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

Copilot · 2025-12-16T12:29:27Z

Summary

This PR adds the --disableLanguageFeature CLI switch and corresponding MSBuild property, enabling developers to selectively disable specific F# language features on a per-project basis. This is useful for enforcing coding standards, maintaining compatibility with older codebases, or gradually migrating code that relies on features being phased out.

What's Added

CLI Switch

New --disableLanguageFeature:<FeatureName> option (repeatable)
Case-insensitive feature names
Error 3879 for invalid feature names

MSBuild Integration

<DisabledLanguageFeatures> ItemGroup property in .fsproj files
Passes through to Fsc task via Microsoft.FSharp.Targets

Implementation Details

Uses reflection to map feature names to LanguageFeature enum cases (no manual mapping required)
Immutable design with WithDisabledFeatures copy-and-update method
Features stored as Set in TcConfigBuilder, converted to array in LanguageVersion
Properly integrated with FSharpChecker cache keys via OtherOptions comparison

Usage Example

CLI:

fsc --disableLanguageFeature:NameOf --disableLanguageFeature:StringInterpolation code.fs

MSBuild (.fsproj):

<ItemGroup>
  <DisabledLanguageFeatures Include="NameOf" />
  <DisabledLanguageFeatures Include="StringInterpolation" />
</ItemGroup>

Testing

6 comprehensive unit tests covering various scenarios
Help baseline tests updated for both FSC and FSI
All tests passing

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 <46543583+T-Gro@users.noreply.github.com>

- Applied fantomas formatting to modified files - Added release notes entry for the new --disableLanguageFeature CLI switch Co-authored-by: T-Gro <46543583+T-Gro@users.noreply.github.com>

- 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 <46543583+T-Gro@users.noreply.github.com>

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 <46543583+T-Gro@users.noreply.github.com>

@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 <46543583+T-Gro@users.noreply.github.com>

@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 <46543583+T-Gro@users.noreply.github.com>

@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 <46543583+T-Gro@users.noreply.github.com>

@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 <46543583+T-Gro@users.noreply.github.com>

src/Compiler/Driver/CompilerOptions.fs

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

tests/FSharp.Compiler.ComponentTests/CompilerOptions/fsc/disableLanguageFeature.fs

…leLanguageFeature.fs

tests/fsharpqa/Source/CompilerOptions/fsi/help/help40-nologo.437.1033.bsl

tests/fsharpqa/Source/CompilerOptions/fsi/help/help40.437.1033.bsl

T-Gro · 2026-01-19T12:57:11Z

@copilot : Make proper 10.0.200 release notes pointing to this PR #19167 .
Also change the PR description to summarize what has been done and why, remove the how and intermediate checklists.

- Created docs/release-notes/.Language/10.0.200.md with the new feature entry - Removed duplicate entry from preview.md - Updated PR reference from "TBD" to actual PR #19167 Addresses comment 3768217127. Co-authored-by: T-Gro <46543583+T-Gro@users.noreply.github.com>

Copilot · 2026-01-19T13:07:11Z

@copilot : Make proper 10.0.200 release notes pointing to this PR #19167 .
Also change the PR description to summarize what has been done and why, remove the how ...

Updated in commit 4903ffc:

Created docs/release-notes/.Language/10.0.200.md with proper PR reference Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features #19167
Removed duplicate entry from preview.md
Updated PR description to be concise summary (what & why, removed implementation checklists)

auduchinok · 2026-01-20T17:45:32Z

I think this may add additional maintenance cost for the tooling with all the possible combinations of language features and versions. @T-Gro Could you please share some context why it was added? Was it because some specific feature was unwanted?

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 <46543583+T-Gro@users.noreply.github.com>

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

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 <46543583+T-Gro@users.noreply.github.com>

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

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

T-Gro reviewed Jan 14, 2026

View reviewed changes

tests/FSharp.Compiler.ComponentTests/CompilerOptions/fsc/disableLanguageFeature.fs Outdated Show resolved Hide resolved

T-Gro and others added 2 commits January 14, 2026 13:54

fsharpqa

ed98a4a

Update tests/FSharp.Compiler.ComponentTests/CompilerOptions/fsc/disab…

53682af

…leLanguageFeature.fs

T-Gro reviewed Jan 14, 2026

View reviewed changes

tests/fsharpqa/Source/CompilerOptions/fsi/help/help40-nologo.437.1033.bsl Outdated Show resolved Hide resolved

T-Gro reviewed Jan 14, 2026

View reviewed changes

tests/fsharpqa/Source/CompilerOptions/fsi/help/help40.437.1033.bsl Outdated Show resolved Hide resolved

T-Gro added 2 commits January 14, 2026 15:46

Apply suggestions from code review

54b60a2

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

86c1e9e

T-Gro marked this pull request as ready for review January 19, 2026 12:55

T-Gro requested a review from a team as a code owner January 19, 2026 12:55

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

90eed1c

T-Gro changed the title ~~[WIP] Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features~~ Add --disableLanguageFeature CLI switch and MSBuild property to selectively disable language features Jan 19, 2026

T-Gro approved these changes Jan 19, 2026

View reviewed changes

Copilot started work on behalf of T-Gro January 19, 2026 12:57 View session

Copilot finished work on behalf of T-Gro January 19, 2026 13:08

T-Gro enabled auto-merge (squash) January 19, 2026 13:34

T-Gro requested a review from abonie January 19, 2026 13:35

T-Gro approved these changes Jan 19, 2026

View reviewed changes

T-Gro added the NO_RELEASE_NOTES Label for pull requests which signals, that user opted-out of providing release notes label Jan 19, 2026

abonie approved these changes Jan 20, 2026

View reviewed changes

T-Gro merged commit d795504 into main Jan 20, 2026
40 of 41 checks passed

This was referenced Jan 20, 2026

[release/10.0.3xx] Source code updates from dotnet/fsharp dotnet/dotnet#4280

Merged

[release/10.0.2xx] Source code updates from dotnet/fsharp dotnet/dotnet#4311

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

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

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

Uh oh!

Copilot AI commented Dec 16, 2025 •

edited

Loading

Uh oh!

T-Gro left a comment

Uh oh!

Uh oh!

github-actions bot commented Jan 14, 2026

Uh oh!

Uh oh!

Uh oh!

Uh oh!

T-Gro commented Jan 19, 2026

Uh oh!

Copilot AI commented Jan 19, 2026

Uh oh!

Uh oh!

auduchinok commented Jan 20, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

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

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

Uh oh!

Conversation

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

Uh oh!

Summary

What's Added

CLI Switch

MSBuild Integration

Implementation Details

Usage Example

Testing

Uh oh!

T-Gro left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

github-actions bot commented Jan 14, 2026

🔧 CLI Command Report

Uh oh!

Uh oh!

Uh oh!

Uh oh!

T-Gro commented Jan 19, 2026

Uh oh!

Copilot AI commented Jan 19, 2026

Uh oh!

Uh oh!

auduchinok commented Jan 20, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

Copilot AI commented Dec 16, 2025 •

edited

Loading