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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Pattern syntax for Vec in respect to assert_matches!()

I create the following custom container type S and some inner type T.

struct T(u32);
struct S(Vec<T>);

impl std::str::FromStr for S {...}

I can't just

type S = Vec<T>;

because impl FromStr for S errors:
error[E0117]: only traits defined in the current crate can be implemented for types defined outside of the crate

This is fine.

Now I'm writing some tests to validate from_str() behaviour.
Using Tracking Issue for assert_matches · Issue #82775 · rust-lang/rust · GitHub to get nice error messages on test failures I write the following

    let s = "1 2 3";
    let parsed = s.parse::<S>();
    assert_matches!(parsed, Ok(S([T(1), T(2), T(3)])));

Aside from whether it makes sense; it's intuitive for me to write it like this.
Unfortunately this doesn't work

error[E0529]: expected an array or slice, found `Vec<T>`
  --> src/main.rs:20:34
   |
20 |     assert_matches!(parsed, Ok(S([T(1), T(2), T(3)])));
   |                                  ^^^^^^^^^^^^^^^^^^ pattern cannot match with input type `Vec<T>`
   |
help: consider using `as_deref` here
   |
20 |     assert_matches!(parsed.as_deref(), Ok(S([T(1), T(2), T(3)])));
   |                           +++++++++++

For more information about this error, try `rustc --explain E0529`.

When I used the compiler hint above, it didn't resolve the issue.
I can however do this:

    assert_matches!(parsed, Ok(S(..)));
    let s_inner = parsed.unwrap().0;
    assert_matches!(s_inner.as_slice(), [T(1), T(2), T(3)]);

So my questions here are:

• How should I've written the code to be idiomatic in the scenario above?
• How should the compiler hint above be changed?

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