design: add design doc for relaxed struct conversions

For golang/go#16085.

Change-Id: I5ec55defa201dfc33774e8d80edd4070c1d3d4de
Reviewed-on: https://go-review.googlesource.com/24192
Reviewed-by: Robert Griesemer <gri@golang.org>

design/16085-conversions-ignore-tags.md

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+# Proposal: Ignore tags in struct type conversions
+
+Author: [Robert Griesemer](gri@golang.org)
+
+Created: June 16, 2016
+
+Last updated: June 16, 2016
+
+Discussion at https://golang.org/issue/16085
+
+## Abstract
+
+This document proposes to relax struct conversions such that struct tags are
+ignored.
+An alternative to the proposal is to add a new function reflect.StructCopy
+that could be used instead.
+
+## Background
+
+The [spec](https://codereview.appspot.com/1698043) and corresponding
+[implementation change](https://golang.org/cl/1667048) submitted almost
+exactly six years ago made [struct tags](https://golang.org/ref/spec#Struct_types)
+an integral part of a struct type by including them in the definition of struct
+[type identity](https://golang.org/ref/spec#Type_identity) and indirectly in
+struct [type conversions](https://golang.org/ref/spec#Conversions).
+
+In retrospect, this change may have been overly restrictive with respect to
+its impact on struct conversions, given the way struct tag use has evolved
+over the years.
+A common scenario is the conversion of struct data coming from, say a database,
+to an _equivalent_ (identical but for its tags) struct that can be JSON-encoded,
+with the JSON encoding defined by the respective struct tags.
+For an example of such a type, see
+https://github.com/golang/text/blob/master/unicode/cldr/xml.go#L6.
+
+The way struct conversions are defined, it is not currently possible to convert
+a value from one struct type to an equivalent one.
+Instead, every field must be copied manually, which leads to more source text,
+and less readable and possibly less efficient code.
+The code must also be adjusted every time the involved struct types change.
+
+https://github.com/golang/go/issues/6858 discusses this in more detail.
+rsc@golang and r@golang suggest that we might be able to relax the rules for
+structs such that struct tags are ignored for conversions, but not for struct
+identity.
+
+## Proposal
+
+The spec states a set of rules for conversions.
+The following rules apply to conversions of struct values (among others):
+
+A non-constant value x can be converted to type T if:
+- x's type and T have identical underlying types
+- x's type and T are unnamed pointer types and their pointer base types have identical underlying types
+
+The proposal is to change these two rules to:
+
+A non-constant value x can be converted to type T if:
+- x's type and T have identical underlying types _if struct tags are ignored (recursively)_
+- x's type and T are unnamed pointer types and their pointer base types have identical underlying types _if struct tags are ignored (recursively)_
+
+Additionally, package reflect is adjusted (Type.ConvertibleTo, Value.Convert)
+to match this language change.
+
+In other words, type identity of structs remains unchanged, but for the purpose
+of struct conversions, type identity is relaxed such that struct tags are
+ignored.
+
+## Compatibility and impact
+
+This is is a backward-compatible language change since it loosens an existing
+restriction:
+Any existing code will continue to compile with the same meaning (*), and some
+code that currently is invalid will become valid.
+
+Programs that manually copy all fields from one struct to another struct with
+identical type but for the (type name and) tags, will be able to use a single
+struct conversion instead.
+
+More importantly, with this change two different (type) views of the same struct
+value become possible via pointers of different types.
+For instance, given:
+
+	type jsonPerson struct {
+		name `json:"name"`
+	}
+
+	type xmlPerson struct {
+		name `xml:"name"`
+	}
+
+we will be able to access a value of *jsonPerson type
+
+	person := new(jsonPerson)
+	// some code that populates person
+
+as a *Txml:
+
+	alias := (*xmlPerson)(&person)
+	// some code that uses alias
+
+This may eliminate the need to copy struct values just to change the tags.
+
+Type identity and conversion tests are also available programmatically, via
+the reflect package.
+The operations of Type.ConvertibleTo and Value.Convert will be relaxed for
+structs with different (or absent) tags:
+
+Type.ConvertibleTo will return true for some arguments where it currently
+returns false.
+This may change the behavior of programs depending on this method.
+
+Value.Convert will convert struct values for which the operation panicked
+before.
+This will only affect programs that relied on (recovered from) that panic.
+
+(*) r@golang points out that a program that is using tags to prevent
+(accidental or deliberate) struct conversion would lose that mechanism.
+Interestingly, package reflect appears to make such use (see type rtype),
+but iant@golang points out that one could obtain the same effect by adding
+differently typed zero-sized fields to the respective structs.
+
+## Discussion
+
+From a language spec point of view, changing struct type identity (rather
+than struct conversions only) superficially looks like a simpler, cleaner,
+and more consistent change: For one, it simplifies the spec, while only
+changing struct conversions requires adding an additional rule.
+
+iant@golang points out (https://github.com/golang/go/issues/11661) that
+leaving struct identity in place doesn’t make much difference in practice:
+It is already impossible to assign or implicitly convert between two
+differently named struct types.
+Unnamed structs are rare, and if accidental conversion is an issue, one can
+always introduce a named struct.
+
+On the other hand, runtime type descriptors (used by reflect.Type, interfaces,
+etc) are canonical, so identical types have the same type descriptor.
+The descriptor provides struct field tags, so identical types must have
+identical tags.
+Thus we cannot at this stage separate struct field tags from the notion of
+type identity.
+
+To summarize: Relaxing struct conversions only but leaving struct type
+identity unchanged is sufficient to enable one kind of data conversion
+that is currently overly tedious, and it doesn’t require larger and more
+fundamental changes to the run time.
+The change may cause a hopefully very small set of programs, which depend
+on package reflect’s conversion-related API, to behave differently.
+
+## Open question
+
+Should tags be ignored at the top-level of a struct only, or recursively
+all the way down?
+For instance, given:
+
+```
+type T1 struct {
+	x int
+	p *struct {
+		name string `foo`
+	}
+}
+
+type T2 struct {
+	x int
+	p *struct {
+		name string `bar`
+	}
+}
+
+var t1 T1
+```
+
+Should the conversion T2(t1) be legal? If tags are only ignored for the
+fields of T1 and T2, conversion is not permitted since the tags attached
+to the type of the p field are different.
+Alternatively, if tags are ignored recursively, conversion is permitted.
+
+On the other hand, if the types were defined as:
+
+```
+type T1 struct {
+	x int
+	p *P1
+}
+
+type T2 struct {
+	x int
+	p *P2
+}
+
+```
+where P1 and P2 are identical structs but for their tags, the conversion
+would not be permitted either way since the p fields have different types
+and thus T1 and T2 have different underlying types.
+
+The proposal suggests to ignore tags recursively, “all the way down”.
+This seems to be the more sensible approach given the stated goal, which
+is to make it easier to convert from one struct type to another, equivalent
+type with different tags.
+For an example where this matters, see https://play.golang.org/p/U73K50YXYk.
+
+Furthermore, it is always possible to prevent unwanted conversions by
+introducing named types, but it would not be possible to enable those
+conversions otherwise.
+
+On the other hand, the current implementation of reflect.Value.Convert
+will make recursive ignoring of struct tags more complicated and expensive.
+crawshaw@golang points out that one could easily use a cache inside the
+reflect package if necessary for performance.
+
+## Implementation
+
+An (almost) complete implementation is in https://golang.org/cl/24190/;
+with a few pieces missing for the reflect package change.
+
+## Alternatives to the language change
+
+Even a backward-compatible language change needs to meet a high bar before
+it can be considered.
+It is not yet clear that this proposal satisfies that criteria.
+
+One alternative is to do nothing.
+That has the advantage of not breaking anything and also doesn’t require
+any implementation effort on the language/library side.
+But it means that in some cases structs have to be explicitly converted
+through field-by-field assignment.
+
+Another alternative that actually addresses the problem is to provide a
+library function.
+For instance, package reflect could provide a new function
+
+```
+func CopyStruct(dst, src Value, mode Mode)
+```
+
+which could be used to copy struct values that have identical types but
+for struct tags.
+A mode argument might be used to control deep or shallow copy, and perhaps
+other modalities.
+A deep copy (following pointers) would be a useful feature that the spec
+change by itself does not enable.
+
+The cost of using a CopyStruct function instead of a direct struct conversion
+is the need to create two reflect.Values, invoking CopyStruct, and (inside
+CopyStruct) the cost to verify type identity but for tags.
+Copying the actual data needs to be done both in CopyStruct but also with a
+direct (language-based) conversion.
+The type verification is likely the most expensive step but identity of
+struct types (with tags ignored) could be cached.
+On the other hand, adonovan@golang points out that the added cost may not
+matter in significant ways since these kinds of struct copies often sit
+between a database request and an HTTP response.
+
+The functional difference between the proposed spec change and a new
+reflect.CopyStruct function is that with CopyStruct an actual copy has to
+take place (as is the case now).
+The spec change on the other hand permits both approaches: a (copying)
+conversion of struct values, or pointers to different struct types that
+point to the same struct value via a pointer conversion.
+The latter may eliminate a copy of data in the first place.