rust-clippy/clippy_lints/src/if_not_else.rs

//! lint on if branches that could be swapped so no `!` operation is necessary
//! on the condition

use rustc::lint::*;
use syntax::ast::*;

use utils::{in_external_macro, span_help_and_lint};

/// **What it does:** Checks for usage of `!` or `!=` in an if condition with an
/// else branch.
///
/// **Why is this bad?** Negations reduce the readability of statements.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// if !v.is_empty() {
///     a()
/// } else {
///     b()
/// }
/// ```
///
/// Could be written:
///
/// ```rust
/// if v.is_empty() {
///     b()
/// } else {
///     a()
/// }
/// ```
declare_lint! {
    pub IF_NOT_ELSE,
    Allow,
    "`if` branches that could be swapped so no negation operation is necessary on the condition"
}

pub struct IfNotElse;

impl LintPass for IfNotElse {
    fn get_lints(&self) -> LintArray {
        lint_array!(IF_NOT_ELSE)
    }
}

impl EarlyLintPass for IfNotElse {
    fn check_expr(&mut self, cx: &EarlyContext, item: &Expr) {
        if in_external_macro(cx, item.span) {
            return;
        }
        if let ExprKind::If(ref cond, _, Some(ref els)) = item.node {
            if let ExprKind::Block(..) = els.node {
                match cond.node {
                    ExprKind::Unary(UnOp::Not, _) => {
                        span_help_and_lint(
                            cx,
                            IF_NOT_ELSE,
                            item.span,
                            "Unnecessary boolean `not` operation",
                            "remove the `!` and swap the blocks of the if/else",
                        );
                    },
                    ExprKind::Binary(ref kind, _, _) if kind.node == BinOpKind::Ne => {
                        span_help_and_lint(
                            cx,
                            IF_NOT_ELSE,
                            item.span,
                            "Unnecessary `!=` operation",
                            "change to `==` and swap the blocks of the if/else",
                        );
                    },
                    _ => (),
                }
            }
        }
    }
}