rust-clippy/src/needless_bool.rs

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

use rustc::plugin::Registry;
use rustc::lint::*;
use rustc::middle::const_eval::lookup_const_by_id;
use rustc::middle::def::*;
use syntax::ast::*;
use syntax::ast_util::{is_comparison_binop, binop_to_string};
use syntax::ptr::P;
use syntax::codemap::Span;

declare_lint! {
    pub NEEDLESS_BOOL,
    Warn,
    "Warn on needless use of if x { true } else { false } (or vice versa)"
}

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

impl LintPass for NeedlessBool {
    fn get_lints(&self) -> LintArray {
        lint_array!(NEEDLESS_BOOL)
    }
    
    fn check_expr(&mut self, cx: &Context, e: &Expr) {
        if let ExprIf(_, ref then_block, Option::Some(ref else_expr)) = e.node {
			match (fetch_bool_block(then_block), fetch_bool_expr(else_expr)) {
				(Option::Some(true), Option::Some(true)) => { cx.span_lint(NEEDLESS_BOOL, e.span, "your if-then-else expression will always return true"); },
				(Option::Some(true), Option::Some(false)) => { cx.span_lint(NEEDLESS_BOOL, e.span, "you can reduce your if-statement to its predicate"); },
				(Option::Some(false), Option::Some(true)) => { cx.span_lint(NEEDLESS_BOOL, e.span, "you can reduce your if-statement to '!' + your predicate"); },
				(Option::Some(false), Option::Some(false)) => { cx.span_lint(NEEDLESS_BOOL, e.span, "your if-then-else expression will always return false"); },
				_ => ()
			}
		}
    }
}

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 { Option::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 { Option::Some(value) } else { Option::None },
		_ => Option::None
	}
}