Or in more complex scenarios:

Common symptoms that precede this error:

- Function pointers with lifetime parameters being assigned to types without them
- Generic functions accepting `fn()` when the caller provides `fn<'a>()`
- Attempting to coerce higher-ranked trait bounds (HRTBs) incorrectly
- Type mismatch between function signatures with and without lifetime annotations

## 2. Root Cause

Rust's coercion system allows certain implicit conversions, but lifetime erasure is generally **not** one of them. When you have a function pointer like `fn(&'a i32)`, you cannot implicitly coerce it to `fn(i32)` because the lifetime carries semantic meaning that would be lost in the erasure.

The "erase" in error message refers to the act of removing type parameters or lifetime annotations. Rust only permits "合法的" (legal) coercions:

| From | To | Legal? |
|------|----|--------|
| `&T` | `&T` where `T: Trait` | ✅ Via unsizing |
| `fn(&'a T)` | `fn(&'a T)` (same signature) | ✅ Identity coercion |
| `fn(&'a T)` | `fn(T)` | ❌ **Illegal erase** |
| `&'a T` | `&'static T` | ✅ Only if `'a: 'static` |
| `&'a mut T` | `&'a T` | ✅ Unsafe coercion |

**E0067** specifically occurs when:

1. **Attempting to remove lifetime parameters from function types** - Going from `fn(&'a i32)` to `fn(i32)` erases the lifetime parameter.
2. **Type parameter erasure in generic contexts** - Trying to coerce a generic type to a concrete type that loses type information.
3. **Improper HRTB handling** - Higher-ranked trait bounds have specific rules that, when violated, can trigger this error.

```rust
// Root cause example 1: Lifetime erasure on function pointers
fn takes_fn_with_lifetime<'a>(f: fn(&'a i32) -> &'a i32) {
    // We cannot do this - it's an illegal erase coercion:
    let g: fn(i32) -> i32 = f; // E0067
}

// Root cause example 2: Trait object coercion issues
trait MyTrait {
    fn call(&self);
}

fn needs_static_trait_object(_: &dyn MyTrait) {}

fn works_with_borrowed<'a>(t: &'a dyn MyTrait) {
    // This might trigger E0067 if lifetime matters
    needs_static_trait_object(t); // Potential E0067 if lifetime constraints violated
}

// Root cause example 3: Generic parameter to concrete type
fn coerce_generic<'a, T: Fn()>(f: T) {
    let _fn: fn() = f; // E0067 if T carries lifetime information
}

fn process<'a>(callback: fn(&'a i32) -> &'a i32) {
    // This creates a conflict when trying to use a static function
    let static_fn: fn(i32) -> i32 = callback; // E0067
}

fn main() {
    fn static_callback(x: &i32) -> &i32 { x }
    process(static_callback);
}

fn process<'a>(callback: fn(&'a i32) -> &'a i32) {
    // Keep the function type with lifetime, or change the signature
    let _ = callback;
}

fn main() {
    fn static_callback<'a>(x: &'a i32) -> &'a i32 { x }
    process(static_callback);
}

fn register_callback<F>(f: F)
where
    F: Fn(),
{
    // Cannot coerce F to fn()
}

fn main() {
    register_callback(|| println!("hello"));
}

fn register_callback(f: fn()) {
    f();
}

fn main() {
    fn closure_wrapper() {
        println!("hello");
    }
    register_callback(closure_wrapper);
}

fn call_fn_ptr(f: fn(i32), x: i32) -> i32 {
    f(x)
}

fn main() {
    let captured = 5;
    let f = || captured; // Closure type: impl Fn() -> i32
    
    // Cannot coerce closure to fn()
    call_fn_ptr(f, 10); // E0067
}

fn call_fn_ptr<F>(f: F, x: i32) -> i32
where
    F: Fn() -> i32,
{
    f()
}

fn main() {
    let captured = 5;
    let f = || captured;
    
    // Use generic or convert closure to proper form
    call_fn_ptr(f, 10); // Works with generic
}

fn accept_fn_ptr(f: fn(&'static str)) {
    f("hello");
}

fn lifetime_fn(s: &str) {
    println!("{}", s);
}

fn main() {
    accept_fn_ptr(lifetime_fn); // E0067 if lifetimes don't match exactly
}

fn accept_fn_ptr(f: fn(&'static str)) {
    f("hello");
}

fn main() {
    // Create a function that accepts 'static
    fn static_wrapper(s: &str) {
        println!("{}", s);
    }
    accept_fn_ptr(static_wrapper); // Works
}

trait Processor {
    fn process(&self, input: &str) -> String;
}

fn process_all<'a, P: Processor>(items: &[&str], processor: P)
where
    P: 'a,
{
    for item in items {
        // Problem: processor might not live 'a
        let result = processor.process(item);
        println!("{}", result);
    }
}

trait Processor {
    fn process(&self, input: &str) -> String;
}

fn process_all<P: Processor + Clone>(items: &[&str], processor: P) {
    // Clone processor for each iteration to avoid lifetime issues
    for item in items {
        let result = processor.clone().process(item);
        println!("{}", result);
    }
}

cargo build 2>&1 | grep -E "(error|warning: E0067)"

$ cargo build
   Compiling my_crate v0.1.0 (/path/to/my_crate)
    Finished dev [unoptimized + debuginfo] target(s) in 0.32s

cargo test 2>&1

// Verify type compatibility
fn main() {
    // This should compile without E0067
    type FnWithLifetime = fn(x: &i32) -> &i32;
    
    fn my_fn<'a>(x: &'a i32) -> &'a i32 { x }
    
    let _: FnWithLifetime = my_fn; // Should work if lifetimes compatible
    
    println!("Type coercion successful");
}

// This will NOT work
let closure = || println!("hello");
let fn_ptr: fn() = closure; // E0067 or E0308

// Be explicit about lifetimes
fn with_static_param(f: fn(val: &'static str)) {
    f("static string");
}

fn main() {
    fn normal_fn(s: &str) { // This is NOT fn(&'static str)
        println!("{}", s);
    }
    with_static_param(normal_fn); // May cause issues
}

trait Printable {
    fn print(&self);
}

fn print_any(p: &dyn Printable) {
    p.print();
}

fn print_static(p: &'static dyn Printable) {
    p.print();
}

fn main() {
    let value = String::from("hello");
    let printable = PrintableStruct { data: value };
    
    // Lifetime conflict - printable is not 'static
    print_static(&printable); // Potential E0067
}

type Callback = fn(x: &i32);

fn register(cb: Callback) {}

fn main() {
    fn with_lifetime<'a>(x: &'a i32) -> &'a i32 { x }
    
    // Error: lifetime mismatch
    register(with_lifetime); // E0067
}

// This compiles fine
fn good<T: Clone>(t: T) -> T {
    t.clone()
}

// This might cause issues with function pointers
fn problematic<F: Fn()>(f: F) -> F {
    f
}

error[E0308]: mismatched types
  --> src/main.rs:5:20
   |
5  |     let f: fn(i32) = |x| x; // E0308 as well as potentially E0067
   |                 --- ^ expected fn pointer, found closure

// Comprehensive example combining related concepts
fn main() {
    // E0067 prevention pattern
    fn create_processor() -> impl Fn(i32) -> i32 {
        move |x| x * 2
    }
    
    let processor = create_processor();
    let result = processor(5);
    println!("Result: {}", result);
}