From 10fa3eb5033b201fe318c206db805df176b3eea2 Mon Sep 17 00:00:00 2001
From: Mangel Maxime <me@mangelmaxime.fr>
Date: Thu, 22 Oct 2026 14:27:35 +0200
Subject: [PATCH] perf(compiler): opt out of telemetry for dotnet build

---
 src/Fable.Compiler/MSBuildCrackerResolver.fs |  8 ++------
 src/Fable.Compiler/ProjectCracker.fs         |  6 +++++-
 src/Fable.Compiler/Util.fs                   | 15 +++++++++++++--
 src/Fable.Compiler/Util.fsi                  |  5 +++++
 4 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/src/Fable.Compiler/MSBuildCrackerResolver.fs b/src/Fable.Compiler/MSBuildCrackerResolver.fs
index 528676e673..52d95b3d3d 100644
--- a/src/Fable.Compiler/MSBuildCrackerResolver.fs
+++ b/src/Fable.Compiler/MSBuildCrackerResolver.fs
@@ -30,9 +30,6 @@ module private MSBuildCrackerResolver =
 
     type FullPath = string
 
-    [<Literal>]
-    let private TelemetryOptOutVariable = "DOTNET_CLI_TELEMETRY_OPTOUT"
-
     let private run_dotnet_msbuild (cwd: string) (fsproj: FullPath) (args: string) (defines: string list) =
         backgroundTask {
             let psi = ProcessStartInfo "dotnet"
@@ -52,9 +49,8 @@ module private MSBuildCrackerResolver =
             // https://github.com/fable-compiler/Fable/issues/4014
             psi.Environment.Add("DOTNET_NOLOGO", "1")
 
-            // The dotnet CLI blocks on sending telemetry at exit, which can take seconds on a restricted network
-            if not (psi.Environment.ContainsKey TelemetryOptOutVariable) then
-                psi.Environment.Add(TelemetryOptOutVariable, "1")
+            for key, value in Fable.Compiler.Util.Process.dotnetEnvVars () do
+                psi.Environment[key] <- value
 
             if not (List.isEmpty defines) then
                 let definesValue = defines |> String.concat ";"
diff --git a/src/Fable.Compiler/ProjectCracker.fs b/src/Fable.Compiler/ProjectCracker.fs
index 9cf5b403ec..f381563ab6 100644
--- a/src/Fable.Compiler/ProjectCracker.fs
+++ b/src/Fable.Compiler/ProjectCracker.fs
@@ -158,7 +158,11 @@ type CrackerOptions(cliArgs: CliArgs, evaluateOnly: bool) =
 
             pendingDllBuilds[normalizedDllPath] <-
                 Task.Run(fun () ->
-                    Process.runSyncWithOutput projDir "dotnet" [ "build"; "-c"; cliArgs.Configuration; "--no-restore" ]
+                    Process.runSyncWithOutputEnv
+                        (Process.dotnetEnvVars ())
+                        projDir
+                        "dotnet"
+                        [ "build"; "-c"; cliArgs.Configuration; "--no-restore" ]
                 )
 
     member _.WaitForBuiltDlls() =
diff --git a/src/Fable.Compiler/Util.fs b/src/Fable.Compiler/Util.fs
index 9b938f5cbb..27ced7521a 100644
--- a/src/Fable.Compiler/Util.fs
+++ b/src/Fable.Compiler/Util.fs
@@ -474,13 +474,24 @@ module Process =
     let runSync (workingDir: string) (exePath: string) (args: string list) =
         runSyncWithEnv [] workingDir exePath args
 
-    let runSyncWithOutput workingDir exePath args =
-        let p = startProcess true [] workingDir exePath args
+    /// Environment for the `dotnet` processes Fable starts for itself, not for the user's --run commands
+    let dotnetEnvVars () =
+        // The dotnet CLI blocks on sending telemetry at exit, which can take seconds on a restricted network
+        if isNull (Environment.GetEnvironmentVariable "DOTNET_CLI_TELEMETRY_OPTOUT") then
+            [ "DOTNET_CLI_TELEMETRY_OPTOUT", "1" ]
+        else
+            []
+
+    let runSyncWithOutputEnv envVars workingDir exePath args =
+        let p = startProcess true envVars workingDir exePath args
         // Don't wait indefinitely to run process
         // This call is used to build local plugins, if the binary is used by another process this process will never end.
         p.WaitForExit 7000 |> ignore
         p.StandardOutput.ReadToEnd()
 
+    let runSyncWithOutput workingDir exePath args =
+        runSyncWithOutputEnv [] workingDir exePath args
+
 [<RequireQualifiedAccess>]
 module Async =
     let fold f (state: 'State) (xs: 'T seq) =
diff --git a/src/Fable.Compiler/Util.fsi b/src/Fable.Compiler/Util.fsi
index f856957270..4cecd1a3a4 100644
--- a/src/Fable.Compiler/Util.fsi
+++ b/src/Fable.Compiler/Util.fsi
@@ -94,6 +94,11 @@ module Process =
 
     val runSyncWithOutput: string -> string -> string list -> string
 
+    val runSyncWithOutputEnv: envVars: (string * string) list -> string -> string -> string list -> string
+
+    /// Environment for the `dotnet` processes Fable starts for itself, not for the user's --run commands
+    val dotnetEnvVars: unit -> (string * string) list
+
 type PathResolver =
     abstract TryPrecompiledOutPath: sourceDir: string * relativePath: string -> string option