Computational expressions tail calls support, revived (#18804)

Author: majocha

docs/release-notes/.FSharp.Compiler.Service/10.0.100.md

@@ -1,6 +1,7 @@
 ### Added
 * Add opt-in warning attribute not valid for union case with fields [PR #18532](https://github.com/dotnet/fsharp/pull/18532))
 * Add support for `when 'T : Enum` library-only static optimization constraint. ([PR #18546](https://github.com/dotnet/fsharp/pull/18546))
+* Add support for tail calls in computation expressions ([PR #18804](https://github.com/dotnet/fsharp/pull/18804))
 * Add `--typecheck-only` flag support for F# Interactive (FSI) scripts to type-check without execution. ([Issue #18686](https://github.com/dotnet/fsharp/issues/18686))
 
 ### Fixed

docs/release-notes/.Language/preview.md

@@ -10,6 +10,7 @@
 * Scoped Nowarn: added the #warnon compiler directive ([Language suggestion #278](https://github.com/fsharp/fslang-suggestions/issues/278), [RFC FS-1146 PR](https://github.com/fsharp/fslang-design/pull/782), [PR #18049](https://github.com/dotnet/fsharp/pull/18049))
 * Allow `let!`, `use!`, `and!` type annotations without requiring parentheses (([PR #18508](https://github.com/dotnet/fsharp/pull/18508) and [PR #18682](https://github.com/dotnet/fsharp/pull/18682)))
 * Exception names are now validated for illegal characters using the same mechanism as types/modules/namespaces ([Issue #18763](https://github.com/dotnet/fsharp/issues/18763))
+* Support tail calls in computation expressions ([PR #18804](https://github.com/dotnet/fsharp/pull/18804))
 
 ### Fixed
 

src/Compiler/Checking/Expressions/CheckComputationExpressions.fs

@@ -1807,4 +1807,5 @@ featureAllowLetOrUseBangTypeAnnotationWithoutParens,"Allow let! and use! type an
 3875,lexWarnDirectiveMustHaveArgs,"Warn directives must have warning number(s) as argument(s)"
 3876,lexWarnDirectivesMustMatch,"There is another %s for this warning already in line %d."
 3877,lexLineDirectiveMappingIsNotUnique,"The file '%s' was also pointed to in a line directive in '%s'. Proper warn directive application may not be possible."
-3878,tcAttributeIsNotValidForUnionCaseWithFields,"This attribute is not valid for use on union cases with fields."
+3878,tcAttributeIsNotValidForUnionCaseWithFields,"This attribute is not valid for use on union cases with fields."
+featureReturnFromFinal,"Support for ReturnFromFinal/YieldFromFinal in computation expressions to enable tailcall optimization when available on the builder."

src/Compiler/FSComp.txt

@@ -103,6 +103,7 @@ type LanguageFeature =
     | BetterAnonymousRecordParsing
     | ScopedNowarn
     | AllowTypedLetUseAndBang
+    | ReturnFromFinal
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -238,6 +239,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.AllowTypedLetUseAndBang, languageVersion100
                 LanguageFeature.UnmanagedConstraintCsharpInterop, languageVersion100
                 LanguageFeature.AllowAccessModifiersToAutoPropertiesGettersAndSetters, languageVersion100
+                LanguageFeature.ReturnFromFinal, languageVersion100
 
                 // F# preview (still preview in 10.0)
                 LanguageFeature.FromEndSlicing, previewVersion // Unfinished features --- needs work
@@ -409,6 +411,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.BetterAnonymousRecordParsing -> FSComp.SR.featureBetterAnonymousRecordParsing ()
         | LanguageFeature.ScopedNowarn -> FSComp.SR.featureScopedNowarn ()
         | LanguageFeature.AllowTypedLetUseAndBang -> FSComp.SR.featureAllowLetOrUseBangTypeAnnotationWithoutParens ()
+        | LanguageFeature.ReturnFromFinal -> FSComp.SR.featureReturnFromFinal ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fs

@@ -94,6 +94,7 @@ type LanguageFeature =
     | BetterAnonymousRecordParsing
     | ScopedNowarn
     | AllowTypedLetUseAndBang
+    | ReturnFromFinal
 
 /// LanguageVersion management
 type LanguageVersion =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -186,6 +186,8 @@ type SynExprYieldOrReturnFromTrivia =
         YieldOrReturnFromKeyword: range
     }
 
+    static member Zero: SynExprYieldOrReturnFromTrivia
+
 /// Represents additional information for SynExpr.AnonRecd
 [<NoEquality; NoComparison>]
 type SynExprAnonRecdTrivia =