[WIP] Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features #19167

Copilot · 2025-12-16T12:29:27Z

Implementation Status - Debugging Complete, Baselines Updated

Working on completing the --disableLanguageFeature CLI switch implementation.

Recent Changes:

Updated FSI help baselines (help40.437.1033.bsl, help40-nologo.437.1033.bsl)
Confirmed cache key handling works correctly (OtherOptions comparison)
All unit tests passing (6/6)

Changes Made:

Test Results:

Ran 6 tests: 6 passed, 0 failed
Duration: 4 seconds

Cache Key Analysis:

FSharpProjectOptions cache keys already handle disabled features correctly because AreSameForChecking compares OtherOptions arrays (line 180 in FSharpCheckerResults.fs), and --disableLanguageFeature:Feature is passed via OtherOptions. No additional cache key changes needed.

Next Steps:

Run fantomas formatting
Final verification of all tests
Code review

Original prompt

Motivation:
Enable a way to selectively disable specific F# language features on a per-project basis by providing a repeatable command line switch, even when the language version would otherwise enable the feature. This is useful for regression-testing, troubleshooting, or as an off-switch for problematic or experimental features.

Scope:

A repeatable CLI argument is added: --disableLanguageFeature:

For every entry, parse the argument to the LanguageFeature DU. Validate (fail with an error listing valid feature names if not present).

Temporarily collect/hold the array of disabled features during CLI/config processing until LanguageVersion is constructed.

Pass the array into LanguageVersion at all construction sites.

LanguageVersion stores the array and updates SupportsFeature: it returns false if the feature is listed in the disables array, regardless of language version.

Use Array.contains for the disables check. No Sets/Lists/Dicts are needed.

All feature-gating through SupportsFeature: no callsite changes.

Add help text for the new CLI switch (FSComp.txt and in CompilerOptions).

Add a negative ComponentTest for a previously positive feature, showing that disabling the feature (e.g. WhileBang) causes a compile error even in preview mode.

Rerun and update CLI/help output tests (e.g., fsc-help.txt) as needed.

Paths to touch:

src/Compiler/Driver/CompilerOptions.fs

src/Compiler/Facilities/LanguageFeatures.fs

src/Compiler/Facilities/LanguageFeatures.fsi

src/fsharp/FSharp.Compiler.Interactive.Settings/FSComp.txt

tests/FSharp.Compiler.ComponentTests/Language/WhileBangTests.fs

tests/fsharp/**/fsc-help.txt (and/or similar help output checks)

Constraints:

No significant churn or callsite changes; only extend existing flow.

Disabled feature array only lives outside LanguageVersion as a temporary transport.

Error diagnostics for invalid feature names must be user-friendly, with a list of valid LanguageFeature options.

Test must be minimal; reuse existing test files, just add one negative case for disables.

Keep the implementation idiomatic and minimal.

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

Motivation:
Enable a way to selectively disable specific F# language features on a per-project basis by providing a repeatable command line switch, even when the language version would otherwise enable the feature. This is useful for regression-testing, troubleshooting, or as an off-switch for problematic or experimental features.

Scope:

A repeatable CLI argument is added: --disableLanguageFeature:

For every entry, parse the argument to the LanguageFeature DU. Validate (fail with an error listing valid feature names if not present).

Temporarily collect/hold the array of disabled features during CLI/config processing until LanguageVersion is constructed.

Pass the array into LanguageVersion at all construction sites.

LanguageVersion stores the array and updates SupportsFeature: it returns false if the feature is listed in the disables array, regardless of language version.

Use Array.contains for the disables check. No Sets/Lists/Dicts are needed.

All feature-gating through SupportsFeature: no callsite changes.

Add help text for the new CLI switch (FSComp.txt and in CompilerOptions).

Add a negative ComponentTest for a previously positive feature, showing that disabling the feature (e.g. WhileBang) causes a compile error even in preview mode.

Rerun and update CLI/help output tests (e.g., fsc-help.txt) as needed.

Paths to touch:

src/Compiler/Driver/CompilerOptions.fs

src/Compiler/Facilities/LanguageFeatures.fs

src/Compiler/Facilities/LanguageFeatures.fsi

src/fsharp/FSharp.Compiler.Interactive.Settings/FSComp.txt

tests/FSharp.Compiler.ComponentTests/Language/WhileBangTests.fs

tests/fsharp/**/fsc-help.txt (and/or similar help output checks)

Constraints:

No significant churn or callsite changes; only extend existing flow.

Disabled feature array only lives outside LanguageVersion as a temporary transport.

Error diagnostics for invalid feature names must be user-friendly, with a list of valid LanguageFeature options.

Test must be minimal; reuse existing test files, just add one negative case for disables.

Keep the implementation idiomatic and minimal.

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

- Added TryParseFeature method to LanguageVersion to parse feature names from strings - Added disabledLanguageFeatures field to TcConfigBuilder to store disabled features - Added SupportsFeature method to TcConfigBuilder that checks both langVersion and disabled features - Added --disableLanguageFeature CLI option in CompilerOptions (repeatable) - Added error message for unrecognized feature names (error 3879) - Created comprehensive tests for the new functionality Co-authored-by: T-Gro <[email protected]>

- Applied fantomas formatting to modified files - Added release notes entry for the new --disableLanguageFeature CLI switch Co-authored-by: T-Gro <[email protected]>

- Modified LanguageVersion to hold disabled features and check them in SupportsFeature - Added SetDisabledFeatures method to LanguageVersion - Updated both langversion and disableLanguageFeature options to call SetDisabledFeatures - This ensures disabled features are respected regardless of option order - Applied code formatting Co-authored-by: T-Gro <[email protected]>

T-Gro

@copilot : Read all comments

As requested, removed the SupportsFeature method from TcConfigBuilder since it's not used anywhere and is redundant - callers can directly use langVersion.SupportsFeature() Co-authored-by: T-Gro <[email protected]>

@T-Gro

…re parsing - Changed LanguageVersion to use immutable array of disabled features instead of mutable set - Added optional constructor parameter for disabled features array - Added WithDisabledFeatures method that returns a new LanguageVersion instance - Replaced manual feature name mapping with reflection-based TryParseFeature using BindingFlags.NonPublic - Updated CompilerOptions to use immutable pattern with Array.append - Changed disabledLanguageFeatures in TcConfigBuilder from Set to array - Updated tests to use typecheck instead of compile and assert exact error codes - Applied code formatting Addresses feedback from @T-Gro about making the design immutable and using reflection. Co-authored-by: T-Gro <[email protected]>

@T-Gro

- Added null check in TryParseFeature using Option.bind before downcasting - Changed disabledLanguageFeatures from array to Set in CompilerConfig (better semantics) - Updated CompilerOptions to use Set.add and convert to array when calling WithDisabledFeatures - Applied code formatting Addresses comments from @T-Gro about null handling and using Set in the builder. Co-authored-by: T-Gro <[email protected]>

@T-Gro

Updated compiler_help_output.bsl to include the new --disableLanguageFeature CLI option in help output. Addresses comment from @T-Gro about updating help tests. Co-authored-by: T-Gro <[email protected]>

@T-Gro

- Added disabledLanguageFeatures field to Fsc task - Added DisabledLanguageFeatures property (takes ITaskItem array) - Updated command line builder to pass each feature with --disableLanguageFeature flag - Follows same pattern as other MSBuild properties This allows .fsproj files to use: <DisabledLanguageFeatures>NameOf;StringInterpolation</DisabledLanguageFeatures> Partial implementation of comment 2627788945 from @T-Gro. Still need to update .targets files. Co-authored-by: T-Gro <[email protected]>

src/Compiler/Driver/CompilerOptions.fs

T-Gro · 2026-01-14T08:33:31Z

/run ilverify

github-actions · 2026-01-14T08:43:54Z

🔧 CLI Command Report

Command: /run ilverify
Outcome: success

✅ Command succeeded, no changes needed.

…thub.com/dotnet/fsharp into copilot/add-disable-language-feature-cli

Initial plan

3beb767

Copilot AI assigned Copilot and T-Gro Dec 16, 2025

github-project-automation bot added this to F# Compiler and Tooling Dec 16, 2025

github-project-automation bot moved this to New in F# Compiler and Tooling Dec 16, 2025

Copilot started work on behalf of T-Gro December 16, 2025 12:29 View session

Copilot AI and others added 3 commits December 16, 2025 13:01

Apply code formatting and update release notes

7f3bcf8

- Applied fantomas formatting to modified files - Added release notes entry for the new --disableLanguageFeature CLI switch Co-authored-by: T-Gro <[email protected]>

Copilot AI changed the title ~~[WIP] Add command line argument to disable F# language features~~ Add --disableLanguageFeature CLI switch to selectively disable language features Dec 16, 2025

Copilot finished work on behalf of T-Gro December 16, 2025 13:22

Copilot AI requested a review from T-Gro December 16, 2025 13:22

Copilot started work on behalf of T-Gro December 16, 2025 14:50 View session

T-Gro requested changes Dec 16, 2025

View reviewed changes

github-project-automation bot moved this from New to In Progress in F# Compiler and Tooling Dec 16, 2025

Remove unused SupportsFeature method from TcConfigBuilder

2852d9c

As requested, removed the SupportsFeature method from TcConfigBuilder since it's not used anywhere and is redundant - callers can directly use langVersion.SupportsFeature() Co-authored-by: T-Gro <[email protected]>

Copilot AI requested a review from T-Gro December 16, 2025 15:06

Copilot finished work on behalf of T-Gro December 16, 2025 15:06

Copilot started work on behalf of T-Gro December 16, 2025 15:06 View session

Copilot finished work on behalf of T-Gro December 16, 2025 15:31

Copilot started work on behalf of T-Gro December 17, 2025 16:42 View session

Copilot AI and others added 3 commits December 17, 2025 16:59

Update help baseline with --disableLanguageFeature option

7c0dc12

Updated compiler_help_output.bsl to include the new --disableLanguageFeature CLI option in help output. Addresses comment from @T-Gro about updating help tests. Co-authored-by: T-Gro <[email protected]>

Copilot AI changed the title ~~Add --disableLanguageFeature CLI switch to selectively disable language features~~ Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features Dec 17, 2025

Copilot AI requested a review from T-Gro December 17, 2025 17:10

Copilot finished work on behalf of T-Gro December 17, 2025 17:10

T-Gro reviewed Dec 18, 2025

View reviewed changes

src/Compiler/Driver/CompilerOptions.fs Show resolved Hide resolved

Copilot started work on behalf of T-Gro December 18, 2025 08:41 View session

Merge branch 'main' into copilot/add-disable-language-feature-cli

4d3594d

dotnet deleted a comment from Copilot AI Jan 14, 2026

T-Gro added 2 commits January 14, 2026 09:58

fsharpqa tests

daaca5a

Merge branch 'copilot/add-disable-language-feature-cli' of https://gi…

f31cb7b

…thub.com/dotnet/fsharp into copilot/add-disable-language-feature-cli

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

[WIP] Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features #19167

[WIP] Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features #19167

Copilot AI commented Dec 16, 2025 •

edited

Loading

Uh oh!

T-Gro left a comment

Uh oh!

Uh oh!

T-Gro commented Jan 14, 2026

Uh oh!

github-actions bot commented Jan 14, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

[WIP] Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features #19167

Are you sure you want to change the base?

[WIP] Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features #19167

Conversation

Copilot AI commented Dec 16, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Implementation Status - Debugging Complete, Baselines Updated

Recent Changes:

Changes Made:

Test Results:

Cache Key Analysis:

Next Steps:

Uh oh!

T-Gro left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

T-Gro commented Jan 14, 2026

Uh oh!

github-actions bot commented Jan 14, 2026

🔧 CLI Command Report

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

Copilot AI commented Dec 16, 2025 •

edited

Loading