// Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.


use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use crate::rustc::{declare_tool_lint, lint_array};
use crate::rustc::hir::*;
use crate::utils::{match_qpath, paths, snippet, span_lint_and_then};
use crate::rustc_errors::Applicability;

/// **What it does:** Lint for redundant pattern matching over `Result` or
/// `Option`
///
/// **Why is this bad?** It's more concise and clear to just use the proper
/// utility function
///
/// **Known problems:** None.
///
/// **Example:**
///
/// ```rust
/// if let Ok(_) = Ok::<i32, i32>(42) {}
/// if let Err(_) = Err::<i32, i32>(42) {}
/// if let None = None::<()> {}
/// if let Some(_) = Some(42) {}
/// ```
///
/// The more idiomatic use would be:
///
/// ```rust
/// if Ok::<i32, i32>(42).is_ok() {}
/// if Err::<i32, i32>(42).is_err() {}
/// if None::<()>.is_none() {}
/// if Some(42).is_some() {}
/// ```
///
declare_clippy_lint! {
    pub IF_LET_REDUNDANT_PATTERN_MATCHING,
    style,
    "use the proper utility function avoiding an `if let`"
}

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

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

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
        if let ExprKind::Match(ref op, ref arms, MatchSource::IfLetDesugar { .. }) = expr.node {
            if arms[0].pats.len() == 1 {
                let good_method = match arms[0].pats[0].node {
                    PatKind::TupleStruct(ref path, ref pats, _) if pats.len() == 1 => {
                        if let PatKind::Wild = pats[0].node {
                            if match_qpath(path, &paths::RESULT_OK) {
                                "is_ok()"
                            } else if match_qpath(path, &paths::RESULT_ERR) {
                                "is_err()"
                            } else if match_qpath(path, &paths::OPTION_SOME) {
                                "is_some()"
                            } else {
                                return;
                            }
                        } else {
                            return;
                        }
                    },

                    PatKind::Path(ref path) if match_qpath(path, &paths::OPTION_NONE) => "is_none()",

                    _ => return,
                };

                span_lint_and_then(
                    cx,
                    IF_LET_REDUNDANT_PATTERN_MATCHING,
                    arms[0].pats[0].span,
                    &format!("redundant pattern matching, consider using `{}`", good_method),
                    |db| {
                        let span = expr.span.with_hi(op.span.hi());
                        db.span_suggestion_with_applicability(
                            span,
                            "try this",
                            format!("if {}.{}", snippet(cx, op.span, "_"), good_method),
                            Applicability::MachineApplicable, // snippet
                        );
                    },
                );
            }
        }
    }
}