Remove always-on language feature flag: WarningWhenInliningMethodImplNoInlineMarkedFunction

[T-Gro]

LanguageFeature.WarningWhenInliningMethodImplNoInlineMarkedFunction has shipped since F# 8.0 and is enabled for every selectable language version (the minimum accepted --langversion is 8.0). Its feature flag can therefore never be turned off, so it is dead configuration. This issue removes the flag and collapses the guarded code to its enabled behaviour.

What to remove

• The WarningWhenInliningMethodImplNoInlineMarkedFunction case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureWarningWhenInliningMethodImplNoInlineMarkedFunction string in FSComp.txt
• Every SupportsFeature LanguageFeature.WarningWhenInliningMethodImplNoInlineMarkedFunction / checkLanguageFeatureAndRecover … WarningWhenInliningMethodImplNoInlineMarkedFunction guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

Consequence: --disableLanguageFeature:WarningWhenInliningMethodImplNoInlineMarkedFunction is no longer a recognised feature name.

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 203 in ea778bb

LanguageFeature.WarningWhenInliningMethodImplNoInlineMarkedFunction, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1597 in ea778bb

featureWarningWhenInliningMethodImplNoInlineMarkedFunction,"Raises warnings when 'let inline ... =' is used together with [<MethodImpl(MethodImplOptions.NoInlining)>] attribute. Function is not getting inlined."

Part of #20139