convert remaining PascalCase enum and union fields to snake_case

[andrewrk]

Currently we have this problem with union(enum):

zig/std/zig/ast.zig

Lines 110 to 130 in aff7b38

	pub const Error = union(enum) {
	InvalidToken: InvalidToken,
	ExpectedVarDeclOrFn: ExpectedVarDeclOrFn,
	ExpectedVarDecl: ExpectedVarDecl,
	ExpectedAggregateKw: ExpectedAggregateKw,
	UnattachedDocComment: UnattachedDocComment,
	ExpectedEqOrSemi: ExpectedEqOrSemi,
	ExpectedSemiOrLBrace: ExpectedSemiOrLBrace,
	ExpectedColonOrRParen: ExpectedColonOrRParen,
	ExpectedLabelable: ExpectedLabelable,
	ExpectedInlinable: ExpectedInlinable,
	ExpectedAsmOutputReturnOrType: ExpectedAsmOutputReturnOrType,
	ExpectedCall: ExpectedCall,
	ExpectedCallOrFnProto: ExpectedCallOrFnProto,
	ExpectedSliceOrRBracket: ExpectedSliceOrRBracket,
	ExtraAlignQualifier: ExtraAlignQualifier,
	ExtraConstQualifier: ExtraConstQualifier,
	ExtraVolatileQualifier: ExtraVolatileQualifier,
	ExpectedPrimaryExpr: ExpectedPrimaryExpr,
	ExpectedToken: ExpectedToken,
	ExpectedCommaOrEnd: ExpectedCommaOrEnd,

Now, if you use Error.InvalidToken are you referring to the type, or the enum value? It's ambiguous. This causes switch expressions to resort to awkward uses of @TagType:

zig/std/zig/ast.zig

Lines 133 to 141 in aff7b38

	switch (self.*) {
	// TODO https://github.com/ziglang/zig/issues/683
	@TagType(Error).InvalidToken => |*x| return x.render(tokens, stream),
	@TagType(Error).ExpectedVarDeclOrFn => |*x| return x.render(tokens, stream),
	@TagType(Error).ExpectedVarDecl => |*x| return x.render(tokens, stream),
	@TagType(Error).ExpectedAggregateKw => |*x| return x.render(tokens, stream),
	@TagType(Error).UnattachedDocComment => |*x| return x.render(tokens, stream),
	@TagType(Error).ExpectedEqOrSemi => |*x| return x.render(tokens, stream),
	@TagType(Error).ExpectedSemiOrLBrace => |*x| return x.render(tokens, stream),

However if we had a different style guide, the enum value would be Error.invalid_token and the type would be Error.InvalidToken.

This is less of a problem now that #683 is mostly solved, but there is still the ambiguity.

This sort of conflicts with #995.

[Akuli]

How about UPPER_CASE for enum fields? Many other languages do that too.

[andrewrk]

I'll note that @thejoshwolfe even before this proposal already broke from the style guide to do this: https://github.com/thejoshwolfe/legend-of-swarkland/blob/ace37864bf4754851bfe0c5256077171c2db695f/src-core/protocol.zig#L8-L28

I'll also note that the enums in @import("builtin") adhere to this, but that's more for consistency with the LLVM names.

[thejoshwolfe]

for unions specifically, the options are very similar to fields, so matching the style of field names makes more sense than matching the style of type names. For enums, the options are most similar to global constants, and i believe the style there is snake case as well. For union enums, we have to choose one style for both.

In no way does it make sense for the options to match the style of a type name.

[andrewrk]

After months of silent deliberation and sleeping on it, this is now accepted.

I'm in no hurry to break everybody's code, but this is the new style for the standard library.

Exceptions:

• If there is some pre-existing naming convention, use that instead.
• If there are Zig language keywords desired to be used in the enum, using capitalization to work around them would be reasonable.
• Projects and applications are generally encouraged to use whatever style they prefer; the style guide is provided as a "tie-breaker" in order to provide a default when there is ambivalence.