I don't know what quotemstr was specifically talking about, but here's my own take.

The ideal error handling is inferred algebraic effects like in Koka[1]. This allows you to add a call to an error-throwing function 15 layers down the stack and it's automatically propagated into the type signatures of all functions up the stack (and you can see the inferred effects with a language server or other tooling, similar to Rust's inferred types).

Consider the following Rust functions:

    fn f1() -> Result<(), E1> {...}
    fn f2() -> Result<(), E2> {...}
    fn f3() -> Result<(), E3> {...}
    fn f4() -> Result<(), E4> {f1()?; f2()?;}
    fn f5() -> Result<(), E5> {f1()?; f3()?;}
    fn f6() -> Result<(), E6> {f4()?; f5()?;}

Now, how do you define E4, E5 and E6? The "correct" way is to use sum types, i.e., `enum E4 {E1(E1), E2(E2)}`, `enum E5 {E1(E1), E3(E3)}` and `enum E6 {E1(E1), E2(E2), E3(E3)}` with the appropriate From traits. The problem is that this involves a ton of boilerplate even with thiserror handling some stuff like the From traits.

Since this is such a massive pain, Rust programs tend to instead either define a single error enum type that has all possible errors in the crate, or just use opaque errors like the anyhow crate. The downside is that these approaches lose type information: you no longer know that a function can't return some specific error (unless it returns no errors at all, which is rare), which is ultimately not so different from those languages where you have to guard against bizarre runtime errors.

Worse yet, if f1 has to be changed such that it returns 2 new errors, then you need to go through all error types in the call stack and flatten the new errors manually into E4, E5 and E6. If you don't flatten errors, then you end up rebuilding the call stack in error types, which is a whole different can of worms.

Algebraic effects just handle all of this more conveniently. That said, an effect system like Koka's isn't viable in a systems programming language like Rust, because optimizing user-defined effects is difficult. But you could have a special compiler-blessed effect for exceptions; algebraic checked exceptions, so to speak. Rust already does this with async.

[1] https://koka-lang.github.io