Implementing Inline Expansion for Function Calls in Rust

This challenge focuses on a fundamental aspect of compilers and code generation: inline expansion. Inline expansion is an optimization technique where the body of a function is inserted directly at the call site, rather than executing the function through a traditional call stack mechanism. This can lead to performance improvements by reducing function call overhead and enabling further optimizations on the expanded code.

Problem Description

Your task is to implement a simplified version of inline expansion for function calls within a hypothetical Rust program representation. You will be given a representation of a program, including function definitions and function calls. Your goal is to process this program and replace each function call with the actual code of the function being called.

What needs to be achieved: Create a process that takes a program structure and performs inline expansion for all direct function calls.

Key requirements:

1. Identify Function Calls: The implementation must correctly identify function call sites.
2. Locate Function Definitions: For each call, it must find the corresponding function definition.
3. Replace Call with Body: The code body of the called function should be inserted at the call site.
4. Handle Arguments: Function arguments passed during the call must be correctly substituted for the function's parameters within the expanded body.
5. Recursive Expansion: If an inlined function itself contains further function calls, these should also be recursively inlined.

Expected behavior: After processing, the program should be transformed such that no explicit function call instructions remain for the functions that were inlined. The logic of the called functions should be directly embedded where they were invoked.

Important edge cases to consider:

• Functions with no arguments: Simple cases where no argument substitution is needed.
• Functions with multiple arguments: Ensure correct mapping of arguments to parameters.
• Nested function calls: The expansion process must handle calls within calls.
• Functions called multiple times: Each call site should be expanded independently.
• Functions not defined: Although not explicitly required by the prompt, a robust solution might consider how to handle calls to undefined functions (e.g., error or skip). For this challenge, assume all called functions are defined.

Examples

Example 1:

Input Program Structure:

// Function definition
Function {
    name: "add",
    parameters: ["a", "b"],
    body: Block {
        statements: [
            Return {
                expression: BinaryOperation {
                    left: Variable { name: "a" },
                    operator: "+",
                    right: Variable { name: "b" }
                }
            }
        ]
    }
}

// Main execution block
Block {
    statements: [
        VariableDeclaration { name: "x", value: Number { value: 5 } },
        VariableDeclaration { name: "y", value: Number { value: 10 } },
        Assignment {
            variable: "result",
            expression: FunctionCall {
                function_name: "add",
                arguments: [
                    Variable { name: "x" },
                    Variable { name: "y" }
                ]
            }
        }
    ]
}

Output Program Structure (after inline expansion):

// Main execution block (modified)
Block {
    statements: [
        VariableDeclaration { name: "x", value: Number { value: 5 } },
        VariableDeclaration { name: "y", value: Number { value: 10 } },
        // Inline expansion of add(x, y)
        // Original body: return a + b;
        // With substitution: return x + y;
        Assignment {
            variable: "result",
            expression: BinaryOperation {
                left: Variable { name: "x" },
                operator: "+",
                right: Variable { name: "y" }
            }
        }
    ]
}

Explanation: The add function was called with x and y. The body of add (a + b) was inserted. The parameter a was replaced by the first argument x, and parameter b was replaced by the second argument y. The return statement is implicitly handled by the assignment in this simplified model.

Example 2:

Input Program Structure:

Function {
    name: "multiply_and_add",
    parameters: ["x", "y", "z"],
    body: Block {
        statements: [
            VariableDeclaration { name: "temp_mult", value: BinaryOperation {
                left: Variable { name: "x" },
                operator: "*",
                right: Variable { name: "y" }
            }},
            Return {
                expression: BinaryOperation {
                    left: Variable { name: "temp_mult" },
                    operator: "+",
                    right: Variable { name: "z" }
                }
            }
        ]
    }
}

Block {
    statements: [
        VariableDeclaration { name: "val1", value: Number { value: 2 } },
        VariableDeclaration { name: "val2", value: Number { value: 3 } },
        VariableDeclaration { name: "val3", value: Number { value: 4 } },
        Assignment {
            variable: "final_result",
            expression: FunctionCall {
                function_name: "multiply_and_add",
                arguments: [
                    Variable { name: "val1" },
                    Variable { name: "val2" },
                    Variable { name: "val3" }
                ]
            }
        }
    ]
}

Output Program Structure (after inline expansion):

Block {
    statements: [
        VariableDeclaration { name: "val1", value: Number { value: 2 } },
        VariableDeclaration { name: "val2", value: Number { value: 3 } },
        VariableDeclaration { name: "val3", value: Number { value: 4 } },
        // Inline expansion of multiply_and_add(val1, val2, val3)
        // Original body:
        //   let temp_mult = x * y;
        //   return temp_mult + z;
        // With substitution:
        //   let temp_mult = val1 * val2;
        //   return temp_mult + val3;
        Assignment {
            variable: "final_result",
            expression: BinaryOperation {
                left: Variable { name: "temp_mult" }, // Note: temp_mult is a local variable within the inlined code
                operator: "+",
                right: Variable { name: "val3" }
            }
        }
        // The VariableDeclaration for temp_mult would also be part of the inlined block.
        // A more complete representation might show the temp_mult declaration directly before its use.
    ]
}

Explanation: The multiply_and_add function with three arguments was inlined. The arguments val1, val2, and val3 were substituted for x, y, and z respectively. Local variables within the inlined function (like temp_mult) are preserved.

Example 3: (Recursive Expansion)

Input Program Structure:

Function {
    name: "subtract",
    parameters: ["a", "b"],
    body: Block {
        statements: [
            Return {
                expression: BinaryOperation {
                    left: Variable { name: "a" },
                    operator: "-",
                    right: Variable { name: "b" }
                }
            }
        ]
    }
}

Function {
    name: "calculate",
    parameters: ["p", "q"],
    body: Block {
        statements: [
            VariableDeclaration { name: "intermediate", value: FunctionCall {
                function_name: "subtract",
                arguments: [
                    Variable { name: "p" },
                    Number { value: 1 }
                ]
            }},
            Return {
                expression: Variable { name: "intermediate" }
            }
        ]
    }
}

Block {
    statements: [
        Assignment {
            variable: "final_val",
            expression: FunctionCall {
                function_name: "calculate",
                arguments: [
                    Number { value: 10 },
                    Number { value: 5 }
                ]
            }
        }
    ]
}

Output Program Structure (after full inline expansion):

Block {
    statements: [
        Assignment {
            variable: "final_val",
            expression: BinaryOperation { // Inlined subtract(p - 1) inside calculate(10, 5)
                left: BinaryOperation { // Inlined subtract(10 - 1)
                    left: Number { value: 10 },
                    operator: "-",
                    right: Number { value: 1 }
                },
                operator: "-", // This operator would not exist in the original calculate body, indicating that the expression returned by calculate has been replaced. This is a simplification in representation.
                right: Number { value: 5 } // This represents the 'q' argument of calculate.
            }
        }
    ]
}

Explanation: First, calculate(10, 5) is inlined. Its body contains a call to subtract(p, 1). This inner call to subtract is then recursively inlined with p being 10 and b being 1. The result is the direct embedding of the subtraction logic where the original calculate call was. The representation shows how the nested structure simplifies. A more literal expansion would involve the intermediate variable declaration.

Constraints

• The program representation is a simplified Abstract Syntax Tree (AST) that you will define or be provided with. You can assume a well-formed AST.
• Function names are unique.
• Parameter names within a function are unique.
• Variables are always declared before use (standard Rust scoping rules).
• No complex control flow (loops, if/else) within function bodies for this challenge, only sequential statements and returns.
• Assume no name collisions between function parameters and local variables in the caller's scope.
• The expansion should be a transformation that produces a new AST, not an in-place modification of the original.

Notes

• You'll need to define the data structures (structs and enums) that represent your Rust AST.
• Think about how to handle variable scoping when expanding. When a function is inlined, its parameters become local to the expanded code. If the function body uses local variables, they are also part of the inlined code.
• The process of substitution for arguments will be crucial. You might want to build a map of parameter names to argument expressions.
• Consider using recursion to handle nested calls. A function that performs inline expansion could call itself on the expressions within a FunctionCall node.