Rollup of 6 pull requests

[frewsxcv]

• Successful merges: reduce stack requirements for floating-point formatting #41509, syntax: Parse trait object types starting with a lifetime bound #41542, Disable ref hint for pattern in let and adding ui tests #40402 #41564, rustbuild improvements #41577, use diff crate for compile-fail test diagnostics #41474 #41588, Add a distcheck for rust-src completeness #41608
• Failed merges:

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @sfackler (or someone else) soon.

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[frewsxcv]

@bors r+ p=10

[bors]

📌 Commit 59d60d2 has been approved by frewsxcv

[bors]

⌛ Testing commit 59d60d2 with merge 94b4599...

[bors]

💔 Test failed - status-travis