Add warning FS3885 for let-in with multi-line body in sequence expressions

[Copilot]

Addresses the long-standing issue where let ... in with an explicit in keyword in indentation-sensitive F# code causes the parser to greedily capture all subsequent lines as part of the let body, leading to unexpected scoping behavior.

Following @dsyme's guidance to raise warnings for this formulation, this PR adds a new compiler warning FS3885 gated behind LanguageFeature.WarnOnLetInSequenceExpression (F# 11.0).

Problem

do
    let x = 1 in x + 1
    x  // incorrectly becomes part of the let body

The parser creates LetOrUse(let x = 1, Sequential(x + 1, x)) instead of the expected Sequential(LetOrUse(let x = 1, x + 1), x), causing x on the second line to resolve to the local binding rather than the outer scope.

Changes Made

• New language feature: Added LanguageFeature.WarnOnLetInSequenceExpression in LanguageFeatures.fs/fsi, gated at F# 11.0
• Warning FS3885: Added warning message in FSComp.txt that fires when let ... in with an explicit in keyword has a Sequential body extending past the in keyword's line
• Detection logic: In CheckExpressions.fs, checks trivia.InKeyword = Some inRange and whether the continuation expression starts on a later line than the in keyword
• Regression tests: 4 tests in WarnExpressionTests.fs covering warning in do blocks, function bodies, no false positives for single-line usage, and no warning on older language versions
• Release notes: Documented in 11.0.100.md

Testing

• ✅ All 4 new regression tests pass
• ✅ All 520 ErrorMessages namespace tests pass
• ✅ All 630 BasicGrammarElements namespace tests pass
• ✅ SurfaceAreaTest passes
• ✅ CodeQL reports no security issues

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[T-Gro]

@copilot : Try again

[T-Gro]

@copilot : Try again.

[abonie]

@copilot Try again

[Copilot]

@copilot Try again

Implemented in 4c00982 and 6259b63. Added warning FS3885 (gated behind LanguageFeature.WarnOnLetInSequenceExpression at F# 11.0) that fires when let ... in with an explicit in keyword has a body that extends to subsequent lines — the common pitfall from issue #7091 where the parser greedily captures all subsequent lines as part of the let body.

Changes:

• New LanguageFeature.WarnOnLetInSequenceExpression in LanguageFeatures.fs/fsi
• Warning FS3885 message in FSComp.txt
• Detection in CheckExpressions.fs: checks for explicit in keyword + Sequential body extending past the in line
• 4 regression tests (warning in do block, warning in function body, no warning for single-line let-in, no warning without preview langversion)
• Release notes

All 520 ErrorMessages tests and 630 BasicGrammar tests pass.

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
LanguageFeatures.fsi	docs/release-notes/.Language/preview.md

[auduchinok]

@T-Gro Have you considered changing how this is handled by LexFilter? Can we detect a new line there and drop the let context?

[T-Gro]

I think I would not be able to avoid breaking changes? A warning can still be silenced, a different decision in LexFilter would affect rest of the compilation process as well.

OR you think this can be done safely?

[auduchinok]

I'd vote for a breaking change here. Similar to various cases in fsharp/fslang-suggestions#1273, I think there should be not many cases where such code was written intentionally and compiled successfully.