[Rust] Support multiple namespaces sharing a prefix in the same file

src/Fable.Cli/CHANGELOG.md

@@ -12,6 +12,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * Fix #3571: `[<AttachMembers>]` not compatible with f# member `this.Item` (by @ncave)
 
+### Changed
+
+#### Rust
+
+* Support multiple namespaces sharing a prefix in the same file (by @ncave)
+
 ## 4.4.1 - 2023-10-25
 
 ### Changed

src/Fable.Transforms/Rust/Fable2Rust.fs

@@ -4085,6 +4085,39 @@ module Util =
         let isInternal, isPrivate = getVis com ctx memb.DeclaringEntity memb.IsInternal memb.IsPrivate
         memberAssocItem |> mkAssocItemWithVis isInternal isPrivate
 
+    let mergeNamespaceDecls (com: IRustCompiler) ctx decls =
+        // separate namespace decls from the others
+        let namespaceDecls, otherDecls =
+            decls
+            |> List.partition (function
+                | Fable.ModuleDeclaration d ->
+                    let ent = com.GetEntity(d.Entity)
+                    ent.IsNamespace
+                | _ -> false)
+        // merge namespace decls with the same name into a single decl
+        let namespaceDecls =
+            namespaceDecls
+            |> List.groupBy (function
+                | Fable.ModuleDeclaration d -> d.Name
+                | _ -> failwith "unreachable")
+            |> List.map (fun (key, decls) ->
+                match decls with
+                | [d] -> d // no merge needed
+                | _ ->
+                    let members =
+                        decls
+                        |> List.map (function
+                            | Fable.ModuleDeclaration d -> d.Members
+                            | _ -> [])
+                        |> List.concat
+                    match List.head decls with
+                    | Fable.ModuleDeclaration d ->
+                        Fable.ModuleDeclaration { d with Members = members }
+                    | d -> d
+            )
+        // return merged decls
+        List.append namespaceDecls otherDecls
+
     let transformModuleDecl (com: IRustCompiler) ctx (decl: Fable.ModuleDecl) =
         let ctx = { ctx with ModuleDepth = ctx.ModuleDepth + 1 }
         let memberDecls =
@@ -4093,13 +4126,15 @@ module Util =
             // this prioritizes non-module declaration transforms first,
             // so module imports can be properly deduped top to bottom.
             decl.Members
+            |> mergeNamespaceDecls com ctx
             |> List.map (fun decl ->
                 let lazyDecl = lazy (transformDecl com ctx decl)
                 match decl with
                 | Fable.ModuleDeclaration _ -> () // delay module decl transform
                 | _ -> lazyDecl.Force() |> ignore // transform other decls first
                 lazyDecl)
             |> List.collect (fun lazyDecl -> lazyDecl.Force())
+
         if List.isEmpty memberDecls then
             [] // don't output empty modules
         else
@@ -4141,6 +4176,11 @@ module Util =
         | Fable.ClassDeclaration decl ->
             transformClassDecl com ctx decl
 
+    let transformDeclarations (com: IRustCompiler) ctx decls =
+        decls
+        |> mergeNamespaceDecls com ctx
+        |> List.collect (transformDecl com ctx)
+
     // F# hash function is unstable and gives different results in different runs
     // Taken from fable-library/Util.ts. Possible variant in https://stackoverflow.com/a/1660613
     let stableStringHash (s: string) =
@@ -4370,12 +4410,12 @@ module Compiler =
                 // mkInnerAttr "feature" ["destructuring_assignment"]
         ]
 
-        let entryPointItems = getEntryPointItems com ctx file.Declarations
+        let entryPointItems = file.Declarations |> getEntryPointItems com ctx
         let importItems = com.GetAllImports(ctx) |> transformImports com ctx
-        let declItems = List.collect (transformDecl com ctx) file.Declarations
+        let declItems = file.Declarations |> transformDeclarations com ctx
         let moduleItems = getModuleItems com ctx // global module imports
         let crateItems = importItems @ declItems @ moduleItems @ entryPointItems
-        let innerAttrs = getInnerAttributes com ctx file.Declarations
+        let innerAttrs = file.Declarations |> getInnerAttributes com ctx
         let crateAttrs = topAttrs @ innerAttrs
         let crate = mkCrate crateAttrs crateItems
         crate

tests/Rust/tests/common/Util3.fs

@@ -7,23 +7,14 @@
 // Duplicating namespaces doesn't work
 // namespace Fable.Tests.A.C
 
-// // Empty namespaces work
-// namespace Fable.Tests.A.D
+// Empty namespaces work
+namespace Fable.Tests.A.D
 
-// // Multiple mamespaces sharing prefix work
-// namespace Fable.Tests.A.B
-// type Helper =
-//     static member Add2(x) = x + 2
+// Multiple mamespaces sharing prefix work
+namespace Fable.Tests.A.B
+type Helper =
+    static member Add2(x) = x + 2
 
-// namespace Fable.Tests.A.C
-// type Helper =
-//     static member Add5(x) = Fable.Tests.A.B.Helper.Add2(x + 3)
-
-//TODO: Support multiple namespaces in the same file
-namespace Fable.Tests.A
-module B =
-    type Helper =
-        static member Add2(x) = x + 2
-module C =
-    type Helper =
-        static member Add5(x) = B.Helper.Add2(x + 3)
+namespace Fable.Tests.A.C
+type Helper =
+    static member Add5(x) = Fable.Tests.A.B.Helper.Add2(x + 3)