refactor: split Helper.fs into root modules and declare InternalsVisibleTo in the project file

[xperiandri]

Proposed Changes

Two fixes from the review of #27 (FlatList.Parallel), made in a pull request of their own so that #27 can be stacked on it. It is based on #42, which lets FAKE write the assembly information as MSBuild properties. Both are comments on Helper.fs: "Move to fsproj with modern syntax <ItemGroup><InternalsVisibleTo..." and "Split to separate files with root modules." The review of this pull request added the small changes that are listed after them.

• Helper.fs is split into the folder src/FSharp.Collections.Immutable/Helper/, one file per module, listed in the project file in the order of their dependencies: ImmutableCollectionUtil and Comparers (both [<AutoOpen>]), ErrorStrings, Raise, ArrayChecks and Arrays. Each file is a root module with the full name, as the other files of the library are (module internal FSharp.Collections.Immutable.Raise), and keeps the accessibility, the attributes, the nested types and the XML documentation of its module. It is a move: the bodies lose four spaces of indentation, each file opens the namespaces that it uses, and nothing else changes. The full names of the modules and of their members stay, so no other source file changes.
• The #if INTERACTIVE ... namespace global conditional at the top of Helper.fs is dropped. A root module has no namespace declaration, and nothing loads Helper.fs into F# Interactive: no script or build file mentions it.
• The library project gives the test assembly access to the internals with <InternalsVisibleTo Include="$(AssemblyBaseName).Tests" />, so that no source file needs an assembly attribute for it. The item works as it is, because the SDK generates the assembly attributes since build: let FAKE write the assembly information as MSBuild properties #42.
• .github/copilot-instructions.md shows the Helper/ folder in the solution structure.
• From the review: each constant of ErrorStrings names the resource that it copies, and the one text that is in no resource, ListsHaveDifferentLengths, is removed (88c5acd). readIsDefault is a piped helper (6dc9420). The messages of Raise are interpolated strings (985832d). The InternalsVisibleTo item is where the review asked for it, and the target is gone (8dd45dc). The properties that the item needed are gone too, because the review asked for FAKE to write the assembly information as MSBuild properties, which build: let FAKE write the assembly information as MSBuild properties #42 does (c355652). isDefaultImmutableArray tests IsValueType first, so a class is rejected by one flag (33164c5). The comment of every algorithm in Arrays starts with the summary of the FSharp.Core function that it mirrors and says why the algorithm exists (a8d940c, c14a85c).

Types of changes

What types of changes does your code introduce to FSharp.Collections.Immutable?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

None of these: a refactoring and a change of the project file. No public function changes.

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

No behavior changes, and the proof does not rest on the existing tests alone (2447 per target framework, all green):

• To review the move, let git follow the lines: git diff --color-moved=dimmed-zebra --color-moved-ws=allow-indentation-change 5795c21 61aebe1 -- src/FSharp.Collections.Immutable/Helper.fs src/FSharp.Collections.Immutable/Helper. Of the changed lines, 891 show as moved; the 17 removed and 10 added lines that remain are the dropped conditional, the module headers and the opens. Taking four spaces off every indented line of the old file leaves the same few differences, so nothing else changed in the bodies.
• Every open of the new files is needed: removing any of them fails to compile.
• The built assemblies (net8.0 and net10.0) were listed by reflection before and after the move: every type, constructor, method, field, property and event, public and non-public, with attributes, signatures and parameter names, and for 851 method bodies a hash of the IL in which every metadata token is replaced by the name that it stands for. The lists are equal, and so is the generated XML documentation. The assemblies differ in what the compiler emits per source file, an empty <StartupCode$FSharp-Collections-Immutable>.$... class (one for Helper.fs before, one per module and one for the generated attribute file now), and in the new InternalsVisibleTo assembly attribute.
• The messages of Raise were compared before and after the change to interpolated strings: a script called every function with 31 argument combinations in six cultures (invariant, en-US, de-DE, sv-SE, ar-SA, fa-IR), and the 186 results (exception type, parameter name and message) are identical.
• Every text of ErrorStrings was looked up in the string resource tables of FSharp.Core 10.1, System.Private.CoreLib 10.0 and System.Collections.Immutable 10.0: 14 are verbatim resources of FSCore.resx, one is ArgumentOutOfRange_NeedNonNegNum of the .NET libraries, and the one that is in none, ListsHaveDifferentLengths, no code uses.

The InternalsVisibleTo item needs the SDK to generate the assembly attributes. Before #42, AssemblyInfo.fs came from the build script and Directory.Build.props switched the generation off, so the item alone did nothing, and a test file that used an internal module failed with FS0039. This pull request first carried properties that switched the generation on for the library and switched off the nine attributes that AssemblyInfo.fs provided (8dd45dc). The review asked for FAKE to write AssemblyInfo.Build.props instead, which #42 does, and the last commit of this pull request (c355652) removes the properties. The attributes of both target frameworks are those of #42 plus InternalsVisibleTo("FSharp.Collections.Immutable.Tests"), and the test file that failed builds.

#27 is rebased onto this branch: Helper/ParallelArrays.fs takes the place of the ParallelArrays block that it added to Helper.fs, and its assembly attribute is gone from the source.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The changes preserve helper implementations and dependency order, accommodate existing assembly metadata generation, and have no identified blocking issues.

No open issues 🎉

What changed in this PR

Prepares the library for #27 by separating shared helpers into root modules and configuring test access to internals without changing public APIs.

Changes:

• Splits Helper.fs into six files, preserving implementations and module identities.
• Adds project-level InternalsVisibleTo generation alongside existing assembly metadata.
• Documents the new helper layout.

File	Description
src/​FSharp.Collections.Immutable/​Helper/​Raise.fs	Extracts exception helpers.
src/​FSharp.Collections.Immutable/​Helper/​ImmutableCollectionUtil.fs	Extracts argument checks and array wrappers.
src/​FSharp.Collections.Immutable/​Helper/​ErrorStrings.fs	Extracts exception messages.
src/​FSharp.Collections.Immutable/​Helper/​Comparers.fs	Extracts the cached default comparer.
src/​FSharp.Collections.Immutable/​Helper/​Arrays.fs	Extracts shared array algorithms.
src/​FSharp.Collections.Immutable/​Helper/​ArrayChecks.fs	Extracts array-length checks.
src/​FSharp.Collections.Immutable/​Helper.fs	Removes the consolidated helper file.
src/​FSharp.Collections.Immutable/​FSharp.Collections.Immutable.fsproj	Registers helpers in dependency order and generates test-access metadata.
.github/​copilot-instructions.md	Documents the helper folder structure.

_{Review effort: Balanced}