rust/clippy_lints/src/ok_if_let.rs

use crate::utils::{match_type, method_chain_args, paths, snippet, span_help_and_lint};
use if_chain::if_chain;
use rustc::declare_lint_pass;
use rustc::hir::*;
use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use rustc_session::declare_tool_lint;

declare_clippy_lint! {
    /// **What it does:*** Checks for unnecessary `ok()` in if let.
    ///
    /// **Why is this bad?** Calling `ok()` in if let is unnecessary, instead match
    /// on `Ok(pat)`
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```ignore
    /// for i in iter {
    ///     if let Some(value) = i.parse().ok() {
    ///         vec.push(value)
    ///     }
    /// }
    /// ```
    /// Could be written:
    ///
    /// ```ignore
    /// for i in iter {
    ///     if let Ok(value) = i.parse() {
    ///         vec.push(value)
    ///     }
    /// }
    /// ```
    pub IF_LET_SOME_RESULT,
    style,
    "usage of `ok()` in `if let Some(pat)` statements is unnecessary, match on `Ok(pat)` instead"
}

declare_lint_pass!(OkIfLet => [IF_LET_SOME_RESULT]);

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for OkIfLet {
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
        if_chain! { //begin checking variables
            if let ExprKind::Match(ref op, ref body, ref source) = expr.kind; //test if expr is a match
            if let MatchSource::IfLetDesugar { .. } = *source; //test if it is an If Let
            if let ExprKind::MethodCall(_, _, ref result_types) = op.kind; //check is expr.ok() has type Result<T,E>.ok()
            if let PatKind::TupleStruct(QPath::Resolved(_, ref x), ref y, _)  = body[0].pat.kind; //get operation
            if method_chain_args(op, &["ok"]).is_some(); //test to see if using ok() methoduse std::marker::Sized;

            then {
                let is_result_type = match_type(cx, cx.tables.expr_ty(&result_types[0]), &paths::RESULT);
                let some_expr_string = snippet(cx, y[0].span, "");
                if print::to_string(print::NO_ANN, |s| s.print_path(x, false)) == "Some" && is_result_type {
                    span_help_and_lint(cx, IF_LET_SOME_RESULT, expr.span,
                    "Matching on `Some` with `ok()` is redundant",
                    &format!("Consider matching on `Ok({})` and removing the call to `ok` instead", some_expr_string));
                }
            }
        }
    }
}