[derive] Document MSRV as 1.56, update MSRV policy

[joshlf]

Update our MSRV policy to promise that zerocopy-derive will always
support its MSRV, but that getting this MSRV to work in practice may
require pinning dependencies (in particular, syn) which have more lax
MSRV policies.

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• 👉 [derive] Document MSRV as 1.56, update MSRV policy #3500

Latest Update: v2 — Compare vs v1

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.85%. Comparing base (53a3fbf) to head (0aa56d3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3500   +/-   ##
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  Coverage   91.85%   91.85%           
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  Files          20       20           
  Lines        6093     6093           
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  Hits         5597     5597           
  Misses        496      496           

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[joshlf]

@jswrenn Note, quoting #3495 (comment):

I've opened #3500, which does the same as #3498 but also documents a new MSRV policy: That we will always support some set of package resolutions which satisfy the current 1.56 MSRV. Note that despite this policy, the plan is still to have a different, more relaxed MSRV policy when we release zerocopy 0.9.

@jswrenn How do you feel about this proposed policy? Merging #3500 will have the effect of closing us off from upgrading our syn dep to 3.0, which will affect our ability to support unsafe fields.