lang: Parse #[account] attribute arguments with syn

[acheroncrypto]

Problem

#[account] macro arguments are parsed by converting the tokens to string and interpreting:

anchor/lang/attribute/account/src/lib.rs

Lines 70 to 94 in f6a8042

	let mut namespace = "".to_string();
	let mut is_zero_copy = false;
	let mut unsafe_bytemuck = false;
	let args_str = args.to_string();
	let args: Vec<&str> = args_str.split(',').collect();
	if args.len() > 2 {
	panic!("Only two args are allowed to the account attribute.")
	}
	for arg in args {
	let ns = arg
	.to_string()
	.replace('\"', "")
	.chars()
	.filter(|c| !c.is_whitespace())
	.collect();
	if ns == "zero_copy" {
	is_zero_copy = true;
	unsafe_bytemuck = false;
	} else if ns == "zero_copy(unsafe)" {
	is_zero_copy = true;
	unsafe_bytemuck = true;
	} else {
	namespace = ns;
	}
	}

This makes adding named arguments e.g. discriminator = <EXPR> challenging.

Furthermore, any argument other than zero_copy and zero_copy(unsafe) is interpreted as custom namespace, even when the argument is not a string.

Summary of changes

• Parse the arguments using syn
• Unknown non-string arguments (e.g. #[account(my_namespace)]) are no longer interpreted as custom namespaces, and they return an error instead

---

Note: This PR is part of a greater effort explained in #3097.

[vercel]

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