Better limit estimation in 2.x

[ZoeyR]

Limit estimation for enums in particular can grossly underestimate the actual allocated size.

in

enum Foo {
    Bar(u8),
    Baz(SomeLargeType),
}

bincode could end up thinking only 5 bytes were claimed (4 for the discriminant, 1 for the u8), but enums take up the size of their largest variant.

Possibilites:

• Overclaim enums for their full size
  • downside of potentially returning limit errors before actual allocated memory reaches the limit
• Only claim for allocated types
  • stack allocated types will always blow the stack if they are too large regardless of input. Its heap allocation that can cause issues.

[Evrey]

This is also important to state in the spec: #517

As it is now, the spec implies only allocating just enough bytes for the currently set variant.

[stale]

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