Why doesn't this code compile Rust Playground

struct Foo(usize);

impl From<&str> for Foo {
  fn from(value: &str) -> Self {
    Self(value.len())
  }
}

fn main() {
  let a: &str = "asdf";
  let b: &String = &String::from("asdf");

  let a_len = Foo::from(a);
  let b_len = Foo::from(b);  // Error
}

with

help: the trait `From<&String>` is not implemented for `Foo`
      but trait `From<&str>` is implemented for it

If I understand correctly from the reference on type coercions, a function call is a "coercion site" and the transformation "&T → &U where T: Deref<Target=U>" is a "coercion type". I assume this is special because trait methods don't count as "coercion sites"? Writing let b_len: Foo = b.into() also does not work, even though (I would assume) that counts as a method call expression, where auto-deref is also performed?

I would love to understand this more.

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