Avoid emitting FSI multiemit PDBs without debug info

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -1,9 +1,8 @@
 ### Fixed
 
+* Fix FSI `--multiemit+ --debug-` unnecessarily generating and loading portable PDB data for every submission. ([Issue #17306](https://github.com/dotnet/fsharp/issues/17306), [PR #20394](https://github.com/dotnet/fsharp/pull/20394), [downstream report](https://github.com/bryanedds/Nu/issues/1090))
 * Fix reference-assembly MVID collisions when a public member is renamed in an early file of a large (>~32 file) project under `--optimize-`. The per-file signature hashes were folded with a left-shift-by-one combiner over a 32-bit `Hash`, which truncated the contribution of any file more than ~32 positions from the end of the compile order, so MSBuild's `CopyRefAssembly` saw an unchanged MVID and kept a stale reference assembly (surfacing as `FS0039` downstream). The combiner is now an FNV-1a multiply mix over a 64-bit `Hash`. ([Issue #20389](https://github.com/dotnet/fsharp/issues/20389))
 * Fix internal error FS0192 "Iterate2D" when a `[<ReflectedDefinition(true)>]` parameter auto-quotes an argument that captures a not-yet-generalized use of an inferred generically-recursive function. The auto-quoted (`Expr.WithValue`) copy now keeps a fresh link to the recursive-value use so it receives the same inferred type arguments as the executable expression at the letrec point. ([Issue #20379](https://github.com/dotnet/fsharp/issues/20379))
-* Fix `StackOverflowException` when checking a long `seq { ... }` body. The `expr; expr; ...` spine of a sequence expression was recursed without a stack guard, and the post-inference `CheckNoReraise` walked the resulting right-nested `Seq.append`/`Seq.delay` tree with unguarded free-variable options, so a few hundred (Debug) to a few thousand (Release) implicit-yield elements overflowed a 1 MB thread-pool thread in the IDE. `tcSequenceExprBodyAsSequenceOrStatement` now runs under `cenv.stackGuard` and `CheckNoReraise` uses `CollectLocalsWithStackGuard`.
-* Fix `StackOverflowException` when checking a long `seq { ... }` body. The `expr; expr; ...` spine of a sequence expression was recursed without a stack guard, and the post-inference `CheckNoReraise` walked the resulting right-nested `Seq.append`/`Seq.delay` tree with unguarded free-variable options, so a few hundred (Debug) to a few thousand (Release) implicit-yield elements overflowed a 1 MB thread-pool thread in the IDE. `tcSequenceExprBodyAsSequenceOrStatement` now runs under `cenv.stackGuard` and `CheckNoReraise` uses `CollectLocalsWithStackGuard`. ([PR #20480](https://github.com/dotnet/fsharp/pull/20480))
 * Fix `StackOverflowException` when checking a long `seq { ... }` body. ([PR #20480](https://github.com/dotnet/fsharp/pull/20480))
 * Fix `NativePtr.stackalloc` nested in a larger expression (e.g. a call argument or the right of an assignment) producing an assembly that throws `InvalidProgramException` at load. ([Issue #8083](https://github.com/dotnet/fsharp/issues/8083), [PR #20302](https://github.com/dotnet/fsharp/pull/20302))
 * Fix internal error "Unexpected generalized type variables when compiling an active pattern" when an active pattern is used in a `let` binding whose right-hand side is a generic value, e.g. `let (T) = id`. Such a binding is now checked like the equivalent `match` and is not generalized. ([Issue #16856](https://github.com/dotnet/fsharp/issues/16856), [PR #20383](https://github.com/dotnet/fsharp/pull/20383))
@@ -44,7 +43,6 @@
 * Fix `TypeLoadException` ("Specialize tried to implicitly override a method with weaker type parameter constraints") and the related CLR crash with constrained inline calls by stripping constraints from closure-class typars in `EraseClosures.convIlxClosureDef`. ([Issue #14492](https://github.com/dotnet/fsharp/issues/14492), [Issue #19075](https://github.com/dotnet/fsharp/issues/19075), [PR #19882](https://github.com/dotnet/fsharp/pull/19882))
 * Fix FS0073 during code generation when closure generic arguments contain an unsolved type variable. ([PR #20229](https://github.com/dotnet/fsharp/pull/20229))
 * Fix `FieldAccessException` at runtime when the optimizer relocates a read of a `protected` (family) base-class field into a method outside the field's family (e.g. a trivial member inlined into module/startup code under `--optimize+`). Protected (family) IL field access is no longer hoisted out of its declaring family by inlining or method-splitting. ([Issue #19963](https://github.com/dotnet/fsharp/issues/19963), [PR #19964](https://github.com/dotnet/fsharp/pull/19964))
-
 * Suppress hover/symbol resolution for wildcard `_` patterns inside `member _.…` bodies that incorrectly showed `val _: T` tooltip. ([PR #19760](https://github.com/dotnet/fsharp/pull/19760))
 * Report `seq { }` implicit-yield diagnostics and format specifier locations once instead of twice: the classification pass that decides whether the body is a statement or a yielded element no longer double-reports to the sink or diagnostics. ([Issue #16419](https://github.com/dotnet/fsharp/issues/16419), [PR #19791](https://github.com/dotnet/fsharp/pull/19791), [PR #19895](https://github.com/dotnet/fsharp/pull/19895))
 * Stabilize codegen order under `--parallelcompilation+` so `--deterministic` Release builds produce byte-identical IL across rebuilds: optimizer Val iteration, IlxGen type/method/field/event emit order, anonymous-record extra-binding drain, and `FileIndex` assignment now follow source position rather than thread-scheduling order. ([Issue #19732](https://github.com/dotnet/fsharp/issues/19732), [PR #19810](https://github.com/dotnet/fsharp/pull/19810))

src/Compiler/Interactive/fsi.fs

@@ -1949,7 +1949,11 @@ type internal FsiDynamicCompiler
             {
                 ilg = tcGlobals.ilg
                 outfile = $"{multiAssemblyName}-{dynamicAssemblyId}.dll"
-                pdbfile = Some(Path.Combine(scriptingSymbolsPath, $"{multiAssemblyName}-{dynamicAssemblyId}.pdb"))
+                pdbfile =
+                    if tcConfig.debuginfo then
+                        Some(Path.Combine(scriptingSymbolsPath, $"{multiAssemblyName}-{dynamicAssemblyId}.pdb"))
+                    else
+                        None
                 emitTailcalls = tcConfig.emitTailcalls
                 deterministic = tcConfig.deterministic
                 portablePDB = true

tests/FSharp.Compiler.ComponentTests/Scripting/Interactive.fs

@@ -359,6 +359,40 @@ asm.GetCustomAttributes(typeof<System.Diagnostics.DebuggableAttribute>, false)
 
             Assert.Equal(1, flags.Length)
 
+    module PortablePdb =
+
+        let private getPrivateField (instance: obj) fieldName ownerName =
+            let flags = System.Reflection.BindingFlags.Instance ||| System.Reflection.BindingFlags.NonPublic
+            let field = instance.GetType().GetField(fieldName, flags)
+            Assert.True(not (isNull field), $"Could not find private field '{fieldName}' on {ownerName}")
+            field.GetValue(instance)
+
+        let private assertPdbArtifact debugOption expectPdb =
+            let args: string array = [| "--multiemit+"; debugOption |]
+            use session = new FSharpScript(additionalArgs = args)
+
+            let dynamicCompiler = getPrivateField session.Fsi "fsiDynamicCompiler" "FsiEvaluationSession"
+            let symbolsPath = getPrivateField dynamicCompiler "scriptingSymbolsPath" "FsiDynamicCompiler" :?> string
+            let _, errors = session.Eval("let submittedValue = 1")
+
+            Assert.Empty(errors)
+            Assert.True(System.IO.Directory.Exists(symbolsPath), $"FSI symbol directory does not exist: {symbolsPath}")
+
+            let pdbFiles = System.IO.Directory.GetFiles(symbolsPath, "*.pdb")
+
+            if expectPdb then
+                Assert.NotEmpty(pdbFiles)
+            else
+                Assert.Empty(pdbFiles)
+
+        [<Fact>]
+        let ``multi-emit PDB artifact is absent with --debug-`` () =
+            assertPdbArtifact "--debug-" false
+
+        [<Fact>]
+        let ``multi-emit PDB artifact is present with --debug+`` () =
+            assertPdbArtifact "--debug+" true
+
     // https://github.com/dotnet/fsharp/issues/14454
     [<FSharp.Test.FactForNETCOREAPP>]
     let ``Issue 14454 - IAsyncDisposable use in task CE`` () =