spec: define that structs are compared field-by-field as listed in source code

[mdempsky]

gc handles struct/array comparisons by short-circuiting if it finds any unequal
fields/elements, and this behavior is noticeable because the Go spec requires
comparisons to panic in some cases; e.g., see http://play.golang.org/p/5jqSUAT1xC

However, unlike short-circuiting for evaluating "a && b", it doesn't
seem that short-circuiting of field/element comparisons is specified by the spec. 
Arguably, the spec currently requires that instead both comparisons in the above program
should panic.

Not a major issue, but thought I'd file an issue to note it.  A couple possible ways to
address it:

1. Ignore it since it probably doesn't matter in practice.
2. Specify gc's behavior since it's intuitive and easy to explain.
3. Specify a set of allowable behaviors (e.g., allow short-circuiting or not; and/or
allow any particular ordering for element/field comparisons).
4. Change gc to not (visibly) short-circuit comparisons that involve comparing interface
types; e.g., comparing two [512]int arrays can still short-circuit, but comparing two
struct{a int; b, c interface{}; d int} structs would need to always compare the b and c
fields, and a and d could be compared conditionally.

[griesemer]

It seems to me that the same problems that might occur with unspecified struct field comparison order would apply to array comparisons. I'll send out a CL specifying both, in the spirit of this issue. We can discuss on the issue if we want to keep the clarification for arrays.

[gopherbot]

Change https://go.dev/cl/449536 mentions this issue: spec: clarify struct field and array element comparison order

[mdempsky]

It seems to me that the same problems that might occur with unspecified struct field comparison order would apply to array comparisons.

I feel like my comments aren't being heard.

My original issue report from 2014 mentions structs and arrays equally: they were both mentioned in the issue title and the very first sentence of the issue body. Whenever I replied mentioning structs, I wrote "struct/array" (1, 2, 3).

@rsc retitled it in June 2020 to remove the mention of arrays. I explicitly asked that same day if that was intentional, and never got a response.

I asked again 6 days ago after @ianlancetaylor pinged the issues, and still I'm not getting any acknowledgement.

[ianlancetaylor]

I agree with @griesemer that this should apply to arrays as well as structs.