src/tools/clippy/clippy_lints/src/infallible_destructuring_match.rs

use super::utils::{get_arg_name, match_var, remove_blocks, snippet_with_applicability, span_lint_and_sugg};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir::*;
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};

declare_clippy_lint! {
    /// **What it does:** Checks for matches being used to destructure a single-variant enum
    /// or tuple struct where a `let` will suffice.
    ///
    /// **Why is this bad?** Just readability – `let` doesn't nest, whereas a `match` does.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust
    /// enum Wrapper {
    ///     Data(i32),
    /// }
    ///
    /// let wrapper = Wrapper::Data(42);
    ///
    /// let data = match wrapper {
    ///     Wrapper::Data(i) => i,
    /// };
    /// ```
    ///
    /// The correct use would be:
    /// ```rust
    /// enum Wrapper {
    ///     Data(i32),
    /// }
    ///
    /// let wrapper = Wrapper::Data(42);
    /// let Wrapper::Data(data) = wrapper;
    /// ```
    pub INFALLIBLE_DESTRUCTURING_MATCH,
    style,
    "a `match` statement with a single infallible arm instead of a `let`"
}

declare_lint_pass!(InfallibleDestructingMatch => [INFALLIBLE_DESTRUCTURING_MATCH]);

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for InfallibleDestructingMatch {
    fn check_local(&mut self, cx: &LateContext<'a, 'tcx>, local: &'tcx Local<'_>) {
        if_chain! {
            if let Some(ref expr) = local.init;
            if let ExprKind::Match(ref target, ref arms, MatchSource::Normal) = expr.kind;
            if arms.len() == 1 && arms[0].guard.is_none();
            if let PatKind::TupleStruct(QPath::Resolved(None, ref variant_name), ref args, _) = arms[0].pat.kind;
            if args.len() == 1;
            if let Some(arg) = get_arg_name(&args[0]);
            let body = remove_blocks(&arms[0].body);
            if match_var(body, arg);

            then {
                let mut applicability = Applicability::MachineApplicable;
                span_lint_and_sugg(
                    cx,
                    INFALLIBLE_DESTRUCTURING_MATCH,
                    local.span,
                    "you seem to be trying to use `match` to destructure a single infallible pattern. \
                     Consider using `let`",
                    "try this",
                    format!(
                        "let {}({}) = {};",
                        snippet_with_applicability(cx, variant_name.span, "..", &mut applicability),
                        snippet_with_applicability(cx, local.pat.span, "..", &mut applicability),
                        snippet_with_applicability(cx, target.span, "..", &mut applicability),
                    ),
                    applicability,
                );
            }
        }
    }
}