use rustc::lint::*;
use rustc::ty;
use rustc::hir::*;
use utils::{match_def_path, paths, span_help_and_lint, opt_def_id};

/// **What it does:** Checks for creation of references to zeroed or uninitialized memory.
///
/// **Why is this bad?** Creation of null references is undefined behavior.
///
/// **Known problems:** None. 
///
/// **Example:**
/// ```rust
/// let bad_ref: &usize = std::mem::zeroed();
/// ```

declare_lint! {
    pub INVALID_REF,
    Warn,
    "creation of invalid reference"
}

const ZERO_REF_SUMMARY: &str = "reference to zeroed memory";
const UNINIT_REF_SUMMARY: &str = "reference to uninitialized memory";
const HELP: &str = "Creation of a null reference is undefined behavior; see https://doc.rust-lang.org/reference/behavior-considered-undefined.html"; 

pub struct InvalidRef; 

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

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for InvalidRef {
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
        if_let_chain!{[
            let ExprCall(ref path, ref args) = expr.node,
            let ExprPath(ref qpath) = path.node,
            args.len() == 0,
            let ty::TyRef(..) = cx.tables.expr_ty(expr).sty, 
            let Some(def_id) = opt_def_id(cx.tables.qpath_def(qpath, path.hir_id)),
        ], {
            let msg = if match_def_path(cx.tcx, def_id, &paths::MEM_ZEROED) | match_def_path(cx.tcx, def_id, &paths::INIT) {
                ZERO_REF_SUMMARY
            } else if match_def_path(cx.tcx, def_id, &paths::MEM_UNINIT) | match_def_path(cx.tcx, def_id, &paths::UNINIT) {
                UNINIT_REF_SUMMARY
            } else {
                return;
            };
            span_help_and_lint(cx, INVALID_REF, expr.span, msg, HELP);
        }}        
        return;
    }
}