[MangelMaxime]

Description

Consumers that walk the typed tree through the symbol API (Fable, analyzers, language servers) get a fresh FSharpMemberOrFunctionOrValue at every call site and a fresh FSharpEntity on every DeclaringEntity, TypeDefinition or AttributeType access, and each symbol recomputes CurriedParameterGroups, Attributes and the like on every call. In a Fable compile of its own test suite those queries were 17 % of all allocated bytes. GetHashCode on a member also hashed its LogicalName, so dictionaries keyed on members collide on every overload.

Within one SymbolEnv, symbols for the same definition are now the same object (generic entities excepted, since each use gets fresh type arguments), their expensive queries are cached, and GetHashCode uses the definition stamp, consistent with the existing Equals. No public API change, and fsc never loads the symbol API.

Workload	Before	After
SymbolApiBenchmarks.WalkSymbols (added in this PR)	21.3 ms / 60.9 MB	18.9 ms / 55.4 MB
FSharpPlus, tooltip-style queries over all 90k symbol uses	143 ms / 165 MB	108 ms / 142 MB
Fable tests/Js/Main, symbol API allocations	993 MB	737 MB
Fable tests/Js/Main, compile time	4083 ms	3940 ms

Across seven real Fable projects compile time drops 0 to 4 % with byte-identical output.

Checklist

• Test cases added

• Performance benchmarks added in case of performance changes

• Release notes entry updated:

  Please make sure to add an entry with short succinct description of the change as well as link to this pull request to the respective release notes file, if applicable.

  Release notes files:

  • If anything under src/Compiler has been changed, please make sure to make an entry in docs/release-notes/.FSharp.Compiler.Service/<version>.md, where <version> is usually "highest" one, e.g. 42.8.200
  • If language feature was added (i.e. LanguageFeatures.fsi was changed), please add it to docs/release-notes/.Language/preview.md
  • If a change to FSharp.Core was made, please make sure to edit docs/release-notes/.FSharp.Core/<version>.md where version is "highest" one, e.g. 8.0.200.

  Information about the release notes entries format can be found in the documentation.
  Example:

  • More inlines for Result module. (PR #16106)
  • Correctly handle assembly imports with public key token of 0 length. (Issue #16359, PR #16363)
    *while! (Language suggestion #1038, PR #14238)

  If you believe that release notes are not necessary for this PR, please add NO_RELEASE_NOTES label to the pull request.

[github-actions]

✅ Release notes checked

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.200.md

[auduchinok]

There are many symbol environments created (and also kept in IDE scenarios). How much can this add to the retained memory?

[MangelMaxime]

It seems to be very little on the envs an IDE keeps. The member cache is only filled by the expression converter, and ImplementationFile / AssemblyContents build a fresh env per call, so on a file's TypeCheckInfo env it stays empty.

The entity cache fills through DeclaringEntity, TypeDefinition and AttributeType, bounded by the distinct non-generic entities the file touches, and the typed tree already keeps those entities alive.

Measured on FSharpPlus: every file checked with ParseAndCheckFileInProject and all 88 results held, then tooltip-style queries over all 90k symbol uses, twice:

	base	this PR
heap after checking 88 files	342.0 MB	342.0 MB
growth after querying all symbol uses	+0.7 MB	+1.7 MB
cache entries	-	0 members, 1897 entities (max 177 per file)

About 1 MB for the whole project held open, and the second pass adds nothing. The dictionaries are now created lazily, so an env that never shares a symbol doesn't pay for them.

[T-Gro]

🤖 🕵️ AI review — verify independently.

[T-Gro]

🤖 🕵️ [P2] Symbol inspection now throws on a missing transitive dependency — Shared's fallback eagerly reads Typars instead of returning the unresolved entity.

// Library exposes ModuleApi.missing : Dim1Dependency.Gone;
// its transitive dim1-Dependency.dll is unavailable to FCS.
let inspect (library: FSharp.Compiler.Symbols.FSharpAssembly) =
    let api =
        library.Contents.Entities
        |> Seq.find (fun e -> e.DisplayName = "ModuleApi")
    let value =
        api.MembersFunctionsAndValues
        |> Seq.find (fun v -> v.DisplayName = "missing")
    value.FullType.TypeDefinition.IsUnresolved
    // Base: true; PR: InvalidOperationException.

[T-Gro]

In general, not just this PR:

Can you pls make PR descriptions more smaller and clearer on what a PR fixes, and less narrating implementation details?

This repo should have a skill for PR descriptions.

(Problem statement + brief problem approach + numerical evidence in case of perf improvements ;; that's all that is needed.)

[MangelMaxime]

Sorry, I missed the PR skill and wanted to give enough context for where the changes came from.

The description is rewritten with it now, and the bot's finding is fixed with a regression test.