oxc-project/oxc · warning

Found a comment that would permit fallthrough, but case cann

Error message

Found a comment that would permit fallthrough, but case cannot fall through.

What it means

The inverse diagnostic of oxlint's no-fallthrough rule: a case contains a comment that matches the fallthrough pattern (e.g. '// falls through'), but the case cannot actually fall through because it ends with break/return/throw or is the last clause. The comment is dead and misleading, so the rule reports it and asks you to remove it or make the fallthrough real.

Source

Thrown at crates/oxc_linter/src/rules/eslint/no_fallthrough.rs:44

    AstNode,
    context::LintContext,
    rule::{DefaultRuleConfig, Rule},
};

fn no_fallthrough_case_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Expected a `break` statement before `case`.")
        .with_help("Use a `break` statement to prevent fallthrough, or add a comment to indicate intentional fallthrough.")
        .with_label(span)
}

fn no_fallthrough_default_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Expected a `break` statement before `default`.")
        .with_help("Use a `break` statement to prevent fallthrough, or add a comment to indicate intentional fallthrough.")
        .with_label(span)
}

fn no_unused_fallthrough_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn(
        "Found a comment that would permit fallthrough, but case cannot fall through.",
    )
    .with_help(
        "Remove the fallthrough comment or add code that allows fallthrough (e.g. remove `break`).",
    )
    .with_label(span)
}

#[derive(Default, Debug, Clone, Deserialize, JsonSchema)]
#[serde(rename_all = "camelCase", default, deny_unknown_fields)]
struct NoFallthroughConfig {
    /// Custom regex pattern to match fallthrough comments.
    #[serde(default, deserialize_with = "deserialize_comment_pattern")]
    comment_pattern: Option<Regex>,
    /// Whether to allow empty case clauses to fall through.
    allow_empty_case: bool,
    /// Whether to report unused fallthrough comments.
    report_unused_fallthrough_comment: bool,

View on GitHub (pinned to e1e7af627c)

Solutions

1. Delete the stale // falls through comment.
2. If fallthrough was intended, remove the break/return that prevents it.
3. Reword the comment so it no longer matches commentPattern when it describes something else.

Example fix

// before
case 2:
  flush();
  // falls through
  break;

// after
case 2:
  flush();
  break;

npx oxlint -A all -D no-fallthrough src/  # catches unused fallthrough comments too

When it happens

Trigger: case 2: flush(); // falls through followed by break; on the next line; a last case or default: ending with a fallthrough comment; a return added above an old fallthrough comment during a bugfix, leaving the marker behind.

Common situations: A bugfix adds a return/break but keeps the old marker; cases are reordered so a marked case becomes terminal; fallthrough comments are copy-pasted across cases during merges.

Related errors

• Expected a `break` statement before `case`.
• Expected a `break` statement before `default`.
• Unexpected comment inline with code
• Unexpected '{term}' comment: {display}
• Checking `switch` discriminant against NaN will never match

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/2afa99a8acd4821a. Report an issue: GitHub.