Warn when record update expression changes all fields

[kerams]

Implements fsharp/fslang-suggestions#603.

There has been some opposition to the suggestion. Should this maybe be off by default or part of warning level 4?

[kerams]

Build failures stem from this diagnostic being triggered when compiling FSharp.Core:

/// Produce a new execution context for a composite asyncmember_.WithContinuations(cont,econt)=
AsyncActivation<'U>// <------------- ok, 2 fields, construction syntax{
cont = cont
aux ={ contents.aux with econt = econt }}/// Produce a new execution context for a composite asyncmemberctxt.WithContinuations(cont,econt,ccont)=
AsyncActivation<'T>{ contents with// <------------- warning, 2 fields, update syntax
cont = cont
aux ={ ctxt.aux with
econt = econt
ccont = ccont
}}

[vzarytovskii]

Can you please put it off by default in current versions and on under preview version? This way, I guess, we can have it off now, but optionally enabled if users want it, and on by default in preview/8?

[psfinaki]

@kerams you are really doing an amazing job with these PRs, thanks a lot. Considering the level of the warning, IMO can be whatever since yeah we don't expect this to happen often to be anyhow noisy.

[vzarytovskii]

@kerams you are really doing an amazing job with these PRs, thanks a lot. Considering the level of the warning, IMO can be whatever since yeah we don't expect this to happen often to be anyhow noisy.

It should not be enabled by default for current language versions, a rule of thumb is usually "if it will break anyone's compilation simply by upgrading to a minor SDK, then it go under language flag"

[T-Gro]

I like the new approach via the exception type - it can be also reused for other scenarios where we want the same ( combination of langfeature + still allowing explicit opt-in for older versions).

Good.