rrubocop 0.7.5

10x faster RuboCop drop-in. Same output format and configs, without maintaining RuboCop gems.
Documentation 
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//! Style/SafeNavigation — prefer &. over explicit nil checks (breadth).

use tree_sitter::Node;

use crate::cop::shared::{node_bytes, node_text};
use crate::cop::{Cop, CopConfig};
use crate::diagnostic::Diagnostic;
use crate::parse::source::SourceFile;

pub struct SafeNavigation;

impl Cop for SafeNavigation {
    fn name(&self) -> &'static str {
        "Style/SafeNavigation"
    }

    fn interested_node_kinds(&self) -> &'static [&'static str] {
        &["binary"]
    }

    fn check_node(
        &self,
        source: &SourceFile,
        node: Node<'_>,
        _config: &CopConfig,
        diagnostics: &mut Vec<Diagnostic>,
        _corrections: Option<&mut Vec<crate::correction::Correction>>,
    ) {
        if !is_and_safe_nav(source, node) || lhs_reassigned_in_method(source, node) {
            return;
        }
        let (line, col) = source.offset_to_line_col(node.start_byte());
        diagnostics.push(self.diagnostic(
            source,
            line,
            col,
            "Use safe navigation (`&.`) instead of checking `nil` with `&&`.".to_string(),
        ));
    }
}

fn is_and_safe_nav(source: &SourceFile, node: Node<'_>) -> bool {
    let mut cur = node.walk();
    let kids: Vec<_> = node.children(&mut cur).collect();
    if kids.len() < 3 {
        return false;
    }
    if node_bytes(source, kids[1]) != b"&&" {
        return false;
    }
    let right = kids[2];
    if right.kind() != "call" {
        return false;
    }
    let Some(recv) = right.child_by_field_name("receiver") else {
        return false;
    };
    node_bytes(source, kids[0]) == node_bytes(source, recv)
}

fn lhs_reassigned_in_method(source: &SourceFile, node: Node<'_>) -> bool {
    let mut cur = node.walk();
    let kids: Vec<_> = node.children(&mut cur).collect();
    let Some(lhs) = kids.first() else {
        return false;
    };
    let Some(name) = local_name(source, *lhs) else {
        return false;
    };
    let Some(method) = enclosing_method(node) else {
        return false;
    };
    method_reassigns_name(source, method, &name, lhs.start_byte())
}

fn local_name(source: &SourceFile, node: Node<'_>) -> Option<String> {
    (node.kind() == "identifier").then(|| node_text(source, node))
}

fn enclosing_method(node: Node<'_>) -> Option<Node<'_>> {
    let mut p = node.parent();
    while let Some(n) = p {
        if matches!(n.kind(), "method" | "singleton_method") {
            return Some(n);
        }
        p = n.parent();
    }
    None
}

fn method_reassigns_name(
    source: &SourceFile,
    method: Node<'_>,
    name: &str,
    before: usize,
) -> bool {
    let Some(body) = method.child_by_field_name("body") else {
        return false;
    };
    let mut out = false;
    walk_assignments(source, body, name, before, &mut out);
    out
}

fn walk_assignments(
    source: &SourceFile,
    node: Node<'_>,
    name: &str,
    before: usize,
    found: &mut bool,
) {
    if *found {
        return;
    }
    if node.kind() == "assignment" {
        if let Some(left) = node.child_by_field_name("left") {
            if left.kind() == "identifier"
                && node_text(source, left) == name
                && node.start_byte() < before
            {
                *found = true;
                return;
            }
        }
    }
    let mut cur = node.walk();
    for child in node.children(&mut cur) {
        walk_assignments(source, child, name, before, found);
    }
}