perf: cache source lines, stream file bytes, and trim hot-path allocations

src/FsAutoComplete.Core/CompilerServiceInterface.fs

@@ -56,9 +56,8 @@ type CompilerProjectOption =
 
   member x.SourceFilesTagged =
     match x with
-    | BackgroundCompiler(options) -> options.SourceFiles |> Array.toList
-    | TransparentCompiler(snapshot) -> snapshot.SourceFiles |> List.map (fun f -> f.FileName)
-    |> List.map Utils.normalizePath
+    | BackgroundCompiler(options) -> options.SourceFiles |> Array.map Utils.normalizePath |> Array.toList
+    | TransparentCompiler(snapshot) -> snapshot.SourceFiles |> List.map (fun f -> Utils.normalizePath f.FileName)
 
   member x.ReferencedProjectsPath =
     match x with
@@ -189,12 +188,16 @@ type FSharpCompilerServiceChecker
       None
 
   let processFSIArgs args =
-    (([||], [||]), args)
-    ||> Array.fold (fun (args, files) arg ->
+    let argsOut = ResizeArray()
+    let filesOut = ResizeArray()
+
+    for arg in args do
       match arg with
       | StartsWith "--use:" file
-      | StartsWith "--load:" file -> args, Array.append files [| file |]
-      | arg -> Array.append args [| arg |], files)
+      | StartsWith "--load:" file -> filesOut.Add(file)
+      | arg -> argsOut.Add(arg)
+
+    argsOut.ToArray(), filesOut.ToArray()
 
   let (|Reference|_|) (opt: string) =
     if opt.StartsWith("-r:", StringComparison.Ordinal) then

src/FsAutoComplete.Core/FileSystem.fs

@@ -169,6 +169,9 @@ module RoslynSourceText =
 
   type RoslynSourceTextFile(fileName: string<LocalPath>, sourceText: SourceText) =
 
+    let cachedLines =
+      lazy (sourceText.Lines |> Seq.toArray |> Array.map (fun l -> l.ToString()))
+
     let walk
       (
         x: IFSACSourceText,
@@ -250,8 +253,7 @@ module RoslynSourceText =
       member x.TotalRange: Range =
         (Range.mkRange (UMX.untag fileName) Position.pos0 ((x :> IFSACSourceText).LastFilePosition))
 
-      member x.Lines: string array =
-        sourceText.Lines |> Seq.toArray |> Array.map (fun l -> l.ToString())
+      member x.Lines: string array = cachedLines.Value
 
       member this.GetText(range: Range) : Result<string, string> =
         range.ToRoslynTextSpan(sourceText) |> sourceText.GetSubText |> string |> Ok
@@ -488,7 +490,24 @@ type FileSystem(actualFs: IFileSystem, tryFindFile: string<LocalPath> -> Volatil
         >> Log.addContext "hash" (file.Source.GetHashCode())
       )
 
-      file.Source.ToString() |> System.Text.Encoding.UTF8.GetBytes)
+      // Write source text to bytes in chunks via CopyTo, avoiding a full intermediate string allocation
+      let source = file.Source :> FSharp.Compiler.Text.ISourceText
+      let length = source.Length
+      let ms = new MemoryStream()
+      let utf8NoBom = System.Text.UTF8Encoding(encoderShouldEmitUTF8Identifier = false)
+      use sw = new StreamWriter(ms, utf8NoBom, bufferSize = 4096, leaveOpen = true)
+      let chunkSize = 8192
+      let charBuffer = Array.zeroCreate (min length chunkSize)
+      let mutable offset = 0
+
+      while offset < length do
+        let count = min (length - offset) charBuffer.Length
+        source.CopyTo(offset, charBuffer, 0, count)
+        sw.Write(charBuffer, 0, count)
+        offset <- offset + count
+
+      sw.Flush()
+      ms.ToArray())
 
   /// translation of the BCL's Windows logic for Path.IsPathRooted.
   ///

src/FsAutoComplete/LspServers/AdaptiveFSharpLspServer.fs

@@ -763,10 +763,14 @@ type AdaptiveFSharpLspServer
                   | Error e -> return Error e
                 }
 
-              let getCompletions forceGetTypeCheckResultsStale =
+              let getCompletions forceGetTypeCheckResultsStale rereadFile =
                 asyncResult {
 
-                  let! volatileFile = state.GetOpenFileOrRead filePath
+                  let! volatileFile =
+                    if rereadFile then
+                      state.GetOpenFileOrRead filePath
+                    else
+                      async { return Ok volatileFile }
 
                   let! lineStr =
                     volatileFile.Source
@@ -815,8 +819,12 @@ type AdaptiveFSharpLspServer
                 match e with
                 | "Should not have empty completions" ->
                   // If we don't get any completions, assume we need to wait for a full typecheck
-                  getCompletions state.GetOpenFileTypeCheckResults
-                | _ -> getCompletions state.GetOpenFileTypeCheckResultsCached
+                  // No need to re-read the file — only the typecheck results matter
+                  getCompletions state.GetOpenFileTypeCheckResults false
+                | "TextDocumentCompletion was sent before TextDocumentDidChange" ->
+                  // File content is stale, re-read on next attempt
+                  getCompletions state.GetOpenFileTypeCheckResultsCached true
+                | _ -> getCompletions state.GetOpenFileTypeCheckResultsCached true
 
               let getCodeToInsert (d: DeclarationListItem) =
                 match d.NamespaceToOpen with
@@ -850,7 +858,7 @@ type AdaptiveFSharpLspServer
                   (TimeSpan.FromMilliseconds(15.))
                   100
                   handleError
-                  (getCompletions state.GetOpenFileTypeCheckResultsCached)
+                  (getCompletions state.GetOpenFileTypeCheckResultsCached false)
                 |> AsyncResult.ofStringErr
               with
               | None -> return! LspResult.success (None)

src/FsAutoComplete/LspServers/AdaptiveServerState.fs

@@ -1773,7 +1773,7 @@ type AdaptiveState
       let tags =
         [ SemanticConventions.fsac_sourceCodePath, box (UMX.untag file.Source.FileName)
           SemanticConventions.projectFilePath, box (options.ProjectFileName)
-          "source.text", box (file.Source.String)
+          "source.length", box (file.Source.Length)
           "source.version", box (file.Version)
 
           ]