rust-sdk-pitfalls

name	rust-sdk-pitfalls
description	Critical pitfalls and safety rules for Miden Rust SDK development. Covers felt arithmetic security, comparison operators, argument limits, storage naming, no-std setup, asset layout, P2ID roots, NoteType construction, note-to-component call boundaries, and note input immutability. Use when reviewing, debugging, or writing Miden contract code.

Miden SDK Pitfalls

P1: Felt Arithmetic is Modular (SECURITY CRITICAL)

Severity: Critical — can cause loss of funds

Felt subtraction wraps around the prime field modulus (p = 2^64 - 2^32 + 1) instead of panicking. Subtracting more than available silently produces a huge positive number.

// DANGEROUS — no check before subtraction
let new_balance = current_balance - withdraw_amount;
// If withdraw_amount > current_balance, new_balance ≈ 2^64 (wraps!)

// SAFE — always validate first
assert!(
    current_balance.as_canonical_u64() >= withdraw_amount.as_canonical_u64(),
    "Insufficient balance"
);
let new_balance = current_balance - withdraw_amount;

Rule: ALWAYS check .as_canonical_u64() values before any Felt subtraction.

Max Felt value: The maximum valid Felt is p - 1 = 18446744069414584320, not u64::MAX (18446744073709551615). Using u64::MAX as a sentinel or boundary value causes silent wraparound.

P2: Felt Comparison Operators Are Misleading for Quantity Logic

Severity: High — silently produces incorrect results

<, >, <=, >= on Felt values compare field elements, which differs from natural number ordering. In protocol-level code working with field elements, these comparisons may be intentional. For business logic (balances, amounts, counts), the results are misleading.

// MISLEADING for business logic — compares field elements
if balance > threshold { ... }

// CORRECT for business logic — compare as integers
if balance.as_canonical_u64() > threshold.as_canonical_u64() { ... }

Rule: For quantity/business logic, ALWAYS convert to .as_canonical_u64() before using comparison operators.

P3: Function Argument Limit (4 Words / 16 Felts)

Severity: Medium — causes compilation errors

Functions can receive at most 4 Words (16 Felts) as arguments.

// PROBLEM — too many arguments
fn process(a: Word, b: Word, c: Word, d: Word, e: Word) { ... } // > 4 Words!

// SOLUTION — pass fat types by reference
fn process(a: &Word, b: &Word, c: &Word, d: &Word, e: &Word) { ... }

P4: Storage API Is Typed

Severity: Medium — old examples no longer compile

The old Value / untyped StorageMap API is gone. Account storage is now:

• StorageValue<T> for a single typed slot
• StorageMap<K, V> for typed maps
• get() / set() methods instead of .read() / .write()
• K: WordKey, T: WordValue, V: WordValue

#[component]
struct CounterContract {
    #[storage(description = "single typed slot")]
    counter: StorageValue<Felt>,

    #[storage(description = "typed map")]
    balances: StorageMap<AccountId, Felt>,
}

If you need custom keys or values, implement WordKey / WordValue by converting to and from a single Word.

P5: Storage Slot Naming Convention

Severity: Medium — causes silent default-value reads in tests

Storage slot names follow a strict pattern. Getting it wrong often returns the default value silently.

Pattern: [component_package_or_name]::[snake_case(component_struct)]::[field_name]

Conversion rule: Replace characters outside [A-Za-z0-9_] with _ in the package or component name. The package comes from [package.metadata.component] package = "...", with any @version suffix ignored.

Package in Cargo.toml	Component Struct	Field	Storage Slot Name
miden:counter-account	CounterContract	count_map	miden_counter_account::counter_contract::count_map
miden:bank-account	BankAccount	balances	miden_bank_account::bank_account::balances
miden:bank-account	BankAccount	initialized	miden_bank_account::bank_account::initialized

P6: No-std Environment

Severity: Medium -- causes compilation errors

All contract code must be #![no_std]. Forgetting this or using std types causes build failures.

Required at the top of every contract file: See any contract in contracts/ for the correct pattern (#![no_std] + #![feature(alloc_error_handler)]).

For heap allocation (Vec, String, Box):

extern crate alloc;
use alloc::vec::Vec;

P7: Asset ABI Is Two Words, Not One

Severity: Medium — old asset.inner[...] code is stale

Asset is now:

pub struct Asset {
    pub key: Word,
    pub value: Word,
}

// Reading the amount from a fungible asset
let amount = asset.value[0];

// Persisting or comparing the asset class
let asset_key = asset.key;

Do not assume the old single-word asset layout. Use asset.key and asset.value, or protocol helpers, instead of reconstructing from old asset.inner[...] offsets.

P8: Recipient::compute Was Removed

Severity: Medium — causes compilation errors after upgrading

Building recipients now goes through the note binding:

extern crate alloc;
use alloc::vec;

let recipient = note::build_recipient(
    serial_num,
    script_root,
    vec![recipient_id.suffix, recipient_id.prefix],
);

P9: P2ID Note Root Hardcoding

Severity: Low-Medium — breaks after miden-standards updates

Creating P2ID output notes requires the MAST root digest of the P2ID script. This is typically hardcoded as a constant.

For any note that is being created within the compiler code, the MAST root digest is needed. Below you find the example of a P2ID note

fn p2id_note_root() -> Digest {
    Digest::from_word(
        Word::try_from([
            13362761878458161062_u64,
            15090726097241769395_u64,
            444910447169617901_u64,
            3558201871398422326_u64,
        ])
        .unwrap(),
    )
}

Risk: If miden-standards updates the P2ID script, this digest becomes invalid and withdrawals silently fail.

Mitigation: Use P2idNote::script_root() from miden-standards if available, or verify the hardcoded root matches the current version after dependency updates.

NoteType for P2ID: P2ID output notes created in contract code should use the private note type value via NoteType::from(felt!(2)) (see P10). Using the public note type triggers an opaque "missing details in advice provider" error at execution time. See miden-bank withdraw for the working pattern.

P10: NoteType Variants Unavailable in Compiler SDK

Severity: Medium -- causes compilation errors

Named enum variants (NoteType::Private, NoteType::Public, NoteType::Encrypted) don't exist in contract code. Construct via NoteType::from():

NoteType	Value
Public	NoteType::from(felt!(1))
Private	NoteType::from(felt!(2))
Encrypted	NoteType::from(felt!(3))

See miden-bank bank-account for NoteType::from(note_type) usage.

P11: Note Scripts Cannot Call Native Account Functions

Severity: High -- causes runtime failures

Note scripts cannot call native_account::add_asset() or other native_account:: functions directly. The kernel's authenticate_account_origin check rejects these calls from a note context. Instead, note scripts must call an account component method, which then calls native_account::add_asset() internally.

See miden-bank deposit-note for the correct pattern: the note script calls bank_account::deposit(), which internally calls native_account::add_asset().

P12: Note Inputs Are Immutable After Creation

Severity: Low -- causes incorrect architecture

The Felt slice that the #[note] macro deserializes into self is baked at note creation time and cannot be modified after creation. Design the typed note struct's field set and field order carefully before deployment; any later change is a breaking change for existing notes.