Allow interpolated strings in constant expressions by xperiandri · Pull Request #20753 · dotnet/fsharp

xperiandri · 2026-10-10T07:10:16Z

Under --langversion:preview, an interpolated string whose holes are non-null constant strings with no alignment or format specifier is a constant expression, as in C# 10: [<Literal>] values, attribute arguments and type provider static arguments accept it instead of reporting FS0267, e.g. [<Obsolete($"Use {nameof getItemV2} instead")>]. Everywhere else, quotations included, interpolated strings are unchanged.

Details

Language suggestion Support (a subset of) interpolated strings in Attribute parameters fsharp/fslang-suggestions#1347 (approved in principle), RFC FS-1352.
A hole that breaks a rule is reported by the constant evaluator as FS0267 at that hole, and only where the string is evaluated: an unused branch such as false && ($"{1}" = "1") stays valid. Before the preview language version nothing changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

github-actions · 2026-10-10T07:10:29Z

❗ Release notes required

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@xperiandri,

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Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.200.md	No release notes found or release notes format is not correct

✅ Found changes and release notes in following paths:

Change path Release notes path Description

`src/Compiler/Facilities/LanguageFeatures.fsi` docs/release-notes/.Language/preview.md

Copilot

🟡 Changes recommended

Diagnostic behavior diverges from the RFC, and type-provider static-argument support lacks direct coverage.

3 open findings

What changed in this PR

Adds preview support for constant interpolated strings across literals, attributes, and type-provider static arguments.

Changes:

Adds constant-context tracking and compile-time interpolation folding.
Adds feature gating, FS3925, localization, and release notes.
Adds component tests for literals, attributes, signatures, diagnostics, and quotations.

File	Description
`tests/FSharp.Compiler.ComponentTests/Language/ConstantInterpolatedStringsTests.fs`	Adds feature tests.
`tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj`	Registers the tests.
`src/Compiler/xlf/FSComp.txt.zh-Hant.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.zh-Hans.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.tr.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.ru.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.pt-BR.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.pl.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.ko.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.ja.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.it.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.fr.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.es.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.de.xlf`	Adds localized resources.
`src/Compiler/xlf/FSComp.txt.cs.xlf`	Adds localized resources.
`src/Compiler/FSComp.txt`	Defines FS3925 and feature text.
`src/Compiler/Facilities/LanguageFeatures.fsi`	Declares the feature case.
`src/Compiler/Facilities/LanguageFeatures.fs`	Gates the preview feature.
`src/Compiler/Checking/Expressions/CheckExpressions.fs`	Implements constant-context folding.
`src/Compiler/Checking/CheckDeclarations.fs`	Initializes constant-context state.
`src/Compiler/Checking/CheckBasics.fsi`	Exposes the internal context field.
`src/Compiler/Checking/CheckBasics.fs`	Defines the context field.
`docs/release-notes/.Language/preview.md`	Documents the language feature.
`docs/release-notes/.FSharp.Compiler.Service/11.0.200.md`	Records the FCS change.

🧠 Review effort: Balanced

…-constants-wider-holes Bring in the review fixes of dotnet#20753: only the first hole that breaks a rule is reported, FS3925 says "must be a constant", and a type provider test covers static arguments. The wider hole rules stay as they are. The first-hole test uses aligned holes here, because integer holes are valid on this branch. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

T-Gro

🤖 🕵️ AI review — verify independently.

T-Gro

The diff is still way too big - apply the NoBloat instructions and CodeCompaction skill.

Especially the massive code comments would benefit not just with drastic reduction, but ideally with well named functions/arguments/bindings/types making the intention clear.

If a well named piece of code just tells what the comment says - delete the comment.

Delete the implementation and test narration around constant context, hole evaluation, accumulated state, and provider fixture behavior. Use names such as tryConstantHoleText, allPreviousHolesWereConstant, and StaticStringValueProvider.
Flatten the nested local helpers and mutable accumulator in TcConstantInterpolatedString.
Reuse the existing static-string type-provider fixture and compact the separate diagnostic Facts into named theory rows.
Keep one concise language-feature release note with syntax and restrictions. Delete the duplicate FCS prose and unchanged-quotation commentary.

An interpolated string whose holes are non-null constant strings without alignment or format specifiers now folds to its text where F# requires a constant: [<Literal>] values (also in signatures), attribute arguments and type provider static arguments. It was rejected with FS0267, so a constant built from other constants needed '+'. The checker marks those three contexts in TcEnv and folds the string there. Everywhere else, quotations included, interpolated strings are still lowered to String.Concat. A hole that breaks a rule reports FS3925 at the hole; a hole that is not a constant reports FS0267 at the hole. Gated on the ConstantInterpolatedStrings preview feature. Language suggestion dotnet#1347 RFC FS-1352 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

RFC FS-1352 reports the rule error on the first hole that breaks a rule. Later holes are still type-checked, but are no longer evaluated as constants, so '$"{1}{2}"' gives FS3925 once instead of once per hole. Reword FS3925 to the RFC's "must be a constant": the message is shown when a hole keeps the string from being one. Cover type provider static arguments, after 'const' and through a literal, with a provider that returns its string static argument. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Review follow-up. The hole text comes from a top-level tryConstantHoleText, every hole is type-checked before the first one is evaluated (so a later undefined name is still reported), and the constant path runs only when the ConstantInterpolatedStrings feature is supported: before the preview language version an interpolated string in a literal is checked as before, so a branch that is never evaluated, such as false && ($"{1}" = "1"), stays valid. The type provider test uses a small fixture that checks the argument it receives: the helloWorld provider overflows the stack of the test host when it is compiled in process. One language release note remains. Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

xperiandri · 2026-10-15T14:03:29Z

Done in 4e4506c, rebased on main. 388 added lines instead of 458, 130 of them generated xlf. Top-level tryConstantHoleText; no local helpers, mutables or comments in TcConstantInterpolatedString; one language release note, so the bot now flags src/Compiler. The helloWorld provider can't be reused: compiled in process it overflows the test host stack in QuotationTranslator, so the fixture is a 41-line stateless provider. AI-review findings fixed and tested: the constant path runs only when the feature is supported (false && ($"{1}" = "1") stays valid), and all holes are type-checked before the first is evaluated (FS0039 for missing is back). In preview, an unevaluated interpolation in a literal is still checked as constant; say if that must go too.

An interpolated string in a constant context is folded quietly. A hole that does not fold leaves a string-typed stand-in at the hole, which the constant evaluator rejects with FS0267 when, and only when, it reaches it. So a literal such as false && ($"{1}" = "1") or if true then Root else $"{1}" stays valid in preview as well, as it is before (review by T-Gro). FS3925 is dropped: the error is the existing FS0267 on the first hole that breaks a rule. Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

xperiandri · 2026-10-18T01:20:33Z

Fixed in 76dec45, as a new commit on top. An interpolated string in a constant context is now folded quietly, and a hole that does not fold leaves a stand-in that the constant evaluator rejects with FS0267 at that hole, only if it reaches it. false && ($"{1}" = "1") and if true then "ok" else $"{1}" compile in preview and in 11.2 (test An interpolated string that is not evaluated is not checked as a constant). FS3925 is gone: the error is the existing FS0267 on the first hole that breaks a rule, which also removes its 13 xlf units. The RFC amendment has the same wording (fsharp/fslang-design#859).

T-Gro

🤖🕵️ AI review — verify independently.

T-Gro · 2026-10-18T03:22:29Z

+        ValueNone
+
+let tryConstantHoleText (g: TcGlobals) (formatting: SynInterpolationFormatting) (hole: Expr) =
+    match formatting, tryEvalConstant g hole with


🤖🕵️ FS0193 (Attempted to divide by zero) while checking a non-string hole; Debug FCS aborts (exit 134).

[<Literal>] let Text = $"{1 / 0}"

Copilot AI balanced review requested due to automatic review settings October 10, 2026 07:10

xperiandri requested a review from a team as a code owner October 10, 2026 07:10

github-project-automation Bot added this to F# Compiler and Tooling Oct 10, 2026

xperiandri added a commit to xperiandri/fsharp that referenced this pull request Oct 10, 2026

Link the release notes to PR dotnet#20753

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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Copilot started reviewing on behalf of xperiandri October 10, 2026 07:10

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This was referenced Oct 10, 2026

Accept number, char, bool and %s holes in constant interpolated strings #20754

Draft

RFC FS-1352: accept integer, decimal, char, bool and %s holes fsharp/fslang-design#859

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T-Gro reviewed Oct 13, 2026

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T-Gro self-requested a review October 13, 2026 11:18

T-Gro added the AI-reviewed PR reviewed by AI review council label Oct 13, 2026

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github-project-automation Bot moved this from New to In Progress in F# Compiler and Tooling Oct 14, 2026

xperiandri and others added 4 commits October 15, 2026 01:29

Link the release notes to PR dotnet#20753

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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

xperiandri force-pushed the interpolated-string-constants branch from cc218d6 to 4e4506c Compare October 15, 2026 01:53

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Allow interpolated strings in constant expressions - #20753

Allow interpolated strings in constant expressions#20753
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