Ignore lines starting with #: (RFC-1337)

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -167,6 +167,7 @@
 * Lower string-typed interpolated strings to `System.String.Concat` rather than the reflection-based `printf` engine, making them trim- and NativeAOT-compatible. This generalizes and ungates the previous all-string `String.Concat` optimization, so it now applies to every string-typed interpolation. ([Language suggestion #1108](https://github.com/fsharp/fslang-suggestions/issues/1108), [PR #19971](https://github.com/dotnet/fsharp/pull/19971))
 * Stabilized several `preview` language features into F# 11.0 (`--langversion:11.0`, enabled by default with a .NET 11 SDK): `MethodOverloadsCache`, `ErrorOnMissingSignatureAttribute`, `DirectDelegateConstruction`, `AccessProtectedBaseFieldFromClosure`, and `RecordSpreads`. `FromEndSlicing` intentionally remains in `preview`. ([PR #20199](https://github.com/dotnet/fsharp/pull/20199))
 * Interpolated string holes (e.g. `$"{x}"`) are now formatted with invariant culture (via the `string` operator) instead of the current thread culture. ([PR #19971](https://github.com/dotnet/fsharp/pull/19971))
+* Lines starting with `#:` are now ignored ([Language suggestion 1440](https://github.com/fsharp/fslang-suggestions/issues/1440), [RFC FS-1337](https://github.com/fsharp/fslang-design/pull/830), [PR #20212](https://github.com/dotnet/fsharp/pull/20212))
 
 ### Breaking Changes
 

docs/release-notes/.Language/11.0.md

@@ -14,6 +14,7 @@
 
 ### Changed
 
+* Lines starting with `#:` are now ignored ([Language suggestion 1440](https://github.com/fsharp/fslang-suggestions/issues/1440), [RFC FS-1337](https://github.com/fsharp/fslang-design/pull/830), [PR #20212](https://github.com/dotnet/fsharp/pull/20212))
 * Direct delegate construction ([PR #19993](https://github.com/dotnet/fsharp/pull/19993))
   * A delegate built from a method or function now points straight at that method instead of an intermediate closure, so `delegate.Method` is the real target and no closure class is generated.
   * Two delegates built from the same method and target now compare equal, where the previous closure form produced distinct instances; this also makes `Delegate.Remove` (and `-=` on events) match and remove such a delegate that it previously left in place.

src/Compiler/FSComp.txt

@@ -1846,3 +1846,4 @@ featureImprovedImpliedArgumentNamesPartTwo,"Improved implied argument names with
 featureRecordSpreads,"record type and expression spreads"
 3908,xmlDocIncludeError,"XML documentation include error: %s"
 3908,xmlDocIncludeError2,"XML documentation include error: Unable to include XML fragment '%s' of file '%s' -- %s"
+3909,lexColonDirectiveMustBeFirst,"#: directives must appear as the first non-whitespace characters on a line"

src/Compiler/lex.fsl

@@ -754,6 +754,13 @@ rule token (args: LexArgs) (skip: bool) = parse
      { errorR(Error(FSComp.SR.lexInvalidIdentifier(), lexbuf.LexemeRange))
        Keywords.IdentifierToken args lexbuf "" }
 
+ | "#:" anystring newline
+     { let m = lexbuf.LexemeRange
+       shouldStartLine args lexbuf m (FSComp.SR.lexColonDirectiveMustBeFirst())
+       incrLine lexbuf
+       if not skip then WHITESPACE (LexCont.Token(args.ifdefStack, args.stringNest))
+       else token args skip lexbuf }
+
  | ('#' anywhite* | "#line" anywhite+ ) digit+ anywhite* ('@'? "\"" [^'\n''\r''"']+ '"')? anywhite* newline
      {  let pos = lexbuf.EndPos
         if skip then

tests/FSharp.Compiler.ComponentTests/CompilerDirectives/IgnoreColon.fs

@@ -0,0 +1,22 @@
+namespace CompilerDirectives
+
+open Xunit
+open FSharp.Test.Compiler
+
+module IgnoreColon =
+
+    let source = """
+module test
+#:r test.dll
+[<EntryPoint>]
+let main _ =
+    #:source test.fs
+    0
+"""
+
+    [<Fact>]
+    let ignoreColonDirective () =
+
+        FSharp source
+        |> compile
+        |> withDiagnosticMessage "#: directives must appear as the first non-whitespace characters on a line"

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -33,6 +33,7 @@
     <Compile Include="CompilerDirectives\Line.fs" />
     <Compile Include="CompilerDirectives\Ifdef.fs" />
     <Compile Include="CompilerDirectives\NonStringArgs.fs" />
+    <Compile Include="CompilerDirectives\IgnoreColon.fs" />
     <Compile Include="Conformance\BasicGrammarElements\AccessibilityAnnotations\Basic\Basic.fs" />
     <Compile Include="Conformance\BasicGrammarElements\AccessibilityAnnotations\OnOverridesAndIFaceImpl\OnOverridesAndIFaceImpl.fs" />
     <Compile Include="Conformance\BasicGrammarElements\AccessibilityAnnotations\OnTypeMembers\OnTypeMembers.fs" />