rust/src/needless_bool.rs

//! Checks for needless boolean results of if-else expressions
//!
//! This lint is **warn** by default

use rustc::lint::*;
use rustc_front::hir::*;

use syntax::ast::Lit_::*;

use utils::{span_lint, snippet};

/// **What it does:** This lint checks for expressions of the form `if c { true } else { false }` (or vice versa) and suggest using the condition directly. It is `Warn` by default.
///
/// **Why is this bad?** Redundant code.
///
/// **Known problems:** Maybe false positives: Sometimes, the two branches are painstakingly documented (which we of course do not detect), so they *may* have some value. Even then, the documentation can be rewritten to match the shorter code.
///
/// **Example:** `if x { false } else { true }`
declare_lint! {
    pub NEEDLESS_BOOL,
    Warn,
    "if-statements with plain booleans in the then- and else-clause, e.g. \
     `if p { true } else { false }`"
}

#[derive(Copy,Clone)]
pub struct NeedlessBool;

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

impl LateLintPass for NeedlessBool {
    fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
        if let ExprIf(ref pred, ref then_block, Some(ref else_expr)) = e.node {
            match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
                (Some(true), Some(true)) => {
                    span_lint(cx, NEEDLESS_BOOL, e.span,
                              "this if-then-else expression will always return true");
                }
                (Some(false), Some(false)) => {
                    span_lint(cx, NEEDLESS_BOOL, e.span,
                              "this if-then-else expression will always return false");
                }
                (Some(true), Some(false)) => {
                    let pred_snip = snippet(cx, pred.span, "..");
                    let hint = if pred_snip == ".." { "its predicate".into() } else {
                        format!("`{}`", pred_snip)
                    };
                    span_lint(cx, NEEDLESS_BOOL, e.span, &format!(
                        "you can reduce this if-then-else expression to just {}", hint));
                }
                (Some(false), Some(true)) => {
                    let pred_snip = snippet(cx, pred.span, "..");
                    let hint = if pred_snip == ".." { "`!` and its predicate".into() } else {
                        format!("`!{}`", pred_snip)
                    };
                    span_lint(cx, NEEDLESS_BOOL, e.span, &format!(
                        "you can reduce this if-then-else expression to just {}", hint));
                }
                _ => ()
            }
        }
    }
}

fn fetch_bool_block(block: &Block) -> Option<bool> {
    if block.stmts.is_empty() {
        block.expr.as_ref().and_then(|e| fetch_bool_expr(e))
    } else { None }
}

fn fetch_bool_expr(expr: &Expr) -> Option<bool> {
    match expr.node {
        ExprBlock(ref block) => fetch_bool_block(block),
        ExprLit(ref lit_ptr) => if let LitBool(value) = lit_ptr.node {
            Some(value) } else { None },
        _ => None
    }
}