Allow --langversion:preview to disable ExperimentalAttribut warning. (#8042)

* Merge issues

* More attributes

* Experimental Attribute

* literally the message

* .fsi and [<Literal>] don't mix with lkg compiler.

Author: KevinRansom

src/fsharp/AttributeChecking.fs

@@ -4,6 +4,7 @@
 /// on items from name resolution
 module internal FSharp.Compiler.AttributeChecking
 
+open System
 open System.Collections.Generic
 open FSharp.Compiler.AbstractIL.IL 
 open FSharp.Compiler.AbstractIL.Internal.Library
@@ -246,7 +247,6 @@ let MethInfoHasAttribute g m attribSpec minfo  =
         |> Option.isSome
 
 
-
 /// Check IL attributes for 'ObsoleteAttribute', returning errors and warnings as data
 let private CheckILAttributes (g: TcGlobals) isByrefLikeTyconRef cattrs m = 
     let (AttribInfo(tref,_)) = g.attrib_SystemObsolete
@@ -267,9 +267,16 @@ let private CheckILAttributes (g: TcGlobals) isByrefLikeTyconRef cattrs m =
 
 /// Check F# attributes for 'ObsoleteAttribute', 'CompilerMessageAttribute' and 'ExperimentalAttribute',
 /// returning errors and warnings as data
-let CheckFSharpAttributes g attribs m = 
-    if isNil attribs then CompleteD 
-    else 
+let langVersionPrefix = "--langversion:preview"
+let CheckFSharpAttributes (g:TcGlobals) attribs m =
+    let isExperimentalAttributeDisabled (s:string) =
+        if g.compilingFslib then
+            true
+        else
+            g.langVersion.IsPreviewEnabled && (s.IndexOf(langVersionPrefix, StringComparison.OrdinalIgnoreCase) >= 0)
+
+    if isNil attribs then CompleteD
+    else
         (match TryFindFSharpAttribute g g.attrib_SystemObsolete attribs with
         | Some(Attrib(_, _, [ AttribStringArg s ], _, _, _, _)) ->
             WarnD(ObsoleteWarning(s, m))
@@ -283,28 +290,30 @@ let CheckFSharpAttributes g attribs m =
         | None -> 
             CompleteD
         ) ++ (fun () -> 
-            
+
         match TryFindFSharpAttribute g g.attrib_CompilerMessageAttribute attribs with
-        | Some(Attrib(_, _, [ AttribStringArg s ; AttribInt32Arg n ], namedArgs, _, _, _)) -> 
+        | Some(Attrib(_, _, [ AttribStringArg s ; AttribInt32Arg n ], namedArgs, _, _, _)) ->
             let msg = UserCompilerMessage(s, n, m)
             let isError = 
                 match namedArgs with 
                 | ExtractAttribNamedArg "IsError" (AttribBoolArg v) -> v 
                 | _ -> false 
             if isError && (not g.compilingFslib || n <> 1204) then ErrorD msg else WarnD msg
-                 
         | _ -> 
             CompleteD
         ) ++ (fun () -> 
-            
+
         match TryFindFSharpAttribute g g.attrib_ExperimentalAttribute attribs with
-        | Some(Attrib(_, _, [ AttribStringArg(s) ], _, _, _, _)) -> 
-            WarnD(Experimental(s, m))
-        | Some _ -> 
+        | Some(Attrib(_, _, [ AttribStringArg(s) ], _, _, _, _)) ->
+            if isExperimentalAttributeDisabled s then
+                CompleteD
+            else
+                WarnD(Experimental(s, m))
+        | Some _ ->
             WarnD(Experimental(FSComp.SR.experimentalConstruct (), m))
-        | _ ->  
+        | _ ->
             CompleteD
-        ) ++ (fun () -> 
+        ) ++ (fun () ->
 
         match TryFindFSharpAttribute g g.attrib_UnverifiableAttribute attribs with
         | Some _ -> 

src/fsharp/FSharp.Core/FSharp.Core.fsproj

@@ -10,7 +10,7 @@
     <AllowCrossTargeting>true</AllowCrossTargeting>
     <DefineConstants>$(DefineConstants);FSHARP_CORE</DefineConstants>
     <DefineConstants Condition="'$(Configuration)' == 'Proto'">BUILDING_WITH_LKG;$(DefineConstants)</DefineConstants>
-    <OtherFlags>$(OtherFlags) --warnon:1182 --compiling-fslib --compiling-fslib-40 --maxerrors:20 --extraoptimizationloops:1</OtherFlags>
+    <OtherFlags>$(OtherFlags) --warnon:1182 --compiling-fslib --compiling-fslib-40 --maxerrors:20 --extraoptimizationloops:1 --nowarn:57</OtherFlags>
     <Tailcalls>true</Tailcalls> <!-- .tail annotations always emitted for this binary, even in debug mode -->
     <NGenBinary>true</NGenBinary>
   </PropertyGroup>