Cache attributes read by compiler— replace repeated string matching with O(1) enum lookups (#19394)

* Define WellKnownILAttributes, WellKnownEntityAttributes, WellKnownValAttributes flag enums

Add three [<Flags>] enums for O(1) well-known attribute lookups:
- WellKnownILAttributes (uint32) in AbstractIL/il.fs/.fsi
- WellKnownEntityAttributes (uint64) in TypedTree/TypedTree.fs/.fsi
- WellKnownValAttributes (uint64) in TypedTree/TypedTree.fs/.fsi

Each enum uses power-of-2 bit values with NotComputed at the sign bit.
Purely additive — no existing code modified.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add computeILWellKnownFlags and migrate IL attribute check sites

- Add computeILWellKnownFlags function in TypedTreeOps.fs that maps
  all 19 WellKnownILAttributes enum values to their TcGlobals attrib_* fields
- Add HasWellKnownAttribute extension members on ILAttributesStored,
  ILTypeDef, ILMethodDef, and ILFieldDef
- Migrate 12 IL-domain existence-only check sites from O(N) TryFindILAttribute
  scans to O(1) HasWellKnownAttribute calls
- Fix pre-existing ILAttributesStored sealed class callers (Given -> CreateGiven,
  customAttrsReader -> CreateReader pattern in ilread.fs)
- Keep 3 data-extracting sites (ReflectedDefinition, AutoOpen, InternalsVisibleTo)
  and 2 individual-attribute iteration sites unchanged
- TypeHierarchy.fs sites use TryFindILAttribute on storedAttrs.CustomAttrs
  (scope comparison prevents full HasWellKnownAttribute migration)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix IL Compute And Migration: scope-safe comparison, TypeHierarchy migration, remove dead enums

- Fix computeILWellKnownFlags to use name+enclosing comparison (not full
  ILTypeRef equality with scope) matching isILAttrib semantics. This fixes
  silent failures when BCL reference assemblies use ILScopeRef.Local while
  TcGlobals uses ILScopeRef.Assembly.
- Migrate TypeHierarchy.fs sites #6 and #7 (IsReadOnlyAttribute,
  RequiresLocationAttribute) to use HasWellKnownAttribute.
- Remove unused WellKnownEntityAttributes and WellKnownValAttributes enums
  from TypedTree.fs/TypedTree.fsi (dead code not part of this sprint).
- Update surface area baseline.
- Run dotnet fantomas on all changed files.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add WellKnownEntityAttribs struct and migrate Entity.entity_attribs

Sprint 2: Entity Container And Infrastructure

- Add WellKnownEntityAttributes flags enum (27 flags + NotComputed sentinel)
- Add WellKnownEntityAttribs struct wrapper with O(1) attribute lookup
- Change Entity.entity_attribs field type from Attribs to WellKnownEntityAttribs
- Entity.Attribs still returns Attrib list via .AsList()
- Add Entity.EntityAttribs and Entity.SetEntityAttribs members
- Fix Entity.IsLinked for struct field (null check on AsList())
- Add computeEntityWellKnownFlags and EntityHasWellKnownAttribute in TypedTreeOps
- Update TypedTreePickle.fs to pickle/unpickle through the wrapper
- Update all entity_attribs assignments across the compiler

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Migrate 30 entity-level existence checks to EntityHasWellKnownAttribute

Replace O(N) HasFSharpAttribute calls with O(1) EntityHasWellKnownAttribute
bit tests for entity-level existence-only attribute checks across:
- AugmentWithHashCompare.fs (1 site)
- CheckDeclarations.fs (12 sites)
- ConstraintSolver.fs (4 sites)
- CheckExpressions.fs (1 site)
- NameResolution.fs (9 sites)
- PostInferenceChecks.fs (2 sites)
- IlxGen.fs (2 sites)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Sprint 4: Val Container And Infrastructure - WellKnownValAttribs

- Add WellKnownValAttributes flags enum (25 flags + NotComputed sentinel)
- Add WellKnownValAttribs wrapper struct mirroring WellKnownEntityAttribs
- Change ValOptionalData.val_attribs field type to WellKnownValAttribs
- Add Val.ValAttribs property returning WellKnownValAttribs
- Add Val.SetValAttribs accepting WellKnownValAttribs
- Val.Attribs still returns Attrib list, Val.SetAttribs still accepts Attribs
- Change ArgReprInfo.Attribs field type to WellKnownValAttribs
- Add computeValWellKnownFlags covering all 25 enum flags
- Add ValHasWellKnownAttribute helper in TypedTreeOps
- Update TypedTreePickle for val_attribs and ArgReprInfo serialization
- Fix all downstream usage sites across compiler

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Sprint 05: Migrate Val/ArgReprInfo-level existence checks to well-known attribute flags

- Add ArgReprInfoHasWellKnownAttribute helper in TypedTreeOps.fs/.fsi
- Migrate 11 Val sites to ValHasWellKnownAttribute (O(1) flag test)
- Migrate 19 ArgReprInfo sites to ArgReprInfoHasWellKnownAttribute
- Migrate 7 Attrib list sites using computeValWellKnownFlags
- Migrate 2 Entity sites to EntityHasWellKnownAttribute
- Total: 39 existence-only attribute check sites migrated

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Sprint 6: Hybrid mapping functions and TyconRef migration

Add mapILFlagToEntityFlag and mapILFlagToValFlag mapping functions to
TypedTreeOps.fs for translating WellKnownILAttributes flags to their
Entity and Val equivalents.

Add TyconRefHasWellKnownAttribute unified hybrid check function that
dispatches on IL vs F# vs provided type metadata using O(1) flag tests.

Migrate 5 existence-only TyconRef hybrid sites:
- NameResolution.fs: ExtensionAttribute check
- PostInferenceChecks.fs: IsByRefLikeAttribute and IsReadOnlyAttribute checks
- TypedTreeOps.fs: IsByRefLikeAttribute (isByrefLikeTyconRef) and
  IsReadOnlyAttribute (isTyconRefReadOnly) checks

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Sprint 7: Final validation fixes - conditional compile mapILFlagToAttribInfo

- Wrap mapILFlagToAttribInfo in #if !NO_TYPEPROVIDERS to fix FS1182
  unused value error on netstandard2.0 target
- Simplify ProvidedTypeMetadata case in TyconRefHasWellKnownAttribute
  to reuse TyconRefHasAttribute via the new helper
- All component tests pass (6630/6630)
- All import tests pass (63/63)
- All CompilerCompat cross-version tests pass (6/6)
- All service tests pass (2089/2089)
- All scripting tests pass (99/99)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Sprint 7: Fix isByrefLikeTyconRef to use name-based attribute matching

Revert isByrefLikeTyconRef from TyconRefHasWellKnownAttribute (type-identity)
back to TyconRefHasAttributeByName (name-based) to preserve RFC FS-1053
compatibility. User code can define its own IsByRefLikeAttribute, so this
function must match by attribute name, not by type identity.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Sprint 7: Fix PostInferenceChecks IsByRefLike to use name-based matching per RFC FS-1053

The IsByRefLikeAttribute check in CheckEntityDefn was incorrectly changed to
type-identity matching (TyconRefHasWellKnownAttribute) during Sprint 6. This
breaks user-defined IsByRefLikeAttribute per RFC FS-1053. Reverted to
name-based matching (TyconRefHasAttributeByName) for consistency with
isByrefLikeTyconRef in TypedTreeOps.fs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Rewrite computeEntityWellKnownFlags with multi-level path dispatch

Replace flat if/elif chain of 27 tyconRefEq comparisons with a two-phase
dispatch: first check assembly identity (ccuEq), then array pattern match
on the namespace path. User-defined attributes from unrelated assemblies
are rejected in 1-2 cheap checks instead of 27 comparisons.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Rewrite computeValWellKnownFlags with multi-level path dispatch

Replace flat if/elif chain of tyconRefEq calls with two-phase
assembly/path routing and array pattern matching, matching the
approach used in computeEntityWellKnownFlags. User attributes
are rejected in 1-2 cheap checks instead of O(N×K) comparisons.

All 25 well-known val attributes are covered.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Rewrite computeILWellKnownFlags with prefix + string match dispatch

Replace flat if/elif chain of nameMatch calls against 16 AttribInfo references
with a prefix-based dispatch on ILTypeRef.Name. User-defined attributes bail out
after 1-2 prefix checks instead of scanning all 16+ comparisons.

Key changes:
- Remove 16 AttribInfo extractions from TcGlobals at function top
- Remove inline nameMatch helper
- Group by StartsWith prefix: System.Runtime.CompilerServices (13),
  Microsoft.FSharp.Core (4), then remaining (3)
- Cover SetsRequiredMembersAttribute from both CompilerServices and
  CodeAnalysis namespaces

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Migrate getAugmentationAttribs to cached WellKnownEntityAttributes flags

Replace 7 TryFindFSharpBoolAttribute linear scans with cached
EntityHasWellKnownAttribute lookups. Update all pattern match sites
in CheckAugmentationAttribs, TyconIsCandidateForAugmentationWithCompare,
and TyconIsCandidateForAugmentationWithEquals from bool option to bool.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Migrate remaining existence-only attribute call sites to cached flags

Replace TryFindFSharpBoolAttribute/HasFSharpAttribute/TryFindFSharpAttribute
calls with EntityHasWellKnownAttribute/ValHasWellKnownAttribute at 10 sites:

- CheckDeclarations.fs: StructuralComparison, StructuralEquality
- NameResolution.fs: AutoOpen (CanAutoOpenTyconRef, module auto-open)
- Optimizer.fs: CLIMutable
- IlxGen.fs: CLIMutable, ComVisible (fast negative guard), LiteralAttribute, EntryPoint
- fsi.fs: EntryPoint (fast negative guard)

GeneralizableValueAttribute site left as-is (enum case is on Entity, not Val).
ComVisible uses conservative fast negative guard to preserve exact semantics.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add missing enum cases to WellKnownEntityAttributes and WellKnownValAttributes

Add ClassAttribute, InterfaceAttribute, StructAttribute, MeasureAttribute
to WellKnownEntityAttributes enum (bits 0x8000000u-0x40000000u).

Add NoCompilerInliningAttribute to WellKnownValAttributes enum (0x2000000uL).

Update computeEntityWellKnownFlags and computeValWellKnownFlags to recognize
the new attribute names in FSharp.Core paths.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Bool TrueFalse encoding for three-state attributes

Add _True/_False flag pairs for attributes that carry boolean semantics:

Entity enum: DefaultAugmentationAttribute_True/_False,
  AutoSerializableAttribute_True/_False (widened enum to uint64)

Val enum: ReflectedDefinitionAttribute_True/_False,
  DefaultValueAttribute_True/_False, NoDynamicInvocationAttribute_True/_False

Compute functions now extract bool args from attribute constructors
and set the appropriate flag. Added EntityTryGetBoolAttribute and
ValTryGetBoolAttribute helpers.

Migrated all TODO WELLKNOWN_ATTRIB bool extraction call sites:
- TypedTreeOps.fs: HasDefaultAugmentationAttribute, isSealedTy
- IlxGen.fs: DefaultAugmentation, NoDynamicInvocation, AutoSerializable
- PostInferenceChecks.fs: DefaultAugmentation
- infos.fs: ReflectedDefinition on ArgReprInfo

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix NoDynamicInvocation/ReflectedDefinition default to false (AssumeFalse semantics)

The original code used TryFindFSharpBoolAttributeAssumeFalse for these two
attributes, which defaults to false when no explicit bool argument is present.
The | _ -> fallback incorrectly set _True; corrected to _False.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix ReflectedDefinitionAttribute existence check to include _False flag

The existence check at PostInferenceChecks.fs line 2122 must check both
_True and _False flags since [<ReflectedDefinition>] without explicit
bool arg sets _False (matching TryFindFSharpBoolAttributeAssumeFalse
semantics). Without this fix, bare [<ReflectedDefinition>] on struct
members would not trigger the FS1220 error.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Flatten ILAttributesStored: eliminate Computed DU case

Replace the Computed(ILAttributes * WellKnownILAttributes) DU case with a
separate mutable wellKnownFlags field on ILAttributesStored. This avoids
allocating a new DU case on every GetOrComputeWellKnownFlags call, reducing
GC pressure for IL type definitions, methods, and fields.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fast negative guards and perf improvements for attribute checks

- Add fast negative guard for StructLayoutAttribute in IlxGen.fs
- Add fast negative guard for AttributeUsageAttribute in CheckExpressions.fs
- Optimize CombineCcuContentFragments to avoid list concat when one side is empty
- Use WellKnownEntityAttribs.Add for extension attribute additions
- Remove TODO: WELLKNOWN_ATTRIB comments from guarded sites

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Use WellKnownValAttribs.Empty instead of WellKnownValAttribs.Create([])

Replace WellKnownValAttribs.Create([]) with WellKnownValAttribs.Empty in NicePrint.fs,
CompilerDiagnostics.fs, Symbols.fs, and TypedTreeBasics.fs. This avoids unnecessary
list allocations. Also remove findings.yaml leftover artifact.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* deduplicate logic for bool arg attrs

* Refactor WellKnownEntityAttribs and WellKnownValAttribs into generic WellKnownAttribs<'TItem, 'TFlags>

Extract the near-identical struct implementations into a single generic
struct WellKnownAttribs<'TItem, 'TFlags> defined in a separate file
(WellKnownAttribs.fs/fsi) outside the module rec context.

The two flag enums (WellKnownEntityAttributes, WellKnownValAttributes)
are also moved to the new file to avoid FS0073 bootstrap compiler
limitations with enum<uint64> constraints in module rec.

WellKnownEntityAttribs and WellKnownValAttribs become type aliases with
companion modules for factory functions (Empty, Create, CreateWithFlags).
All instance methods (HasWellKnownAttribute, AsList, Flags, Add, Append,
WithRecomputedFlags) live on the generic struct and work through the
type aliases unchanged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Migrate 10 additional attribute callsites to cached well-known flags

- DllImport: IlxGen.IsValRefIsDllImport and TailCallChecks to ValHasWellKnownAttribute
- CLIEventAttribute: Add flag, add ValCompileAsEvent, migrate 4 val-based callers
- ComVisibleAttribute: Split into True/False flags, migrate IlxGen bool check
- ComImportAttribute: Add True flag, migrate isComInteropTy
- CompilationRepresentation: Add ModuleSuffix/PermitNull/Instance/Static entity flags,
  migrate TyconHasUseNullAsTrueValueAttribute
- ObsoleteAttribute: Add to WellKnownILAttributes + WellKnownEntityAttributes,
  add CheckILAttributesForUnseenStored using O(1) flag check,
  migrate NameResolution.IsTyconUnseenObsoleteSpec

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove dead TryFindFSharpBoolAttributeAssumeFalse helper

All callers migrated to decodeBoolAttribFlag in compute functions.
Inline TryFindFSharpBoolAttribute (default=true) directly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Phase A: Use already-computed entity flags in Phase1G

Replace redundant HasFSharpAttribute/TryFindFSharpBoolAttribute calls with
direct flag tests on the entityFlags that are already computed 3 lines above.

- SealedAttribute: split into _True/_False flag pair for three-state semantics
- MeasureableAttribute: add to WellKnownEntityAttributes enum
- hasMeasureAttr, hasMeasureableAttr, hasAllowNullLiteralAttr: test entityFlags bits
- structLayoutAttr: kept as TryFindFSharpInt32Attribute (needs LayoutKind int)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Phase B: Compute entity flags ad-hoc in Phase1B and early typar check

Replace HasFSharpAttribute with computeEntityWellKnownFlags + bit test
in TcTyconDefnCore_Phase1B and TyparsAllHaveMeasureDeclEarlyCheck.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Phase C: Compute val flags ad-hoc in CheckExpressions

Replace HasFSharpAttribute/HasFSharpAttributeOpt with computeValWellKnownFlags
+ bit tests for: LiteralAttribute (3 sites), DllImportAttribute (1 site),
NoCompilerInliningAttribute (1 site). ComputeInlineFlag computes flags once
for both NoCompilerInlining and DllImport checks.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Phase D: Compute val flags in GenParamAttribs

Replace HasFSharpAttribute/HasFSharpAttributeOpt with computeValWellKnownFlags
+ bit tests for InAttribute, OutAttribute, OptionalAttribute in GenParamAttribs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Phase E: Compute val flags ad-hoc for field-level attrs

- CheckDeclarations field attrs: compute computeValWellKnownFlags once for
  DefaultValueAttribute, VolatileFieldAttribute, ThreadStatic, ContextStatic
- PostInferenceChecks: DefaultValueAttribute_True flag for zero-init check
- TypedTreeOps: DefaultValueAttribute_False flag for field filtering
- IlxGen NonSerializedAttribute: kept as-is (single site, merged list)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix review findings: MeasureableAttribute name, AllowNullLiteral True/False

- Fix MeasureableAttribute: match on 'MeasureAnnotatedAbbreviationAttribute'
  (the actual FSharp.Core type name) instead of 'MeasureableAttribute'
- AllowNullLiteralAttribute: split into _True/_False pair to preserve
  three-state semantics ([<AllowNullLiteral(false)>] vs absence)
- Fix all references to renamed enum cases

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add attribsHaveEntityFlag/attribsHaveValFlag helpers, refactor callsites

Address review findings: extract inline helpers for ad-hoc flag testing
on raw Attrib lists. Refactor all verbose computeXWellKnownFlags + bit test
patterns to use the new helpers. Fix ThreadStatic/ContextStatic to use
combined ||| pattern.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove dead TryFindFSharpBoolAttribute helper (zero callers remain)

All bool attribute checks now use cached flags via decodeBoolAttribFlag
in compute functions or attribsHaveValFlag ad-hoc helpers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove dead code and migrate remaining CompilationRepresentation/CompileAsEvent

Dead code removed:
- isThreadOrContextStatic (zero callers)
- TryFindILAttributeOpt (zero callers)
- enum_CompilationRepresentationAttribute_* constants (all 3, zero callers)

Migrated to flags:
- CompileAsEvent now uses attribsHaveValFlag internally
- ModuleNameIsMangled now uses attribsHaveEntityFlag
- MemberIsCompiledAsInstance Instance/Static uses computeEntityWellKnownFlags

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Delete HasFSharpAttributeOpt (zero callers), migrate CheckFSharpAttributesForObsolete

Eliminated helpers:
- HasFSharpAttributeOpt: last caller inlined, function deleted
- TryFindFSharpBoolAttribute: already deleted (prior commit)
- TryFindFSharpBoolAttributeAssumeFalse: already deleted (prior commit)

Migrated:
- CheckFSharpAttributesForObsolete: ObsoleteAttribute + IsByRefLikeAttribute
  now use attribsHaveEntityFlag instead of HasFSharpAttribute/Opt
- IlxGen NonSerialized: inlined HasFSharpAttributeOpt to HasFSharpAttribute

Remaining HasFSharpAttribute callers (7): 3 typar-level, 3 assembly-level,
1 NonSerialized fallback — all deliberately deferred.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Prototype: classifyEntityAttrib + tryFindEntityAttribByFlag

Factor computeEntityWellKnownFlags into two layers:
- classifyEntityAttrib: classify a SINGLE attrib, return its flag
- computeEntityWellKnownFlags: OR all flags (uses classifyEntityAttrib)
- tryFindEntityAttribByFlag: find first attrib matching a flag

This enables enum-powered data extraction: callers use the WellKnownEntityAttributes
enum to both check existence (O(1) via cached flags) AND find the actual attrib
for data extraction (O(N) only on hit, via classifyEntityAttrib).

Demonstrate with 2 migrations:
- IlxGen StructLayoutAttribute: tryFindEntityAttribByFlag replaces TryFindFSharpAttribute
- CheckExpressions AttributeUsageAttribute: same pattern

This commit is isolated and revertable.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Migrate NonSerializedAttribute to val flags

Add NonSerializedAttribute to WellKnownValAttributes enum + compute function.
Replace last HasFSharpAttribute caller with attribsHaveValFlag.

HasFSharpAttribute callers now: 6 (3 typar-level, 3 assembly-level — deferred).
HasFSharpAttributeOpt callers: 0 (fully deleted).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add tryFindEntityAttribString/Int32, delete TryFindFSharpInt32Attribute

Add enum-powered data extraction helpers that use classifyEntityAttrib
for O(1) negative guard, then extract string/int data on hit.

Migrate StructLayoutAttribute int extraction in CheckDeclarations from
TryFindFSharpInt32Attribute to tryFindEntityAttribInt32.

TryFindFSharpInt32Attribute now has zero callers and is deleted.

This commit is isolated and revertable.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add (|EntityAttrib|_|) and (|ValAttrib|_|) active patterns for enum-powered data extraction

Factor computeEntityWellKnownFlags into classifyEntityAttrib (per-attrib) + fold.
Factor computeValWellKnownFlags into classifyValAttrib (per-attrib) + fold.

Active patterns use classifyEntityAttrib/classifyValAttrib to find the
matching attrib by flag, then callers destructure the full Attrib in the
pattern match — existence check AND data extraction in one expression.

Before:
  match TryFindFSharpAttribute g g.attrib_MethodImplAttribute attrs with
  | Some(Attrib(_, _, [ AttribInt32Arg flags ], _, _, _, _)) -> flags
  | _ -> 0x0

After:
  match attrs with
  | ValAttrib g WellKnownValAttributes.MethodImplAttribute (Attrib(_, _, [ AttribInt32Arg flags ], _, _, _, _)) -> flags
  | _ -> 0x0

Migrated 7 callsites:
- IlxGen: MethodImplAttribute, PreserveSigAttribute, FieldOffsetAttribute, StructLayoutAttribute
- CheckExpressions: HasMethodImplNoInliningAttribute, AttributeUsageAttribute
- CheckDeclarations: StructAttribute on ArgReprInfo
- infos.fs: CallerMemberNameAttribute

Added val flags: MethodImplAttribute, PreserveSigAttribute, FieldOffsetAttribute
Deleted: TryFindFSharpInt32Attribute (zero callers), g.attrib_CallerMemberNameAttribute

This commit is isolated and revertable.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add typed extraction APs: EntityAttribInt, EntityAttribString, ValAttribInt, ValAttribString

Slim active patterns that extract the common single-arg data directly:

Before:
  | ValAttrib g WellKnownValAttributes.MethodImplAttribute
      (Attrib(_, _, [ AttribInt32Arg flags ], _, _, _, _)) -> flags

After:
  | ValAttribInt g WellKnownValAttributes.MethodImplAttribute flags -> flags

Migrated: MethodImplAttribute (×2), FieldOffsetAttribute, StructLayoutAttribute.
Full (|EntityAttrib|_|) and (|ValAttrib|_|) remain for complex cases
needing named args or multiple constructor args.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Make typed APs compose on top of base (|EntityAttrib|_|) / (|ValAttrib|_|)

EntityAttribInt/String now match via EntityAttrib internally instead of
calling tryFindEntityAttribByFlag independently. Same for Val variants.
Proper layering: base AP is the building block, typed APs are sugar.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* A1: Add 7 entity/val flag cases, migrate 11 TryFindFSharpAttribute callers to APs

WellKnownEntityAttributes: CompilerMessage, Experimental, Unverifiable,
EditorBrowsable, CompiledName added to enum + classifyEntityAttrib.
WellKnownValAttributes: CompiledName, WarnOnWithoutNullArgument added.

Migrated to EntityAttrib/ValAttrib/EntityAttribString/ValAttribString APs:
- AttributeChecking.fs: Obsolete, CompilerMessage(×2), Experimental,
  Unverifiable, EditorBrowsable
- CheckDeclarations.fs: CompiledName
- CheckExpressions.fs: CompiledName(×2), WarnOnWithoutNullArgument(×2)
- fsi.fs: EntryPoint

TryFindFSharpAttribute callers: 15 → 8

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* B1: Add WellKnownAssemblyAttributes, migrate all string attribute callers

New enum WellKnownAssemblyAttributes with AutoOpen, InternalsVisibleTo,
AssemblyCulture, AssemblyVersion. classifyAssemblyAttrib + (|AssemblyAttribString|_|).

Migrated 7 TryFindFSharpStringAttribute callers:
- fsc.fs: InternalsVisibleTo
- IncrementalBuild.fs: AutoOpen, IVT, AssemblyCulture, AssemblyVersion
- TransparentCompiler.fs: AssemblyCulture, AssemblyVersion

TryFindFSharpStringAttribute callers: 10 → 0 (can be deleted)
TryFindLocalizedFSharpStringAttribute callers: 2 → 0 (can be deleted)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Delete dead helpers and g.attrib_ members after AP migration

Dead helpers removed (zero callers):
- TryFindFSharpStringAttribute
- TryFindLocalizedFSharpStringAttribute

Dead g.attrib_ members removed (zero callers outside TcGlobals):
- attrib_CompilerMessageAttribute
- attrib_ExperimentalAttribute
- attrib_UnverifiableAttribute
- attrib_InternalsVisibleToAttribute
- attrib_WarnOnWithoutNullArgumentAttribute
- attrib_ComponentModelEditorBrowsableAttribute

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* C3: Shared combinators tryFindAttribByClassifier + attribsHaveFlag

Extract inline combinators parameterized by classify function and zero value.
Entity/Val/Assembly tryFind and attribsHave functions become thin wrappers:

  let tryFindEntityAttribByFlag g flag attribs =
      tryFindAttribByClassifier classifyEntityAttrib WellKnownEntityAttributes.None g flag attribs

  let attribsHaveValFlag g flag attribs =
      attribsHaveFlag classifyValAttrib WellKnownValAttributes.None g flag attribs

Eliminates 3×duplicated tryFind + 2×duplicated attribsHave implementations.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Migrate remaining IsMatchingFSharpAttribute/TryFindFSharpAttribute to flag-based APIs

- Add DebuggerDisplayAttribute to WellKnownEntityAttributes and classifyEntityAttrib
- Add MarshalAsAttribute to WellKnownValAttributes and classifyValAttrib
- Export classifyEntityAttrib, classifyValAttrib, filterOutWellKnownAttribs,
  and tryFindValAttribByFlag from TypedTreeOps.fsi
- NicePrint.fs: Replace 13 serial filters with single filterOutWellKnownAttribs call
- NicePrint.fs: Replace OptionalArgumentAttribute filter
- IlxGen.fs: Migrate 14 filter sites and 4 TryFind data-extraction sites
- CheckExpressions.fs: Replace MeasureAttribute filter
- CheckDeclarations.fs: Replace ConditionalAttribute tryFind
- TypedTreeOps.fs: Replace ExtensionAttribute tryFind
- NameResolution.fs: Replace IsMatchingFSharpAttribute for StructAttribute
- infos.fs: Replace DefaultParameterValueAttribute TryFindOpt

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Filter chain optimization + delete dead helpers and 18 g.attrib_ members

Major changes:
- filterOutWellKnownAttribs: single-pass filter using classify functions
  replaces 30+ serial IsMatchingFSharpAttribute filter chains
- NicePrint.fs: 13 serial O(N) filters → 1 filterOutWellKnownAttribs call
- IlxGen.fs: 12 filter sites migrated to filterOutWellKnownAttribs
- Data extraction: TryFindFSharpAttributeOpt callers migrated to ValAttrib APs

Dead code removed:
- TryFindFSharpAttributeOpt, IsMatchingFSharpAttributeOpt (zero callers)
- 18 g.attrib_ TcGlobals members (zero callers)

g.attrib_ members: 59 → 41

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add IL-level classifyILAttrib + (|ILAttribDecoded|_|) AP, migrate 9 TryDecodeILAttribute sites

Factor computeILWellKnownFlags into classifyILAttrib (per-attrib) + fold.
Add tryFindILAttribByFlag and (|ILAttribDecoded|_|) active pattern for
enum-powered IL attribute data extraction.

New WellKnownILAttributes flags: CompilerFeatureRequired, Experimental,
RequiredMember, NullableContext, AttributeUsage.

Migrated 9 TryDecodeILAttribute callsites:
- AttributeChecking.fs: CompilerFeatureRequired, Experimental, Obsolete(×3), IsByRefLike
- import.fs: NullableAttribute, NullableContextAttribute
- infos.fs: ReflectedDefinition

Also migrated TryFindILAttribute callers:
- CheckExpressions.fs: SetsRequiredMembersAttribute → tryFindILAttribByFlag
- infos.fs: RequiredMemberAttribute → tryFindILAttribByFlag

Deleted 6 g.attrib_ members (zero callers): NullableAttribute_opt,
NullableContextAttribute_opt, CompilerFeatureRequiredAttribute,
SetsRequiredMembersAttribute, RequiredMemberAttribute, IlExperimentalAttribute.

g.attrib_ members: 41 → 35

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Delete attrib_IlExperimentalAttribute (zero callers)

g.attrib_ members: 95 → 35 (60 deleted total)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Audit cleanup: hasFlag helper, CheckFlag dedup, ILAttributes extension, docs

- Add inline hasFlag helper to replace 37 verbose &&& <> None patterns
- Add CheckFlag method to WellKnownAttribs struct, dedup 3 cache functions
- Add ILAttributes.HasWellKnownAttribute extension for non-caching checks
- Fix ILPropInfo.IsRequired to use cached HasWellKnownAttribute
- Simplify CheckRequiredProperties to use ILAttributes extension
- Document WellKnownAssemblyAttributes uint32 choice
- Update surface area baseline for new WellKnownILAttributes members
- Fix DefaultValue(false) regression: zeroInit must match both True and False flags

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add MethInfoHasWellKnownAttribute for O(1) checks on overload resolution hot path

Migrate NoEagerConstraintApplicationAttribute (2 callers in MethodCalls +
CheckExpressions) and ExtensionAttribute (NameResolution) from linear-scan
MethInfoHasAttribute to cached-flag MethInfoHasWellKnownAttribute. The new
function dispatches to ILMethodDef.HasWellKnownAttribute (cached IL flags)
for ILMeth and ValHasWellKnownAttribute (cached val flags) for FSMeth, with
ProvidedMeth fallback to the old API.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Council fixes: resolveAttribPath DU, merge IL maps, private HasWellKnownAttribute, cleanup

- Refactor resolveAttribPath from callback to struct(bclPath, fsharpCorePath)
  return value. Eliminates mutable-captured-by-closure in all 3 classifiers,
  removes 1 nesting level, makes control flow linear and purely functional.
- Merge mapILFlagToAttribInfo + mapILFlagToEntityFlag into single mapILFlag
  returning struct(EntityFlag * AttribInfo option). One match, no drift risk.
- Remove HasWellKnownAttribute from .fsi — callers must use CheckFlag.
- Remove dead _g parameter from CheckCompilerFeatureRequiredAttribute.
- Add NoEagerConstraintApplicationAttribute to WellKnownValAttributes for
  complete MethInfoHasWellKnownAttribute coverage on FSMeth path.
- Add comment on dual-namespace SetsRequiredMembersAttribute.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Round 2 council fixes: extract WarnOnWithoutNull helper, dedup CheckILUnseen, tryGetAssemblyAttribString

- Extract tryGetWarnOnWithoutNullMessage helper from duplicated 8-line
  decode logic at two CheckExpressions.fs callsites
- CheckILAttributesForUnseen now uses ILAttributes.HasWellKnownAttribute
  (non-caching) — same logic as CheckILAttributesForUnseenStored, no decode
- Add tryGetAssemblyAttribString for single-attrib string extraction,
  eliminating [attr] singleton-list wrapping in IncrementalBuild.fs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Final cleanup: dead code, encapsulation, AllowNullLiteral migration, IlxGen single-scan

Dead code removed:
- WellKnownAttribs.Append (zero callers)
- ILAttributesStored.CreateGiven(idx,attrs) two-arg overload (zero callers)
- ILAttributesStored.GetCustomAttrs(int32) shim (ignored arg, fix import.fs)
- MetadataIndex + GetOrComputeWellKnownFlags removed from il.fsi (no external callers)

Encapsulation:
- CheckFlag now returns needsWriteBack bool, removing need to expose .Flags
- .Flags removed from WellKnownAttribs.fsi

Migration:
- TyconRefAllowsNull replaces 5 TryFindTyconRefBoolAttribute AllowNullLiteral
  sites with cached flag path (IL: HasWellKnownAttribute, F#: EntityTryGetBoolAttribute)

Efficiency:
- GenParamAttribs: gate tryFindValAttribByFlag with flag check, merge MarshalAs
  filter into single pass, pass valFlags to GenMarshal
- GenMarshal: accept valFlags, skip all scans when MarshalAs absent
- ComputeMethodImplAttribs: compute flags once, gate attribsHave + filterOut

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add ValueAsStaticPropertyAttribute to WellKnownValAttributes for O(1) lookup

Replace List.exists with string comparison on hot path in
Val.IsCompiledAsStaticPropertyWithoutField and PostInferenceChecks
with cached WellKnownValAttributes flag check.

- Add ValueAsStaticPropertyAttribute flag at bit 39 in WellKnownValAttributes
- Add classifyValAttrib entry for the attribute
- Simplify Val.IsCompiledAsStaticPropertyWithoutField to use HasWellKnownAttribute
- Replace inline string scan in PostInferenceChecks with ValHasWellKnownAttribute
- Expose HasWellKnownAttribute and Flags in WellKnownAttribs.fsi

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add HasWellKnownAttribute members on Entity and Val for transparent caching

Add convenience members that encapsulate the CheckFlag + write-back
pattern directly on Entity and Val types. This eliminates the need for
callers to manually remember the write-back step when checking well-known
attribute flags.

- Entity.HasWellKnownAttribute(flag, computeFlags) encapsulates cache update
- Val.HasWellKnownAttribute(flag, computeFlags) encapsulates cache update
- EntityHasWellKnownAttribute now forwards to entity.HasWellKnownAttribute
- ValHasWellKnownAttribute now forwards to v.HasWellKnownAttribute
- EntityTryGetBoolAttribute uses entity.HasWellKnownAttribute for first check
- Both members declared in .fsi signature files

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add WellKnownMethAttribute struct to bundle correlated attribute check parameters

Introduce WellKnownMethAttribute struct that bundles ILFlag, ValFlag, and
AttribInfo for well-known attribute checks on MethInfo. Add
MethInfoHasWellKnownAttributeSpec overload that takes the bundled struct,
delegating to the original function. Update all 3 callsites to use the
new struct-based API, eliminating the risk of passing mismatched flags.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add ValTryGetBoolAttribute for three-state bool attribute queries on Val

Mirrors EntityTryGetBoolAttribute to provide symmetric API for querying
True/False/NotPresent attribute states on Val nodes. Supports flag pairs:
ReflectedDefinition, DefaultValue, NoDynamicInvocation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove unused parameters from AttributeChecking functions

- Remove unused _g from CheckILExperimentalAttributes (private)
- Remove unused _m from CheckILAttributesForUnseenStored and update .fsi
- Remove unused _m from CheckFSharpAttributesForUnseen and update .fsi
- Prefix newly-unused m params with _ in PropInfoIsUnseen and IsValUnseen
- Add cross-reference doc comments between CheckILAttributesForUnseen variants
- Update all callsites in NameResolution.fs and AttributeChecking.fs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Rewrite TryFindAutoOpenAttr and TryFindInternalsVisibleToAttr to use classifyILAttrib

Replace hand-rolled isILAttribByName checks with the modern classifyILAttrib
flag-based API for consistency with the rest of the attribute classification
infrastructure. Remove unused tname_AutoOpenAttr binding.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add well-known attribute API navigation guide comment block

Add a comprehensive navigation comment block before the attribute helpers
section in TypedTreeOps.fs. The comment documents all well-known attribute
APIs organized by category: existence checks, ad-hoc checks, data extraction
active patterns, bool queries, IL-level operations, cross-metadata dispatch,
and classification functions. Notes that MethInfoHasWellKnownAttribute lives
in AttributeChecking.fs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix verifier issues: BSL baseline, CompilerCompat test bugs, trailing newlines

- Commit updated surface area baseline removing stale ILAttributesStored entries
- Fix reserved keyword 'sealed' used as variable name in CompilerCompat Program.fs
- Fully qualify StructRecord.Y field for cross-project resolution
- Add trailing newlines to Library.fs and AttributeUsage.fs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add TailCallAttribute to WellKnownValAttributes for O(1) flag-based lookup

- Add TailCallAttribute = (1uL <<< 40) to WellKnownValAttributes enum (.fs and .fsi)
- Add TailCallAttribute classification in classifyValAttrib under Microsoft.FSharp.Core
- Remove HasTailCallAttrib from TcGlobals; replace with hasTailCallAttrib helper in TailCallChecks
- Helper uses O(1) flag lookup with fallback for user-defined shadow attributes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Consolidate TypeProviderAssemblyAttribute detection into classifyAssemblyAttrib

Add WellKnownAssemblyAttributes.TypeProviderAssemblyAttribute flag and
classify it under Microsoft.FSharp.Core.CompilerServices in
classifyAssemblyAttrib. Fold detection into the existing assembly
attribute classification loop in IncrementalBuild.fs and
TransparentCompiler.fs, eliminating a duplicate List.exists scan over
topAttrs.assemblyAttrs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fast-path DefaultMemberAttribute in indexer resolution

Extract shared tryBindTyconRefAttributeCore with WellKnownILAttributes voption
parameter. Both TryBindTyconRefAttribute and TryBindTyconRefAttributeWithILFlag
delegate to it, eliminating code duplication while preserving the O(1) flag
check fast-path on the IL metadata path.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Remove duplicate DefaultAugmentation(false) test

The test 'DefaultAugmentation(false) suppresses helpers' in AttributeUsage.fs
was a near-duplicate of the more specific 'Is* discriminated union properties
are unavailable with DefaultAugmentation(false)' test in DiscriminatedUnionTests.fs.
Remove the duplicate per TEST-CODE-QUALITY verifier feedback.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* AugmentWishHashCompare - remove 7 bools in matches

* Fix MarshalAs regression: don't filter attribute before GenMarshal reads it

GenParamAttribs was filtering MarshalAsAttribute from the attribs list
before passing it to GenMarshal, which then couldn't find the attribute
data (UnmanagedType value). Remove MarshalAs from the pre-filter mask
and let GenMarshal handle its own filtering.

Fixes: Marshal_fs IL baseline test (all platforms)
Fixes: attributes-FSC_OPTIMIZED/FSI (Windows Desktop)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix fantomas formatting in TypedTree.fsi and TypedTreeOps.fsi

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Adam Boniecki <20281641+abonie@users.noreply.github.com>

Author: 3 people

src/Compiler/AbstractIL/il.fs

@@ -1229,19 +1229,75 @@ type ILAttributes(array: ILAttribute[]) =
 
     static member val internal Empty = ILAttributes([||])
 
-[<NoEquality; NoComparison>]
-type ILAttributesStored =
-
-    /// Computed by ilread.fs based on metadata index
+[<Flags>]
+type WellKnownILAttributes =
+    | None = 0u
+    | IsReadOnlyAttribute = (1u <<< 0)
+    | IsUnmanagedAttribute = (1u <<< 1)
+    | IsByRefLikeAttribute = (1u <<< 2)
+    | ExtensionAttribute = (1u <<< 3)
+    | NullableAttribute = (1u <<< 4)
+    | ParamArrayAttribute = (1u <<< 5)
+    | AllowNullLiteralAttribute = (1u <<< 6)
+    | ReflectedDefinitionAttribute = (1u <<< 7)
+    | AutoOpenAttribute = (1u <<< 8)
+    | InternalsVisibleToAttribute = (1u <<< 9)
+    | CallerMemberNameAttribute = (1u <<< 10)
+    | CallerFilePathAttribute = (1u <<< 11)
+    | CallerLineNumberAttribute = (1u <<< 12)
+    | IDispatchConstantAttribute = (1u <<< 13)
+    | IUnknownConstantAttribute = (1u <<< 14)
+    | RequiresLocationAttribute = (1u <<< 15)
+    | SetsRequiredMembersAttribute = (1u <<< 16)
+    | NoEagerConstraintApplicationAttribute = (1u <<< 17)
+    | DefaultMemberAttribute = (1u <<< 18)
+    | ObsoleteAttribute = (1u <<< 19)
+    | CompilerFeatureRequiredAttribute = (1u <<< 20)
+    | ExperimentalAttribute = (1u <<< 21)
+    | RequiredMemberAttribute = (1u <<< 22)
+    | NullableContextAttribute = (1u <<< 23)
+    | AttributeUsageAttribute = (1u <<< 24)
+    | NotComputed = (1u <<< 31)
+
+type internal ILAttributesStoredRepr =
     | Reader of (int32 -> ILAttribute[])
-
-    /// Already computed
     | Given of ILAttributes
 
-    member x.GetCustomAttrs metadataIndex =
-        match x with
-        | Reader f -> ILAttributes(f metadataIndex)
-        | Given attrs -> attrs
+[<Sealed; NoEquality; NoComparison>]
+type ILAttributesStored private (metadataIndex: int32, initial: ILAttributesStoredRepr) =
+    [<VolatileField>]
+    let mutable repr = initial
+
+    [<VolatileField>]
+    let mutable wellKnownFlags = WellKnownILAttributes.NotComputed
+
+    member _.MetadataIndex = metadataIndex
+
+    member x.CustomAttrs: ILAttributes =
+        match repr with
+        | Given a -> a
+        | Reader f ->
+            let r = ILAttributes(f metadataIndex)
+            repr <- Given r
+            r
+
+    member x.HasWellKnownAttribute(flag: WellKnownILAttributes, compute: ILAttributes -> WellKnownILAttributes) : bool =
+        x.GetOrComputeWellKnownFlags(compute) &&& flag <> WellKnownILAttributes.None
+
+    member x.GetOrComputeWellKnownFlags(compute: ILAttributes -> WellKnownILAttributes) : WellKnownILAttributes =
+        let f = wellKnownFlags
+
+        if f <> WellKnownILAttributes.NotComputed then
+            f
+        else
+            let a = x.CustomAttrs
+            let computed = compute a
+            wellKnownFlags <- computed
+            computed
+
+    static member CreateReader(idx: int32, f: int32 -> ILAttribute[]) = ILAttributesStored(idx, Reader f)
+
+    static member CreateGiven(attrs: ILAttributes) = ILAttributesStored(-1, Given attrs)
 
 let emptyILCustomAttrs = ILAttributes [||]
 
@@ -1256,18 +1312,18 @@ let mkILCustomAttrs l =
     | [] -> emptyILCustomAttrs
     | _ -> mkILCustomAttrsFromArray (List.toArray l)
 
-let emptyILCustomAttrsStored = ILAttributesStored.Given emptyILCustomAttrs
+let emptyILCustomAttrsStored = ILAttributesStored.CreateGiven emptyILCustomAttrs
 
 let storeILCustomAttrs (attrs: ILAttributes) =
     if attrs.AsArray().Length = 0 then
         emptyILCustomAttrsStored
     else
-        ILAttributesStored.Given attrs
+        ILAttributesStored.CreateGiven attrs
 
 let mkILCustomAttrsComputed f =
-    ILAttributesStored.Reader(fun _ -> f ())
+    ILAttributesStored.CreateReader(-1, fun _ -> f ())
 
-let mkILCustomAttrsReader f = ILAttributesStored.Reader f
+let mkILCustomAttrsReader f = ILAttributesStored.CreateReader(-1, f)
 
 type ILCodeLabel = int
 
@@ -1791,7 +1847,7 @@ type ILParameter =
         MetadataIndex: int32
     }
 
-    member x.CustomAttrs = x.CustomAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     override x.ToString() =
         x.Name |> Option.defaultValue "<no name>"
@@ -1809,7 +1865,7 @@ type ILReturn =
 
     override x.ToString() = "<return>"
 
-    member x.CustomAttrs = x.CustomAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     member x.WithCustomAttrs(customAttrs) =
         { x with
@@ -1870,7 +1926,7 @@ type ILGenericParameterDef =
         MetadataIndex: int32
     }
 
-    member x.CustomAttrs = x.CustomAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     /// For debugging
     [<DebuggerBrowsable(DebuggerBrowsableState.Never)>]
@@ -1916,13 +1972,7 @@ type InterfaceImpl =
         mutable CustomAttrsStored: ILAttributesStored
     }
 
-    member x.CustomAttrs =
-        match x.CustomAttrsStored with
-        | ILAttributesStored.Reader f ->
-            let res = ILAttributes(f x.Idx)
-            x.CustomAttrsStored <- ILAttributesStored.Given res
-            res
-        | ILAttributesStored.Given attrs -> attrs
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     static member Create(ilType: ILType, customAttrsStored: ILAttributesStored) =
         {
@@ -2029,7 +2079,7 @@ type ILMethodDef
                  | Some attrs -> attrs)
         )
 
-    member x.CustomAttrs = x.CustomAttrsStored.GetCustomAttrs metadataIndex
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     member x.SecurityDecls = x.SecurityDeclsStored.GetSecurityDecls x.MetadataIndex
 
@@ -2266,7 +2316,7 @@ type ILEventDef
 
     member _.MetadataIndex = metadataIndex
 
-    member x.CustomAttrs = customAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = customAttrsStored.CustomAttrs
 
     member x.With(?eventType, ?name, ?attributes, ?addMethod, ?removeMethod, ?fireMethod, ?otherMethods, ?customAttrs) =
         ILEventDef(
@@ -2342,7 +2392,7 @@ type ILPropertyDef
     member x.Init = init
     member x.Args = args
     member x.CustomAttrsStored = customAttrsStored
-    member x.CustomAttrs = customAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = customAttrsStored.CustomAttrs
     member x.MetadataIndex = metadataIndex
 
     member x.With(?name, ?attributes, ?setMethod, ?getMethod, ?callingConv, ?propertyType, ?init, ?args, ?customAttrs) =
@@ -2418,7 +2468,7 @@ type ILFieldDef
     member _.Offset = offset
     member _.Marshal = marshal
     member x.CustomAttrsStored = customAttrsStored
-    member x.CustomAttrs = customAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = customAttrsStored.CustomAttrs
     member x.MetadataIndex = metadataIndex
 
     member x.With
@@ -2677,8 +2727,6 @@ type ILTypeDef
         metadataIndex: int32
     ) =
 
-    let mutable customAttrsStored = customAttrsStored
-
     let hasFlag flag = additionalFlags &&& flag = flag
 
     new
@@ -2829,13 +2877,7 @@ type ILTypeDef
             customAttrs = defaultArg customAttrs x.CustomAttrsStored
         )
 
-    member x.CustomAttrs: ILAttributes =
-        match customAttrsStored with
-        | ILAttributesStored.Reader f ->
-            let res = ILAttributes(f x.MetadataIndex)
-            customAttrsStored <- ILAttributesStored.Given res
-            res
-        | ILAttributesStored.Given res -> res
+    member x.CustomAttrs: ILAttributes = customAttrsStored.CustomAttrs
 
     member x.SecurityDecls = x.SecurityDeclsStored.GetSecurityDecls x.MetadataIndex
 
@@ -2993,7 +3035,7 @@ type ILNestedExportedType =
         MetadataIndex: int32
     }
 
-    member x.CustomAttrs = x.CustomAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     override x.ToString() = "exported type " + x.Name
 
@@ -3017,7 +3059,7 @@ and [<NoComparison; NoEquality>] ILExportedTypeOrForwarder =
 
     member x.IsForwarder = x.Attributes &&& enum<TypeAttributes> 0x00200000 <> enum 0
 
-    member x.CustomAttrs = x.CustomAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     override x.ToString() = "exported type " + x.Name
 
@@ -3057,7 +3099,7 @@ type ILResource =
         | ILResourceLocation.Local bytes -> bytes.GetByteMemory()
         | _ -> failwith "GetBytes"
 
-    member x.CustomAttrs = x.CustomAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     override x.ToString() = "resource " + x.Name
 
@@ -3104,7 +3146,7 @@ type ILAssemblyManifest =
         MetadataIndex: int32
     }
 
-    member x.CustomAttrs = x.CustomAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     member x.SecurityDecls = x.SecurityDeclsStored.GetSecurityDecls x.MetadataIndex
 
@@ -3151,7 +3193,7 @@ type ILModuleDef =
         | None -> false
         | _ -> true
 
-    member x.CustomAttrs = x.CustomAttrsStored.GetCustomAttrs x.MetadataIndex
+    member x.CustomAttrs = x.CustomAttrsStored.CustomAttrs
 
     override x.ToString() = "assembly " + x.Name
 

src/Compiler/AbstractIL/il.fsi

@@ -878,15 +878,45 @@ type ILAttributes =
 
     static member internal Empty: ILAttributes
 
+[<Flags>]
+type WellKnownILAttributes =
+    | None = 0u
+    | IsReadOnlyAttribute = (1u <<< 0)
+    | IsUnmanagedAttribute = (1u <<< 1)
+    | IsByRefLikeAttribute = (1u <<< 2)
+    | ExtensionAttribute = (1u <<< 3)
+    | NullableAttribute = (1u <<< 4)
+    | ParamArrayAttribute = (1u <<< 5)
+    | AllowNullLiteralAttribute = (1u <<< 6)
+    | ReflectedDefinitionAttribute = (1u <<< 7)
+    | AutoOpenAttribute = (1u <<< 8)
+    | InternalsVisibleToAttribute = (1u <<< 9)
+    | CallerMemberNameAttribute = (1u <<< 10)
+    | CallerFilePathAttribute = (1u <<< 11)
+    | CallerLineNumberAttribute = (1u <<< 12)
+    | IDispatchConstantAttribute = (1u <<< 13)
+    | IUnknownConstantAttribute = (1u <<< 14)
+    | RequiresLocationAttribute = (1u <<< 15)
+    | SetsRequiredMembersAttribute = (1u <<< 16)
+    | NoEagerConstraintApplicationAttribute = (1u <<< 17)
+    | DefaultMemberAttribute = (1u <<< 18)
+    | ObsoleteAttribute = (1u <<< 19)
+    | CompilerFeatureRequiredAttribute = (1u <<< 20)
+    | ExperimentalAttribute = (1u <<< 21)
+    | RequiredMemberAttribute = (1u <<< 22)
+    | NullableContextAttribute = (1u <<< 23)
+    | AttributeUsageAttribute = (1u <<< 24)
+    | NotComputed = (1u <<< 31)
+
 /// Represents the efficiency-oriented storage of ILAttributes in another item.
-[<NoEquality; NoComparison>]
+[<Sealed; NoEquality; NoComparison>]
 type ILAttributesStored =
-    /// Computed by ilread.fs based on metadata index
-    | Reader of (int32 -> ILAttribute[])
-    /// Already computed
-    | Given of ILAttributes
+    member CustomAttrs: ILAttributes
+
+    member HasWellKnownAttribute: flag: WellKnownILAttributes * compute: (ILAttributes -> WellKnownILAttributes) -> bool
 
-    member GetCustomAttrs: int32 -> ILAttributes
+    static member CreateReader: idx: int32 * f: (int32 -> ILAttribute[]) -> ILAttributesStored
+    static member CreateGiven: attrs: ILAttributes -> ILAttributesStored
 
 /// Method parameters and return values.
 [<RequireQualifiedAccess; NoEquality; NoComparison>]