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BuildSmart Contracts (Move)Linter

Aptos Move Lint

The “Aptos Move Lint” tool runs on a Move package to find and warn about common issues in Move programs, helping improve your Move code.

You can run it with the aptos CLI command: aptos move lint.

This tool is currently in beta, so please try it out and submit your feedback, including ideas and requests for new lint rules.

Lint Checks

avoid_copy_on_identity_comparison

Checks for identity comparisons (== or !=) between copied values of type vector or struct (i.e., types for which copy can be expensive). It instead suggests to use reference-based identity comparison instead (i.e., use &x == &y instead of x == y, when the above mentioned conditions meet).

This recommendation is also given in the Move book. Due to automatic copy inference, it may not be obvious when a copy is being made while using == or != on values with types that have the copy ability. This lint identifies cases where extra copies on vectors or structs could be skipped by using reference-based identity comparisons.

blocks_in_conditions

Checks for use of blocks in conditions (e.g., in if, match, and while conditions), which can make code hard to read. An example coding pattern caught by this lint is:

if ({let x = foo(); !x}) { // uses a block in condition
  bar();
}

Such code can usually be rewritten to hoist the block out and above the condition, usually making it more readable.

It is a common Move pattern to provide inline specifications in conditions, especially loop invariants, which requires creating blocks in conditions. We exclude this pattern in the lint check to continue to allow for this specification pattern.

Note that an assert! is translated to a conditional abort, so blocks in assert! condition also are reported by this lint.

needless_bool

Checks for patterns of the form (where x is any arbitrary boolean expression):

  • if (x) true else false, which can be replaced with just x.
  • if (x) false else true, which can be replaced with just !x.
  • if (x) { return true } else { return false }, which can be replaced with just return x.
  • if (x) { return false } else { return true }, which can be replaced with just return !x.
  • if (x) true else true or if (x) false else false, which should be rewritten to remove the redundant branch.

needless_deref_ref

Checks for patterns where references taken are immediately dereferenced, and suggests removing the pair of dereference-reference operators:

  • *&x.f can be simplified to x.f
  • *&mut x.f can be simplified to x.f
  • *&mut x.f = 5; can be simplified to x.f = 5;

needless_mutable_reference

Checks for mutable references or borrows (currently: mutable reference parameters, mutable borrow of locals, borrow_global_mut) that are not used mutably, and suggests to use the immutable reference or borrow instead.

For example, in the function foo below, &mut can be replaced by & because the reference is not mutably used.

fun foo(x: u64): u64 {
  let y = &mut x;
  *y
}

needless_ref_deref

Checks for patterns where immutable reference are taken for a dereference, and suggests removing the pair of reference-dereference operators: &*x can be simplified to x.

needless_ref_in_field_access

Checks for patterns where there are needless references taken when accessing a field of a struct or an enum, and suggests removing the explicit reference taken:

  • (&s).f can be simplified to s.f
  • (&mut s).f = 42; can be simplified to s.f = 42;

simpler_numeric_expression

Checks for various patterns where a simpler numeric expression can be used instead. In all these cases, the code must already type check, and x can be any numeric expression.

  • x & 0, x * 0, 0 & x, 0 * x, 0 << x, 0 >> x, x % 1 can all be replaced with just 0.
  • x | 0, x ^ 0, x >> 0, x << 0, x + 0, x - 0, x / 1, x * 1, 0 | x, 0 ^ x, 0 + x, 1 * x can all be replaced with just x.

unnecessary_boolean_identity_comparison

Checks for boolean identity comparisons of the form:

  • x == true, true == x, which can be replaced with just x.
  • x == false, false == x, which can be replaced with just !x.

In all these cases, x can be any arbitrary boolean expression.

unnecessary_numerical_extreme_comparison

Checks if there are any numerical comparisons with extreme values (i.e., min and max value representable by that numeric type) that are unnecessary or can be made more precise and clear. Depending on the comparison, various recommendations are made.

Consider the following example expressions that are caught by the lint, and the corresponding recommendations made (in all these cases, x is a place holder for a numerical expression of type u8, u16, u32, u64, u128, or u256, and MAX is a place holder for the max value representable for that numeric type):

  • x < 0, 0 > x, x > MAX, MAX < x, are always false, rewrite code to remove this comparison
  • x >= 0, 0 <= x, x <= MAX, MAX >= x, are always true, rewrite code to remove this comparison
  • x <= 0, 0 >= x, x >= MAX, MAX <= x, can all be simplified to use == instead
  • x > 0, 0 < x, x < MAX, MAX > x, can all be clarified to use != instead

while_true

Checks for while (true) { .... } patterns and suggests using the more explicit loop { .... } construct instead.

Suppressing Lint Warnings

To suppress one or more lint checks named check1, check2, … (and so on), you can add the attribute #[lint::skip(check1, check2, ...)] to a function or a module. The linter will then not perform the checks named check1, check2, … (and so on) for that function or module.

For example, the function below would usually get a warning from the linter about a needless_bool, but due to the attribute on the function, the linter does not emit a warning.

#[lint::skip(needless_bool)]
fun violation(): bool {
    if (foo()) true else false
}
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