Consider setting `hints.mostly-unused` in `Cargo.toml`

[FalkWoldmann]

Describe the feature

hints.mostly-unused is a new feature in nightly Rust that allows library crates to declare that most of their API might be unused. As discussed in #113, this is definitely the case for many AWS SDKs. This information allows Rust to optimize compilation time.

The Rust blog by @joshtriplett, introducing the feature, even mentions aws-sdk-ec2 as a good example where this feature halved compilation time. [1]

Adding the following configuration snippet:

[hints]
mostly-unused = true

to the Cargo.toml files enables this optimization for SDK crates, provided the user is using a nightly Rust toolchain. For users not on a nightly toolchain, this attribute is a no-op, making it safe to include.

[1] - https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/

Use Case

Compilation time (#113) is the most up-voted issue.

Proposed Solution

No response

Other Information

No response

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

A note for the community

Community Note

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[aajtodd]

There is already a draft PR for this (smithy-lang/smithy-rs#4208). We will most likely enable it for a few services to start with and gather feedback.

[landonxjames]

A change adding hints.mostly-unused for a small group of crates was released today. You can see more details and the full set of crates in this comment #113 (comment)

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