Compiled ToStrings under -reflectionfree for DUs and Records

[charlesroddie]

This PR implements ToString on user-defined DUs and records, under the --reflectionfree switch, by delegating into the equivalent of LanguagePrimitives.anyToStringShowingNull, as is currently used by Option. Currently only the type name is shown. The new ToString is AOT/trim friendly, provided that the inner types have AOT/trim-friendly ToString methods.

It also implements ToString on Result and Choice types, and consolidates behaviour to match Option.

Fixes fsharp/fslang-suggestions#919

Behavioural changes (ungated)

• Ok(0) now renders as "Ok(0)", matching Some(0) which renders as "Some(0)".

Behavioural changes (subject to --reflectionfree)

• Records and DUs now generate real ToStrings instead of just the type name. The behaviour matches Option for the internal rendering, and this differs from non-reflection-free mode (sprintf "%+A") in the following ways:
  • The floating point number 5. renders as 5.
  • Records render on a single line, avoiding the complex indentation logic in sprintf "%A".
  • Anonymous records render with {| ... |} syntax instead of { ... }

While the exact code of Option couldn't be used directly (since anyToStringShowingNull is internal to FSharp.Core), it was easy to match it, and there are some tests that the behaviour is the same, so that if there are any changes to the above in Option (which there is some ongoing discussion about, e.g. over null rendering), they are synced up with other DU non-reflection-based rendering.

It is obviously easy to make the changes to --reflectionfree mode a decisive improvement, since we are starting with nothing. However the intention would be that the behaviour is good enough to switch as a default in a future version of F#.

Note: this PR and #19971 add an identical v_string_operator_info and mkCallStringOperator.

[github-actions]

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[kerams]

Kickass

[github-actions]

🔍 Tooling Safety Check — Affects-Compiler-Output
Affects-Compiler-Output: IlxGen.fs, prim-types.fs, Optimizer.fs modify IL emission and FSharp.Core runtime

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