Implement Error for TryReserveError

[LenaWil2]

I noticed that the Error trait wasn't implemented for TryReserveError. (#48043)

Not sure if the error messages and code style are 100% correct, it's my first time contributing to the Rust std.

[LenaWil2]

Maybe it's possible to return the LayoutErr as cause and source of the TryReserveError when passed here, although it would make the struct bigger.

[LenaWil2]

I do dislike that this isn't formatted consistently, but rustfmt formatted it this way.

[sfackler]

The cleaner approach would be to move the match out of the write_str call and just assign the string to a variable IMO.

[LenaWil2]

It doesn't help with the formatting though, rustfmt transforms this:

#[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]implDisplayforTryReserveError{fnfmt(&self,fmt:&mut core::fmt::Formatter<'_>,) -> core::result::Result<(), core::fmt::Error>{
fmt.write_str("memory allocation failed")?;let reason = match&self{TryReserveError::CapacityOverflow => " because the computed capacity exceeded the collection's maximum",TryReserveError::AllocError{ .. } => {" because the memory allocator returned a error"}};
fmt.write_str(reason)}}

Into this:

#[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]implDisplayforTryReserveError{fnfmt(&self,fmt:&mut core::fmt::Formatter<'_>,) -> core::result::Result<(), core::fmt::Error>{
fmt.write_str("memory allocation failed")?;let reason = match&self{TryReserveError::CapacityOverflow => {" because the computed capacity exceeded the collection's maximum"}TryReserveError::AllocError{ .. } => " because the memory allocator returned a error",};
fmt.write_str(reason)}}

[LenaWil2]

I tried to submit code in which both statements used braces, but that makes one of the checks fail.

[LenaWil2]

I think it's fine now, btw, it's a really minor issue.

[rust-highfive]

The job x86_64-gnu-llvm-7 of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

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2020-03-11T16:32:46.0800403Z ========================== Starting Command Output ===========================
2020-03-11T16:32:46.0804545Z [command]/bin/bash --noprofile --norc /home/vsts/work/_temp/b8cb0517-4ea3-42c2-b481-e19e00785563.sh
2020-03-11T16:32:46.0805076Z 2020-03-11T16:32:46.0808922Z ##[section]Finishing: Disable git automatic line ending conversion
2020-03-11T16:32:46.0827722Z ##[section]Starting: Checkout rust-lang/rust@refs/pull/69792/merge to s
2020-03-11T16:32:46.0831004Z Task : Get sources
2020-03-11T16:32:46.0831248Z Description : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
2020-03-11T16:32:46.0831530Z Version : 1.0.0
2020-03-11T16:32:46.0831851Z Author : Microsoft
---
2020-03-11T16:32:47.3241867Z ##[command]git remote add origin https://github.com/rust-lang/rust
2020-03-11T16:32:47.3248187Z ##[command]git config gc.auto 0
2020-03-11T16:32:47.3251124Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2020-03-11T16:32:47.3254999Z ##[command]git config --get-all http.proxy
2020-03-11T16:32:47.3262404Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/69792/merge:refs/remotes/pull/69792/merge

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[sfackler]

@bors r+ rollup

[bors]

📌 Commit 2c90a37 has been approved by sfackler