Remove i586-pc-windows-msvc

[beetrees]

Proposal

i586-pc-windows-msvc is the non-SSE version of the i686-pc-windows-msvc target, added back in 2016 in rust-lang/rust#32034 when Rust supported Windows XP. Windows XP/Vista support was subsequently dropped in #378, and Windows 7/8 support for then-existing targets was dropped in #651, which raised "the minimum supported Windows version of current targets to Windows 10". i586-pc-windows-msvc is currently listed as a tier 2 target without std support.

Windows hasn't supported pre-SSE2 CPUs since Windows 8, with Windows 7 apparently dropping support for pre-SSE2 CPUs in a March 2018 update. This means that for Windows 10+ the i586-pc-windows-msvc target (when compared to the i686-pc-windows-msvc target) is a footgun: it provides worse performance with more miscompilations (see rust-lang/rust#114479), without providing any greater CPU support, as the underlying OS requires SSE2 anyway.

Given the lack of purpose for this target on Windows 10+ as detailed above the target also fails the tier 2 target policy requirement that a target "must document [...] why the specific difference in baseline expectations provides sufficient value to justify a separate target". There aren't any target maintains listed for this target, and none of the existing Windows 7 maintainers want to maintain a hypothetical i586-win7-windows-msvc target. Therefore, I propose removing the i586-pc-windows-msvc target.

(Previous discussion is available at rust-lang/rust#137244)

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Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
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• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

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@rfcbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rfcbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler

[Noratrieb]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted