Rollup of 18 pull requests

[Manishearth]

Successful merges:

• Add Ayu theme to rustdoc #71237 (Add Ayu theme to rustdoc)
• Substantial refactor to the design of LineWriter #72808 (Substantial refactor to the design of LineWriter)
• Update RELEASES.md for 1.45.0 #73354 (Update RELEASES.md for 1.45.0)
• rustdoc: insert newlines between attributes #73852 (rustdoc: insert newlines between attributes)
• Document the union keyword #73867 (Document the union keyword)
• add (unchecked) indexing methods to raw (and NonNull) slices #73986 (add (unchecked) indexing methods to raw (and NonNull) slices)
• Fix caching issue when building tools. #74046 (Fix caching issue when building tools.)
• clean up E0718 explanation #74123 (clean up E0718 explanation)
• rustdoc: Allow linking from private items to private types #74147 (rustdoc: Allow linking from private items to private types)
• Detect tuple struct incorrectly used as struct pat #74173 (Detect tuple struct incorrectly used as struct pat)
• Remove an unwrap in layout computation #74227 (Remove an unwrap in layout computation)
• Update llvm-project to latest origin/rustc/10.0-2020-05-05 commit  #74239 (Update llvm-project to latest origin/rustc/10.0-2020-05-05 commit )
• don't mark linux kernel module targets as a unix environment #74257 (don't mark linux kernel module targets as a unix environment)
• Slight reorganization of sys/(fast_)thread_local #74263 (Slight reorganization of sys/(fast_)thread_local)
• typeck: report placeholder type error w/out span #74270 (typeck: report placeholder type error w/out span)
• pprust: support multiline comments within lines #74272 (pprust: support multiline comments within lines)
• #71669: add ui, codegen tests for volatile + nearby int intrinsics #74285 (Missing tests for intrinsics. #71669: add ui, codegen tests for volatile + nearby int intrinsics)
• Added detailed error code explanation for issue E0688 in Rust compiler. #74286 (Added detailed error code explanation for issue E0688 in Rust compiler.)

Failed merges:

r? @ghost