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
  83 impl EarlyLintPass for CollapsibleIf {
  84     fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr) {
  85         if !in_macro(expr.span) {
  86             check_if(cx, expr)
  87         }
  88     }
  89 }
  90
  91 fn check_if(cx: &EarlyContext<'_>, expr: &ast::Expr) {
  92     match expr.node {
  93         ast::ExprKind::If(ref check, ref then, ref else_) => {
  94             if let Some(ref else_) = *else_ {
  95                 check_collapsible_maybe_if_let(cx, else_);
  96             } else {
  97                 check_collapsible_no_if_let(cx, expr, check, then);
  98             }
  99         },
 100         ast::ExprKind::IfLet(_, _, _, Some(ref else_)) => {
 101             check_collapsible_maybe_if_let(cx, else_);
 102         },
 103         _ => (),
 104     }
 105 }
 106
 107 fn block_starts_with_comment(cx: &EarlyContext<'_>, expr: &ast::Block) -> bool {
 108     // We trim all opening braces and whitespaces and then check if the next string is a comment.
 109     let trimmed_block_text = snippet_block(cx, expr.span, "..")
 110         .trim_start_matches(|c: char| c.is_whitespace() || c == '{')
 111         .to_owned();
 112     trimmed_block_text.starts_with("//") || trimmed_block_text.starts_with("/*")
 113 }
 114
 115 fn check_collapsible_maybe_if_let(cx: &EarlyContext<'_>, else_: &ast::Expr) {
 116     if_chain! {
 117         if let ast::ExprKind::Block(ref block, _) = else_.node;
 118         if !block_starts_with_comment(cx, block);
 119         if let Some(else_) = expr_block(block);
 120         if !in_macro(else_.span);
 121         then {
 122             match else_.node {
 123                 ast::ExprKind::If(..) | ast::ExprKind::IfLet(..) => {
 124                     let mut applicability = Applicability::MachineApplicable;
 125                     span_lint_and_sugg(
 126                         cx,
 127                         COLLAPSIBLE_IF,
 128                         block.span,
 129                         "this `else { if .. }` block can be collapsed",
 130                         "try",
 131                         snippet_block_with_applicability(cx, else_.span, "..", &mut applicability).into_owned(),
 132                         applicability,
 133                     );
 134                 }
 135                 _ => (),
 136             }
 137         }
 138     }
 139 }
 140
 141 fn check_collapsible_no_if_let(cx: &EarlyContext<'_>, expr: &ast::Expr, check: &ast::Expr, then: &ast::Block) {
 142     if_chain! {
 143         if !block_starts_with_comment(cx, then);
 144         if let Some(inner) = expr_block(then);
 145         if let ast::ExprKind::If(ref check_inner, ref content, None) = inner.node;
 146         then {
 147             if expr.span.ctxt() != inner.span.ctxt() {
 148                 return;
 149             }
 150             span_lint_and_then(cx, COLLAPSIBLE_IF, expr.span, "this if statement can be collapsed", |db| {
 151                 let lhs = Sugg::ast(cx, check, "..");
 152                 let rhs = Sugg::ast(cx, check_inner, "..");
 153                 db.span_suggestion_with_applicability(
 154                     expr.span,
 155                     "try",
 156                     format!(
 157                         "if {} {}",
 158                         lhs.and(&rhs),
 159                         snippet_block(cx, content.span, ".."),
 160                     ),
 161                     Applicability::MachineApplicable, // snippet
 162                 );
 163             });
 164         }
 165     }
 166 }
 167
 168 /// If the block contains only one expression, return it.
 169 fn expr_block(block: &ast::Block) -> Option<&ast::Expr> {
 170     let mut it = block.stmts.iter();
 171
 172     if let (Some(stmt), None) = (it.next(), it.next()) {
 173         match stmt.node {
 174             ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => Some(expr),
 175             _ => None,
 176         }
 177     } else {
 178         None
 179     }
 180 }