https://errorvault.dev/tags/declarative-macros/ Recent content in Declarative-Macros on ErrorVault — Developer Error Code Dictionary Hugo en-us Sun, 26 Jul 2026 07:56:04 +0800 https://errorvault.dev/rust/rust-e0666-malformed-macro-invocation/ Sun, 26 Jul 2026 07:56:04 +0800 https://errorvault.dev/rust/rust-e0666-malformed-macro-invocation/ <h2 id="1-symptoms">1. Symptoms</h2> <p>When Rust&rsquo;s macro expansion engine encounters a macro invocation that does not match the macro definition&rsquo;s pattern, the compiler emits error E0666. This error manifests with a clear diagnostic message indicating the specific mismatch between what the macro expected and what was provided in the invocation.</p> <p>The compiler output typically includes the macro name, the pattern that failed to match, and the tokens that were actually supplied. The error message structure varies slightly depending on the complexity of the mismatch, but the core diagnostic remains consistent across Rust versions.</p>