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These are some common errors that can happen when developing on Substrate:
Errors like:
Double lang item in crate <crate> (which `std`/ `serde` depends on):...Attempted to define built-in macro more than once,
happen commonly when using std crates in a non-std environment, like Substrate's runtime. std crates can't be used because we compile to WASM. If you run into an error like this and the crate you are using is no-std, make sure you are setting them up correctly. For example, make sure that the dependency is imported with default-features = false or that the std feature is set correctly in the respective Cargo.toml. If you are writing a new module, make sure that it is premised by #![cfg_attr(not(feature = "std"), no_std)].
When trying to use alloc features like vec, you might run into the trouble that the compiler can't find the alloc crate. This feature can be imported from various dependencies like serde and serde_json. To use it make sure to add extern crate alloc; at the top of your file.
An error like the following:
Failed to deserialize `.../minimal_template_runtime.wasm`: UnknownOpcode(192)
might arise while building with the --release flag while you are modifying
the node (debug builds rarely produce it). This might persist even if you complete all
modifications indicated.
In such situations, best thing to do is start a build from scratch by issuing a cargo clean --release command,
delete Cargo.lock, and release-build again. That should do it.
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