Regression in type annotations needed for check between 1.79 and 1.80

[Fishrock123]

Code

I tried this code:

[package]
name = "example"
version = "0.1.0"
edition = "2021"

[dependencies]
time = { version = "=0.3.34", features = ["formatting"] }

cargo check

Code in question for this version: https://github.com/time-rs/time/blob/f11f9b2a0c8d17badd96a8a9167d81466a3fa39b/time/src/format_description/parse/mod.rs#L66-L87

This existing source code should continue to work fine between Rust 1.79 and 1.80.

Instead, this happened:

error[E0282]: type annotations needed for `Box<_>`
  --> /Users/jsenkpiel/.cargo/registry/src/index.crates.io-6f17d22bba15001f/time-0.3.34/src/format_description/parse/mod.rs:83:9
   |
83 |     let items = format_items
   |         ^^^^^
...
86 |     Ok(items.into())
   |              ---- type must be known at this point
   |
help: consider giving `items` an explicit type, where the placeholders `_` are specified
   |
83 |     let items: Box<_> = format_items
   |              ++++++++

Version it worked on

It most recently worked on: 1.79

Version with regression

rustc --version --verbose:

rustc 1.80.0 (051478957 2024-07-21)
binary: rustc
commit-hash: 051478957371ee0084a7c0913941d2a8c4757bb9
commit-date: 2024-07-21
host: aarch64-apple-darwin
release: 1.80.0
LLVM version: 18.1.7

---

I think the issue was fixed by this change in time for 0.3.35. However, it should still not fail between regular Rustc version upgrades.

Apparent fix in time @ 0.3.35: time-rs/time@5b0c627 - time-rs/time#671

[lqd]

See #127343 and #127343 (comment)

[tgross35]

To add some context: type inference is one of the few behaviors that is not covered by Rust's stability guarantees, partially because adding new library items can break it. Obviously no regressions is always the goal, but it does mean that sometimes we accept inference regressions for things that objectively make Rust better - like here.

Being able to specify the type means that there is always a fix that works for all rust versions. Also, time has already accounted for this - just needs an update.