encoding/json/v2: remove `format` tag option

[dsnet]

Proposal Details

With Go 1.28 prospectively having typed struct tags in some form or another, the json/v2 working group decided to remove support for the format tag option since this would be more naturally expressed as a typed struct tag.

The implementation for format will not be eliminated, but instead be guarded behind a goexperiment flag (e.g., GOEXPERIMENT=jsonformat. This allows for a path forward for existing users of the format flag until Go 1.28, which hopefully provides first-class support for equivalent behavior in a different way.

[gabyhelp]

Related Issues

• encoding/json/v2: remove recursive nature of `string` tag option #79065
• proposal: encoding/json: support struct tag for time.Format in JSON Marshaller/Unmarshaller #21990
• proposal: encoding/json[/v2]: custom encoder/decoder struct tag #77395 (closed)
• encoding/json/v2: add new JSON API behind a GOEXPERIMENT=jsonv2 guard #71845 (closed)
• proposal: encoding/json: export tagOptions, parseTag #60770
• proposal: encoding/json/v2: encode time with `format:unix` as integer instead of float #73486
• proposal: encoding/json/v2: struct field tags for directional Marshal/Unmarshal #77423 (closed)
• encoding/json/v2: remove `unknown` tag option and DiscardUnknownMembers #77271 (closed)
• encoding/json/v2: documentation doesn't specify that `format` option has to come last #77547
• proposal: encoding/json/v2: add `format:0xhex` for `[]byte` #75492

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[gopherbot]

Change https://go.dev/cl/773980 mentions this issue: encoding/json/v2: support format tag option behind goexperiment