Rust Escape Analysis: Optimizing Memory Allocation

Escape analysis is a compiler optimization technique that determines whether a variable's lifetime is confined to the current function (doesn't "escape") or if it might be accessed after the function returns. By identifying non-escaping data, Rust can potentially allocate it on the stack instead of the heap, leading to significant performance improvements by avoiding heap allocation overhead and improving cache locality. Your challenge is to implement a simplified escape analysis pass for a subset of Rust code.

Problem Description

Your task is to create a Rust program that simulates a basic escape analysis for a simplified Rust-like language. You will be given an abstract syntax tree (AST) representing a function's body. Your analysis should determine for each local variable if it escapes the current function.

Key Requirements:

• Input: An AST representing a function's body. The AST will include variable declarations, assignments, function calls, returns, and references.
• Output: For each local variable, you need to identify whether it escapes or not.
• Escape Conditions: A variable escapes if:
  • Its address is taken and returned from the function.
  • Its address is taken and passed to another function that might retain it beyond the current function's scope (for simplicity, assume all function calls could cause an escape).
  • It's assigned to a global mutable variable.
  • It's stored in a data structure that might outlive the current function (e.g., a Vec or Box which could be returned).
• Non-Escape Conditions: A variable does not escape if:
  • It's only used within the function and dropped before the function returns.
  • It's assigned to another local variable that doesn't escape.

Expected Behavior:

Your analysis should produce a mapping from variable names to an EscapeState enum, where EscapeState can be Escapes or DoesNotEscape.

Edge Cases:

• Variables declared but never used.
• References to variables that themselves escape.
• Complex nested data structures.

Examples

Example 1:

Input AST Snippet:

FunctionBody {
    declarations: [
        VarDecl { name: "x", init: Literal(5) }, // Integer literal
        VarDecl { name: "y", init: HeapAlloc(Box::new(Literal(10))) }, // Box allocation
        VarDecl { name: "z", init: Literal(true) } // Boolean literal
    ],
    statements: [
        Return(Variable("x")) // Returning x
    ]
}

Output:

{
    "x": Escapes,
    "y": Escapes, // Boxed value might escape if y is returned, but for simplicity, assume heap allocation implies potential escape
    "z": DoesNotEscape // z is declared but never used or returned
}

Explanation: x is returned, so it escapes. y is allocated on the heap using Box, which is considered an escape for this simplified analysis. z is declared but never used or returned.

Example 2:

Input AST Snippet:

FunctionBody {
    declarations: [
        VarDecl { name: "local_val", init: Literal(42) }
    ],
    statements: [
        // No return statement, no function calls that could retain it
    ]
}

Output:

{
    "local_val": DoesNotEscape
}

Explanation: local_val is a simple integer literal and is not returned or passed to any function that might retain it. It will be dropped at the end of the function.

Example 3: (Reference and function call)

Input AST Snippet:

FunctionBody {
    declarations: [
        VarDecl { name: "data", init: Literal(String::from("hello")) },
        VarDecl { name: "ptr_to_data", init: AddressOf(Variable("data")) } // Taking address of data
    ],
    statements: [
        Call(SomeFunction, vec![Variable("ptr_to_data")]) // Passing reference to data to SomeFunction
    ]
}

Output:

{
    "data": Escapes,
    "ptr_to_data": Escapes
}

Explanation: Taking the address of data makes data escape. Passing ptr_to_data to SomeFunction is treated as an escape because SomeFunction could store the reference beyond the current function's scope.

Constraints

• The AST will represent a single function body.
• Variable names are alphanumeric strings.
• The AST will be a simplified representation, omitting complex control flow like loops and complex match statements for the purpose of this challenge.
• Assume that any function call could cause an escape for any argument passed by reference.
• Assume that creating a Box or Vec implies potential escape for the data they contain.
• Performance is not a primary concern; correctness of the escape analysis logic is paramount.

Notes

• You'll need to define your AST structures and the EscapeState enum.
• A common approach is a worklist algorithm or a recursive traversal of the AST.
• Think about how to propagate escape information. If a variable A contains a reference to B, and A escapes, then B also escapes.
• For this simplified challenge, you can make strong assumptions about function calls and heap allocations. In a real compiler, this is much more nuanced.
• Focus on identifying why a variable escapes.