Type Propagation in Generics

[IGI-111]

CS-FSSA-006

When the type inference meets a generic type with a placeholder it creates an unknown generic type.
Normally, when later more infomration is found the type should be updated. However, this is not the case
for the following example:

script;
trait Wrap {
 fn wrap(self) -> Vec<Self>;
}
impl<T> Wrap for Vec<T> {
 fn wrap(self) -> Vec<Self> {
 let mut v = Vec::new();
 v.push(self);
 v
 }
}
fn foo() {
 let mut a: Vec<_> = Vec::new();
 let b: Vec<Vec<_>> = a.wrap();
 let c: Vec<Vec<Vec<u8>>> = b.wrap();
 a.push("str");
}
fn main() {}

Here the types, and more specifically the unique type ids assigned to values a, b and c respectively
aren't properly updated due to the way unification is implemented. As a result, the compiler is not able to
deduce that a cannot be a vector of strings if c is a Vec<Vec<Vec>>.

[tritao]

Seems like this is trait-specific, the following errors out as expected:

library;

struct Vec<T> {
}

impl<T> Vec<T> {
  fn new() -> Self { return Self {}}
  fn push(self, elem: T) {}
  fn wrap(self) -> Vec<Self> {
   let mut v = Vec::new();
   v.push(self);
   v
  }
}

fn main() {
 let mut a: Vec<_> = Vec::new();
 let b: Vec<Vec<_>> = a.wrap();
 let c: Vec<Vec<Vec<u8>>> = b.wrap();
 a.push("str");
}

error
  --> main.sw:20:9
   |
18 | 
19 |  let c: Vec<Vec<Vec<u8>>> = b.wrap();
20 |  a.push("str");
   |         ^^^^^ This parameter was declared as type u8, but argument of type str was provided.
21 | }
   |
____