regression: type annotations needed for Box<_>

[Mark-Simulacrum]

• https://crater-reports.s3.amazonaws.com/beta-1.80-4/index.html
  • See "build failed (unknown)", roughly ~5400 regressions
• https://crater-reports.s3.amazonaws.com/beta-1.80-4/beta-2024-06-22/gh/Abdelrahman7373.My-First-Rust-Website/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.80-4/beta-2024-06-22/gh/AbdesamedBendjeddou.cch2023/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.80-4/beta-2024-06-22/gh/AbhishekSatpathy4848.URL-Shortener/log.txt

Mostly reverse-dependencies of the time crate (before time-rs/time@5b0c627, released in 0.3.35). My sense is there's probably nothing to do here, but lots of crates have older version of time in Cargo.lock. Nominating for libs-api to approve the wide spectrum of regressions. (Caused by #99969).

[INFO] [stdout] error[E0282]: type annotations needed for `Box<_>`
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-6f17d22bba15001f/time-0.3.20/src/format_description/parse/mod.rs:83:9
[INFO] [stdout]    |
[INFO] [stdout] 83 |     let items = format_items
[INFO] [stdout]    |         ^^^^^
[INFO] [stdout] ...
[INFO] [stdout] 86 |     Ok(items.into())
[INFO] [stdout]    |              ---- type must be known at this point
[INFO] [stdout]    |
[INFO] [stdout] help: consider giving `items` an explicit type, where the placeholders `_` are specified
[INFO] [stdout]    |
[INFO] [stdout] 83 |     let items: Box<_> = format_items
[INFO] [stdout]    |              ++++++++
[INFO] [stdout] 
[INFO] [stdout] 

[dtolnay]

We discussed this regression in today's @rust-lang/libs-api team meeting, and agree there's nothing to change on Rust's end. Those repos that have an old version of time in a lockfile will need to update that.

I would love if someone were interested in cataloguing all the "type annotations needed" type inference breakages resulting from the addition of a trait impl through the history of the standard library, and think about building static analysis that could forewarn about code that is at risk of becoming ambiguous (e.g. a += b.as_ref()). It may be something that widely used crates want to enforce in CI.

[cyqsimon]

Backward compatibility is difficult =(

[ayushgupta0610]

So what exactly is the solution to resolve this?

[BurntSushi]

@ayushgupta0610 Answer is here: #127343 (comment)