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")]
impl Display for TryReserveError {
    fn fmt(
        &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")]
impl Display for TryReserveError {
    fn fmt(
        &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.

Click to expand the log.

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