oxc/crates/oxc_linter/src/rules/eslint/no_unreachable.rs

use oxc_ast::{ast::VariableDeclarationKind, AstKind};
use oxc_diagnostics::OxcDiagnostic;
use oxc_macros::declare_oxc_lint;
use oxc_semantic::{
    control_flow::graph::{
        visit::{depth_first_search, Control, DfsEvent, EdgeRef},
        Direction,
    },
    EdgeType, ErrorEdgeKind, InstructionKind,
};
use oxc_span::{GetSpan, Span};

use crate::{context::LintContext, rule::Rule};

fn no_unreachable_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::error("eslint(no-unreachable): Unreachable code.").with_labels([span.into()])
}

/// <https://github.com/eslint/eslint/blob/069aa680c78b8516b9a1b568519f1d01e74fb2a2/lib/rules/no-unreachable.js#L196>
#[derive(Debug, Default, Clone)]
pub struct NoUnreachable;

declare_oxc_lint!(
    /// ### What it does
    ///
    /// Disallow unreachable code after `return`, `throw`, `continue`, and `break` statements
    ///
    NoUnreachable,
    nursery
);

impl Rule for NoUnreachable {
    fn run_once(&self, ctx: &LintContext) {
        let nodes = ctx.nodes();
        let Some(root) = nodes.root_node() else { return };
        let cfg = ctx.semantic().cfg();
        let graph = &cfg.graph;

        // A pre-allocated vector containing the reachability status of all the basic blocks.
        // We initialize this vector with all nodes set to `unreachable` since if we don't visit a
        // node in our paths then it should be unreachable by definition.
        let mut unreachables = vec![true; cfg.basic_blocks.len()];

        // All of the end points of infinite loops we encountered.
        let mut infinite_loops = Vec::new();

        // Set the root as reachable.
        unreachables[root.cfg_id().index()] = false;

        // In our first path we first check if each block is definitely unreachable, If it is then
        // we set it as such, If we encounter an infinite loop we keep its end block since it can
        // prevent other reachable blocks from ever getting executed.
        let _: Control<()> = depth_first_search(graph, Some(root.cfg_id()), |event| {
            if let DfsEvent::Finish(node, _) = event {
                let unreachable = cfg.basic_block(node).unreachable;
                unreachables[node.index()] = unreachable;

                if !unreachable {
                    if let Some(it) = cfg.is_infinite_loop_start(node, nodes) {
                        infinite_loops.push(it);
                    }
                }
            }
            Control::Continue
        });

        // In the second path we go for each infinite loop end block and follow it marking all
        // edges as unreachable unless they have a reachable jump (eg. break).
        for loop_ in infinite_loops {
            // A loop end block usually is also its condition and start point but what is common
            // in all cases is that it may have `Jump` or `Backedge` edges so we only want to
            // follow the `Normal` edges as these are the exiting edges.
            let starts: Vec<_> = graph
                .edges_directed(loop_.1, Direction::Outgoing)
                .filter(|it| matches!(it.weight(), EdgeType::Normal))
                .map(|it| it.target())
                .collect();

            // Search with all `Normal` edges as starting point(s).
            let _: Control<()> = depth_first_search(graph, starts, |event| match event {
                DfsEvent::Discover(node, _) => {
                    let mut incoming = graph.edges_directed(node, Direction::Incoming);
                    if incoming.any(|e| match e.weight() {
                        // `NewFunction` is always reachable
                        | EdgeType::NewFunction
                        // `Finalize` can be reachable if we encounter an error in the loop.
                        | EdgeType::Finalize
                        // Explicit `Error` can also be reachable if we encounter an error in the loop.
                        | EdgeType::Error(ErrorEdgeKind::Explicit) => true,

                        // If we have an incoming `Jump` and it is from a `Break` instruction,
                        // We know with high confidence that we are visiting a reachable block.
                        // NOTE: May cause false negatives but I couldn't think of one.
                        EdgeType::Jump
                            if cfg
                                .basic_block(e.source())
                                .instructions()
                                .iter()
                                .any(|it| matches!(it.kind, InstructionKind::Break(_))) =>
                        {
                            true
                        }
                        _ => false,
                    }) {
                        // We prune this branch if it is reachable from this point forward.
                        Control::Prune
                    } else {
                        // Otherwise we set it to unreachable and continue.
                        unreachables[node.index()] = true;
                        Control::Continue
                    }
                }
                _ => Control::Continue,
            });
        }
        for node in ctx.nodes().iter() {
            // exit early if we are not visiting a statement.
            if !node.kind().is_statement() {
                continue;
            }

            // exit early if it is an empty statement.
            if matches!(node.kind(), AstKind::EmptyStatement(_)) {
                continue;
            }

            if matches! {
                node.kind(),
                AstKind::VariableDeclaration(decl)
                    if matches!(decl.kind, VariableDeclarationKind::Var) && !decl.has_init()
            } {
                // Skip `var` declarations without any initialization,
                // These work because of the JavaScript hoisting rules.
                continue;
            }

            if unreachables[node.cfg_id().index()] {
                ctx.diagnostic(no_unreachable_diagnostic(node.kind().span()));
            }
        }
    }
}

#[test]
fn test() {
    use crate::tester::Tester;

    let pass = vec![
        "function foo() { function bar() { return 1; } return bar(); }",
        "function foo() { return bar(); function bar() { return 1; } }",
        "function foo() { return x; var x; }",
        "function foo() { var x = 1; var y = 2; }",
        "function foo() { var x = 1; var y = 2; return; }",
        "while (true) { switch (foo) { case 1: x = 1; x = 2;} }",
        "while (true) { break; var x; }",
        "while (true) { continue; var x, y; }",
        "while (true) { throw 'message'; var x; }",
        "while (true) { if (true) break; var x = 1; }",
        "while (true) continue;",
        "switch (foo) { case 1: break; var x; }",
        "switch (foo) { case 1: break; var x; default: throw true; };",
        "const arrow_direction = arrow => {  switch (arrow) { default: throw new Error();  };}",
        "var x = 1; y = 2; throw 'uh oh'; var y;",
        "function foo() { var x = 1; if (x) { return; } x = 2; }",
        "function foo() { var x = 1; if (x) { } else { return; } x = 2; }",
        "function foo() { var x = 1; switch (x) { case 0: break; default: return; } x = 2; }",
        "function foo() { var x = 1; while (x) { return; } x = 2; }",
        "function foo() { var x = 1; for (x in {}) { return; } x = 2; }",
        "function foo() { var x = 1; try { return; } finally { x = 2; } }",
        "function foo() { var x = 1; for (;;) { if (x) break; } x = 2; }",
        "A: { break A; } foo()",
        "function* foo() { try { yield 1; return; } catch (err) { return err; } }",
        "function foo() { try { bar(); return; } catch (err) { return err; } }",
        "function foo() { try { a.b.c = 1; return; } catch (err) { return err; } }",
        "class C { foo = reachable; }",
        "class C { foo = reachable; constructor() {} }",
        "class C extends B { foo = reachable; }",
        "class C extends B { foo = reachable; constructor() { super(); } }",
        "class C extends B { static foo = reachable; constructor() {} }",
        "function foo() { var x = 1; for (;x == 1;) { if (x) continue; } x = 2; }",
        "
        if (a) {
            a();
        } else {
          for (let i = 1; i <= 10; i++) {
            b();
          }

          for (let i = 1; i <= 10; i++) {
            c();
          }
        }
        ",
        "
        try {
            throw 'error';
        } catch (err) {
            b();
        }
        c();
        ",
        "
        export const getPagePreviewText = (page) => {
            if (!a) {
                return '';
            }
            while (a && b > c && d-- > 0) {
            }
        };
        ",
        "
        try {
            for (const a of b) {
                c();
            }

            while (true) {
                d();
            }
        } finally {
        }
        ",
        "
        switch (authType) {
          case 1:
            return a();
          case 2:
            return b();
          case 3:
            return c();
        }
        d();
        ",
        "
        try {
          a();
        } catch (e) {
          b();
        } finally {
          c();
        }
        d();
        ",
        "
        try {
            while (true) {
                a();
            }
        } finally {
            b();
        }
        ",
        "
        try {
            a();
        } finally {
            b();
        }
        c();
        ",
        "
        try {
            while (true) {
                a();
            }
        } catch {
            b();
        }
        ",
    ];

    let fail = vec![
        //[{ messageId: "unreachableCode", type: "VariableDeclaration" }]
        "function foo() { return x; var x = 1; }",
        //[{ messageId: "unreachableCode", type: "VariableDeclaration" }]
        "function foo() { return x; var x, y = 1; }",
        "while (true) { break; var x = 1; }",
        //[{ messageId: "unreachableCode", type: "VariableDeclaration" }]
        "while (true) { continue; var x = 1; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { return; x = 1; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { throw error; x = 1; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "while (true) { break; x = 1; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "while (true) { continue; x = 1; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { switch (foo) { case 1: return; x = 1; } }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { switch (foo) { case 1: throw e; x = 1; } }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "while (true) { switch (foo) { case 1: break; x = 1; } }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "while (true) { switch (foo) { case 1: continue; x = 1; } }",
        //[{ messageId: "unreachableCode", type: "VariableDeclaration" }]
        "var x = 1; throw 'uh oh'; var y = 2;",
        // [{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { var x = 1; if (x) { return; } else { throw e; } x = 2; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { var x = 1; if (x) return; else throw -1; x = 2; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { var x = 1; try { return; } finally {} x = 2; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { var x = 1; try { } finally { return; } x = 2; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { var x = 1; do { return; } while (x); x = 2; }",
        // [{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { var x = 1; while (x) { if (x) break; else continue; x = 2; } }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { var x = 1; for (;;) { if (x) continue; } x = 2; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { var x = 1; while (true) { } x = 2; }",
        //[{ messageId: "unreachableCode", type: "ExpressionStatement" }]
        "function foo() { var x = 1; do { } while (true); x = 2; }",
    ];

    Tester::new(NoUnreachable::NAME, pass, fail).test_and_snapshot();
}