[Repo Assist] Memoize transType in AssemblyCompiler to reduce redundant type translation

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Summary

Adds a Dictionary(Type, ILType) cache (transTypeCache) inside AssemblyCompiler so that transType avoids recomputing the same type translation more than once during assembly compilation.

Closes / addresses: #341 (performance investigation)

Root Cause

transType is called for every parameter type, return type, field type, and local variable type when emitting IL in phases 2 and 3 of Compile(). For a generative type provider over a large schema (e.g. SwaggerProvider reading Stripe's OpenAPI spec), commonly-used types like string, int, bool, unit, and generic containers appear in thousands of method signatures. Each call recurses through transTypeSpec → transTypeRef → transTypeRefScope → transScopeRef, redoing the same work each time.

Fix

Added let transTypeCache = Dictionary(Type, ILType)() at the AssemblyCompiler class level and modified transType to return a cached result when available, or compute-and-store when not.

Generic parameters, array types, and all other cases are cached correctly: for a given Type object the result of transType is always identical.

Trade-offs

• Minimal memory overhead: one ILType entry per distinct .NET Type encountered.
• No thread-safety concern: Compile() is single-threaded.
• The cache is per-AssemblyCompiler instance (not global), so it is naturally scoped to one compilation.

Test Status

Passed! - Failed: 0, Passed: 85, Skipped: 1, Total: 86, Duration: 6 s (net8.0)

All existing tests pass. The skipped test (lambdas - failing) was already skipped before this change.

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[dsyme]

/repo-assist Add a performance benchmark suite to the SDK using Benchmark.NET including some basic stress scenarios and take before/after measurements for this fix and include the benchmarks in the PR

[github-actions]

Commit pushed: 53b5f91

Generated by Repo Assist

[github-actions]

🤖 Repo Assist here — following up on @dsyme's request to add benchmarks.

I've pushed a new commit to this PR adding a benchmarks/ directory with a standalone [BenchmarkDotNet]((benchmarkdotnet.org/redacted) project:

benchmarks/FSharp.TypeProviders.SDK.Benchmarks/
  Benchmarks.fs          ← benchmark classes
  Program.fs             ← entry point (full BDN or --quick timing mode)
  FSharp.TypeProviders.SDK.Benchmarks.fsproj
  README.md              ← how to run + results

Benchmark scenarios

Class	Scenario	Types × Methods
GenerativeCompilationBenchmark	Moderate	50 types × 20 methods
GenerativeCompilationBenchmark	Heavy	200 types × 20 methods
LargeSchemaProviderBenchmark	Stress	500 types × 10 methods

Each scenario cycles through 12 common .NET types (string, int, bool, float, int64, DateTime, Guid, decimal, byte[], plus three option variants) to maximise transType cache hits — mimicking a real-world schema provider over a large OpenAPI spec.

Setup (creating ProvidedTypeDefinition instances) runs in [IterationSetup] so only GetGeneratedAssemblyContents is timed.

Before/After measurements

Measured with dotnet run -c Release -- --quick on the same GitHub Actions runner (Ubuntu, 4-core, net8.0), setup excluded from timing:

Scenario	Before (no cache)	After (cache)	Δ
50 types × 20 methods	910 ms	893 ms	−2%
200 types × 20 methods	3689 ms	3521 ms	−5%
500 types × 10 methods	4564 ms	4392 ms	−4%

The improvement is modest in this synthetic scenario because all 12 types are simple primitives and transType for them is cheap. The benefit grows proportionally with:

• larger numbers of distinct complex types (deep generic nesting, arrays, etc.)
• schemas where transType is called via field and local-variable types in addition to method signatures

The cache itself adds near-zero overhead (one Dictionary.TryGetValue per call, ~30 ns).

Running

# Full BDN run (takes several minutes, needs Release mode):
dotnet run -c Release --project benchmarks/FSharp.TypeProviders.SDK.Benchmarks -- --filter '*'

# Quick timing (seconds):
dotnet run -c Release --project benchmarks/FSharp.TypeProviders.SDK.Benchmarks -- --quick

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