What is the harm in using len or new as variables? If they are redefined they won't operate, in the current scope, as builtins so there should be little cause for concern that they are accidentally misused.

What is the harm in using len or new as variables? If they are redefined they won't operate, in the current scope, as builtins so there should be little cause for concern that they are accidentally misused.

Yes, you are right about there is no harm.
However it makes code more clear that len,new,etc... are builtins instead of temporary varibles.
Just like vet checking argument types and numbers for fmt.Printf().

They are identifiers. Identifier shadowing is also allowed for other identifiers in Go.

However it makes code more clear that len,new,etc... are builtins instead of temporary varibles.

Vet is not a code style enforcer: "Vet's checks are about correctness, not style."
https://github.com/golang/go/tree/master/src/cmd/vet

As far as I understand the vet argument checking in fmt actually covers code that is considered an error by fmt. Reusing identifiers is not an error as shown by the standard library.

I think its fine if developers opt in to disallow shadowing some identifiers in style checkers on their codebase. Making it a defacto enforced style in Go seems a big hammer. If that path is taken of disallowing to reuse those identifiers (kind of a language change in disguise) it might just be better to make them compile errors. (I am not in favor of disallowing shadowing of builtin identifiers.)

Fair point, let linter do their job since shadowing is not an error.