Memory Safety Check Skill

Verify Mojo code follows memory safety rules.

When to Use

• Reviewing code for memory safety
• Debugging memory issues
• Before merging code
• Performance testing reveals issues

Memory Ownership

Owned Parameters

fn consume(owned data: Tensor):
    # Takes ownership, original is invalidated
    process(data)
    # data is destroyed here

Borrowed Parameters

fn read_only(borrowed data: Tensor):
    # Read-only access, no ownership transfer
    let value = data[0]
    # data still valid in caller

Inout Parameters

fn modify(inout data: Tensor):
    # Mutable reference
    data[0] = 42
    # Changes visible in caller

Common Issues

1. Use After Move

# ❌ Wrong
fn bad():
    let data = Tensor()
    consume(data^)  # Move ownership
    print(data[0])  # Error: data moved!

# ✅ Correct
fn good():
    let data = Tensor()
    let copy = data  # Copy if needed
    consume(data^)
    print(copy[0])  # OK

2. Lifetime Issues

# ❌ Wrong
fn return_dangling() -> borrowed Tensor:
    let local = Tensor()
    return local  # Returns reference to local!

# ✅ Correct
fn return_owned() -> Tensor:
    let local = Tensor()
    return local^  # Move ownership

3. Mutable Aliasing

# ❌ Wrong
fn alias_problem(inout a: Tensor, borrowed b: Tensor):
    a[0] = b[0]  # If a and b alias, undefined!

# ✅ Correct
fn no_alias(inout a: Tensor, borrowed b: Tensor):
    # Ensure a and b don't alias
    if a.ptr() != b.ptr():
        a[0] = b[0]

Checklist

• No use-after-move
• No dangling references
• No mutable aliasing
• Proper lifetime management
• Ownership clear and documented

See Mojo ownership documentation for details.