RFC FS-1033: Deprecate places where seq can be omitted (#17772)

* Treat `{ 1..10 }` as sequence expression

* deprecate { start..finish } and { start..step..finish }

* fix build part1

* 3873., chkDeprecatePlacesWhereSeqCanBeOmitted

* more tests

* release notes

* reorg seq tests

* Better check

* fix merge conflicts

* Add code fix for adding missing `seq` before `{…}`

* More tests

* Fmt

* update release notes

* more editor tests

---------

Co-authored-by: Brian Rourke Boll <brianrourkeboll@gmail.com>

Author: edgarfgp

docs/release-notes/.FSharp.Compiler.Service/9.0.200.md

@@ -8,7 +8,7 @@
 * Fix nullness inference for member val and other OO scenarios  ([PR #17845](https://github.com/dotnet/fsharp/pull/17845))
 
 ### Added
-
+* Deprecate places where `seq` can be omitted. ([Language suggestion #1033](https://github.com/fsharp/fslang-suggestions/issues/1033), [PR #17772](https://github.com/dotnet/fsharp/pull/17772))
 * Support literal attribute on decimals ([PR #17769](https://github.com/dotnet/fsharp/pull/17769))
 
 ### Changed

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

@@ -1,6 +1,7 @@
 ### Added
 
 * Better generic unmanaged structs handling. ([Language suggestion #692](https://github.com/fsharp/fslang-suggestions/issues/692), [PR #12154](https://github.com/dotnet/fsharp/pull/12154))
+* Deprecate places where `seq` can be omitted. ([Language suggestion #1033](https://github.com/fsharp/fslang-suggestions/issues/1033), [PR #17772](https://github.com/dotnet/fsharp/pull/17772))
 
 ### Fixed
 

docs/release-notes/.VisualStudio/17.13.md

@@ -0,0 +1,8 @@
+### Fixed
+
+### Added
+* Code fix for adding missing `seq`. ([PR #17772](https://github.com/dotnet/fsharp/pull/17772))
+
+### Changed
+
+### Breaking Changes

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -5891,6 +5891,17 @@ and TcExprUndelayed (cenv: cenv) (overallTy: OverallTy) env tpenv (synExpr: SynE
         TcForEachExpr cenv overallTy env tpenv (seqExprOnly, isFromSource, pat, synEnumExpr, synBodyExpr, m, spFor, spIn, m)
 
     | SynExpr.ComputationExpr (hasSeqBuilder, comp, m) ->
+        let isIndexRange = match comp with | SynExpr.IndexRange _ -> true | _ -> false
+        let deprecatedPlacesWhereSeqCanBeOmitted =
+            cenv.g.langVersion.SupportsFeature LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted
+        if
+            deprecatedPlacesWhereSeqCanBeOmitted
+            && isIndexRange
+            && not hasSeqBuilder
+            && not cenv.g.compilingFSharpCore
+        then
+            warning (Error(FSComp.SR.chkDeprecatePlacesWhereSeqCanBeOmitted (), m))
+
         let env = ExitFamilyRegion env
         cenv.TcSequenceExpressionEntry cenv env overallTy tpenv (hasSeqBuilder, comp) m
 

src/Compiler/FSComp.txt

@@ -1783,4 +1783,6 @@ featureEmptyBodiedComputationExpressions,"Support for computation expressions wi
 featureAllowAccessModifiersToAutoPropertiesGettersAndSetters,"Allow access modifiers to auto properties getters and setters"
 3871,tcAccessModifiersNotAllowedInSRTPConstraint,"Access modifiers cannot be applied to an SRTP constraint."
 featureAllowObjectExpressionWithoutOverrides,"Allow object expressions without overrides"
-3872,tcPartialActivePattern,"Multi-case partial active patterns are not supported. Consider using a single-case partial active pattern or a full active pattern."
+3872,tcPartialActivePattern,"Multi-case partial active patterns are not supported. Consider using a single-case partial active pattern or a full active pattern."
+3873,chkDeprecatePlacesWhereSeqCanBeOmitted,"This construct is deprecated. Sequence expressions should be of the form 'seq {{ ... }}'"
+featureDeprecatePlacesWhereSeqCanBeOmitted,"Deprecate places where 'seq' can be omitted"

src/Compiler/Facilities/LanguageFeatures.fs

@@ -94,6 +94,7 @@ type LanguageFeature =
     | ParsedHashDirectiveArgumentNonQuotes
     | EmptyBodiedComputationExpressions
     | AllowObjectExpressionWithoutOverrides
+    | DeprecatePlacesWhereSeqCanBeOmitted
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -219,6 +220,7 @@ type LanguageVersion(versionText) =
                 LanguageFeature.FromEndSlicing, previewVersion // Unfinished features --- needs work
                 LanguageFeature.AllowAccessModifiersToAutoPropertiesGettersAndSetters, previewVersion
                 LanguageFeature.AllowObjectExpressionWithoutOverrides, previewVersion
+                LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -375,6 +377,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.ParsedHashDirectiveArgumentNonQuotes -> FSComp.SR.featureParsedHashDirectiveArgumentNonString ()
         | LanguageFeature.EmptyBodiedComputationExpressions -> FSComp.SR.featureEmptyBodiedComputationExpressions ()
         | LanguageFeature.AllowObjectExpressionWithoutOverrides -> FSComp.SR.featureAllowObjectExpressionWithoutOverrides ()
+        | LanguageFeature.DeprecatePlacesWhereSeqCanBeOmitted -> FSComp.SR.featureDeprecatePlacesWhereSeqCanBeOmitted ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -85,6 +85,7 @@ type LanguageFeature =
     | ParsedHashDirectiveArgumentNonQuotes
     | EmptyBodiedComputationExpressions
     | AllowObjectExpressionWithoutOverrides
+    | DeprecatePlacesWhereSeqCanBeOmitted
 
 /// LanguageVersion management
 type LanguageVersion =