx, check, "..");
 156                 let rhs = Sugg::ast(cx, check_inner, "..");
 157                 db.span_suggestion(
 158                     expr.span,
 159                     "try",
 160                     format!(
 161                         "if {} {}",
 162                         lhs.and(&rhs),
 163                         snippet_block(cx, content.span, ".."),
 164                     ),
 165                     Applicability::MachineApplicable, // snippet
 166                 );
 167             });
 168         }
 169     }
 170 }
 171
 172 /// If the block contains only one expression, return it.
 173 fn expr_block(block: &ast::Block) -> Option<&ast::Expr> {
 174     let mut it = block.stmts.iter();
 175
 176     if let (Some(stmt), None) = (it.next(), it.next()) {
 177         match stmt.node {
 178             ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => Some(expr),
 179             _ => None,
 180         }
 181     } else {
 182         None
 183     }
 184 }