Fix preferreduilang switch leaking into fsi.CommandLineArgs

[Copilot]

Description

The --preferreduilang and /preferreduilang switches were appearing in fsi.CommandLineArgs when positioned after the script file, breaking script argument contracts and causing MSBuild-invoked FSI sessions to leak implementation details.

Changes

Core Implementation (src/Compiler/Interactive/fsi.fs)

• Modified script argument handler to filter and consume preferreduilang switches regardless of position
• Extracts culture value, sets tcConfigB.preferredUiLang and Thread.CurrentThread.CurrentUICulture
• Validates argument format and handles invalid cultures gracefully with specific exception catching

Tests (tests/FSharp.Compiler.ComponentTests/Scripting/PreferredUiLangTests.fs)

• Added 6 regression tests covering switch positions (before/after script), both forms (--//), culture application, and error handling

Example

// Before: switch leaked into CommandLineArgs
// dotnet fsi script.fsx --preferreduilang:es-ES arg1
// fsi.CommandLineArgs = ["script.fsx"; "--preferreduilang:es-ES"; "arg1"]

// After: switch consumed, culture set
// dotnet fsi script.fsx --preferreduilang:es-ES arg1
// fsi.CommandLineArgs = ["script.fsx"; "arg1"]
// Thread.CurrentThread.CurrentUICulture.Name = "es-ES"

Checklist

• [x] Test cases added
• [ ] Performance benchmarks added in case of performance changes
• [x] Release notes entry updated

Original prompt

Implement proper handling of the preferreduilang switch in FSI and add regression tests.

Scope:

1. Consume and apply --preferreduilang//preferreduilang in FSI option parsing so it does not surface in fsi.CommandLineArgs, and set localization accordingly.

   • Add a recognized option in src/Compiler/Interactive/fsi.fs (option table near OptionGeneral("script.fsx arg1 arg2 ...")).
   • Support both /preferreduilang:<culture> and --preferreduilang:<culture>.
   • On parse: set tcConfigB.lcid and Thread.CurrentThread.CurrentUICulture (and possibly CurrentCulture) to the parsed culture; if parsing fails, warn and ignore.
   • Ensure this happens before ReportUserCommandLineArgs so the switch is consumed and not forwarded to scripts.

2. Add tests for argument separation and localization.

   • CLI regression tests (new folder tests/fsharp/core/interactive-args/ or extend existing tests/fsharpqa/Source/InteractiveSession/Misc/CommandLineArgs01*.fs): a) dotnet fsi --preferreduilang:en-US --exec script.fsx a → fsi.CommandLineArgs = ["script.fsx"; "a"] (switch consumed). b) dotnet fsi script.fsx --preferreduilang:en-US a → fsi.CommandLineArgs = ["script.fsx"; "a"] (even after script). c) Localization: dotnet fsi --preferreduilang:es-ES --exec script.fsx where the script triggers a warning/error; assert the culture is set (CurrentUICulture.Name starts with es-ES) and the message is localized or falls back if resources are missing. Consider a second locale (e.g., fr-FR).

3. MSBuild-path regression test.

   • Add a sample project under tests/projects/FSI/PreferreduilangRepro/ that runs a script printing fsi.CommandLineArgs and CurrentUICulture.Name.
   • Invoke via dotnet msbuild (or existing VS integration harness) so the ToolTask-injected /preferreduilang is present.
   • Assert the switch is not in CommandLineArgs and the culture is applied.

4. Notes/guardrails:

   • Do not leak preferreduilang into explicitArgs/CommandLineArgs regardless of position relative to the script token.
   • If culture parsing fails, warn but do not treat it as a script arg.
   • Keep behavior consistent between CLI and MSBuild paths.

References:

• Option parsing & script arg handling: src/Compiler/Interactive/fsi.fs (option table around ~1053–1108; final ReportUserCommandLineArgs around ~1208–1222).
• Public contract for fsi.CommandLineArgs: src/Compiler/Interactive/fsi.fsi (~397–412).
• MSBuild host that may inject /preferreduilang: src/FSharp.Build/Fsi.fs (ToolTask; captured args around ~421–424).

This pull request was created as a result of the following prompt from Copilot chat.

Implement proper handling of the preferreduilang switch in FSI and add regression tests.

Scope:

1. Consume and apply --preferreduilang//preferreduilang in FSI option parsing so it does not surface in fsi.CommandLineArgs, and set localization accordingly.

   • Add a recognized option in src/Compiler/Interactive/fsi.fs (option table near OptionGeneral("script.fsx arg1 arg2 ...")).
   • Support both /preferreduilang:<culture> and --preferreduilang:<culture>.
   • On parse: set tcConfigB.lcid and Thread.CurrentThread.CurrentUICulture (and possibly CurrentCulture) to the parsed culture; if parsing fails, warn and ignore.
   • Ensure this happens before ReportUserCommandLineArgs so the switch is consumed and not forwarded to scripts.

2. Add tests for argument separation and localization.

   • CLI regression tests (new folder tests/fsharp/core/interactive-args/ or extend existing tests/fsharpqa/Source/InteractiveSession/Misc/CommandLineArgs01*.fs): a) dotnet fsi --preferreduilang:en-US --exec script.fsx a → fsi.CommandLineArgs = ["script.fsx"; "a"] (switch consumed). b) dotnet fsi script.fsx --preferreduilang:en-US a → fsi.CommandLineArgs = ["script.fsx"; "a"] (even after script). c) Localization: dotnet fsi --preferreduilang:es-ES --exec script.fsx where the script triggers a warning/error; assert the culture is set (CurrentUICulture.Name starts with es-ES) and the message is localized or falls back if resources are missing. Consider a second locale (e.g., fr-FR).

3. MSBuild-path regression test.

   • Add a sample project under tests/projects/FSI/PreferreduilangRepro/ that runs a script printing fsi.CommandLineArgs and CurrentUICulture.Name.
   • Invoke via dotnet msbuild (or existing VS integration harness) so the ToolTask-injected /preferreduilang is present.
   • Assert the switch is not in CommandLineArgs and the culture is applied.

4. Notes/guardrails:

   • Do not leak preferreduilang into explicitArgs/CommandLineArgs regardless of position relative to the script token.
   • If culture parsing fails, warn but do not treat it as a script arg.
   • Keep behavior consistent between CLI and MSBuild paths.

References:

• Option parsing & script arg handling: src/Compiler/Interactive/fsi.fs (option table around ~1053–1108; final ReportUserCommandLineArgs around ~1208–1222).
• Public contract for fsi.CommandLineArgs: src/Compiler/Interactive/fsi.fsi (~397–412).
• MSBuild host that may inject /preferreduilang: src/FSharp.Build/Fsi.fs (ToolTask; captured args around ~421–424).

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