feat: add cargo_output to eliminate last vestiges of stdout pollution

Author: Gankra

Cargo.toml

@@ -30,6 +30,8 @@ libc = { version = "0.2.62", default-features = false, optional = true }
 
 [features]
 parallel = ["dep:libc", "dep:jobserver", "dep:once_cell"]
+# enables OutputKind::Stderr which requires rustc 1.74.0 -- currently above msrv
+stdout_to_stderr = []
 
 [dev-dependencies]
 tempfile = "3"

src/command_helpers.rs

@@ -15,13 +15,14 @@ use std::{
     },
 };
 
-use crate::{Error, ErrorKind, Object};
+use crate::{Error, ErrorKind, Object, OutputKind};
 
 #[derive(Clone, Debug)]
 pub(crate) struct CargoOutput {
     pub(crate) metadata: bool,
     pub(crate) warnings: bool,
     pub(crate) debug: bool,
+    pub(crate) output: OutputKind,
     checked_dbg_var: Arc<AtomicBool>,
 }
 
@@ -31,6 +32,7 @@ impl CargoOutput {
         Self {
             metadata: true,
             warnings: true,
+            output: OutputKind::Forward,
             debug: std::env::var_os("CC_ENABLE_DEBUG_OUTPUT").is_some(),
             checked_dbg_var: Arc::new(AtomicBool::new(false)),
         }
@@ -65,6 +67,16 @@ impl CargoOutput {
             Stdio::null()
         }
     }
+
+    fn stdio_for_output(&self) -> Stdio {
+        match self.output {
+            OutputKind::Capture => Stdio::piped(),
+            OutputKind::Forward => Stdio::inherit(),
+            OutputKind::Discard => Stdio::null(),
+            #[cfg(feature = "stdout_to_stderr")]
+            OutputKind::Stderr => std::io::stderr().into(),
+        }
+    }
 }
 
 pub(crate) struct StderrForwarder {
@@ -321,9 +333,10 @@ pub(crate) fn run_output(
 ) -> Result<Vec<u8>, Error> {
     let program = program.as_ref();
 
-    cmd.stdout(Stdio::piped());
-
-    let mut child = spawn(cmd, program, cargo_output)?;
+    // We specifically need the output to be captured, so override default
+    let mut captured_cargo_output = cargo_output.clone();
+    captured_cargo_output.output = OutputKind::Capture;
+    let mut child = spawn(cmd, program, &captured_cargo_output)?;
 
     let mut stdout = vec![];
     child
@@ -333,6 +346,7 @@ pub(crate) fn run_output(
         .read_to_end(&mut stdout)
         .unwrap();
 
+    // Don't care about this output, use the normal settings
     wait_on_child(cmd, program, &mut child, cargo_output)?;
 
     Ok(stdout)
@@ -356,7 +370,11 @@ pub(crate) fn spawn(
     cargo_output.print_debug(&format_args!("running: {:?}", cmd));
 
     let cmd = ResetStderr(cmd);
-    let child = cmd.0.stderr(cargo_output.stdio_for_warnings()).spawn();
+    let child = cmd
+        .0
+        .stderr(cargo_output.stdio_for_warnings())
+        .stdout(cargo_output.stdio_for_output())
+        .spawn();
     match child {
         Ok(child) => Ok(child),
         Err(ref e) if e.kind() == io::ErrorKind::NotFound => {

src/lib.rs

@@ -375,6 +375,25 @@ impl Object {
     }
 }
 
+/// Different strategies for handling compiler output (to stdout)
+#[derive(Clone, Debug)]
+pub enum OutputKind {
+    /// Forward the output to this process' stdout (Stdio::inherit)
+    Forward,
+    /// Discard the output (Stdio::null)
+    Discard,
+    /// Capture the result
+    ///
+    /// Currently this is internal-only (it's just redundant with Discard as a public interface)
+    #[doc(hidden)]
+    Capture,
+    /// Forward the output to this process' stderr
+    ///
+    /// Requires Rust 1.74.0
+    #[cfg(feature = "stdout_to_stderr")]
+    Stderr,
+}
+
 impl Build {
     /// Construct a new instance of a blank set of configuration.
     ///
@@ -1196,6 +1215,16 @@ impl Build {
         self
     }
 
+    /// Define whether compiler output (to stdout) should be emitted. Defaults to `Forward`
+    /// (forward compiler stdout to this process' stdout)
+    ///
+    /// Some compilers emit errors to stdout, so if you *really* need stdout to be clean
+    /// you should also set this to `Discard`.
+    pub fn cargo_output(&mut self, cargo_output: OutputKind) -> &mut Build {
+        self.cargo_output.output = cargo_output;
+        self
+    }
+
     /// Adds a native library modifier that will be added to the
     /// `rustc-link-lib=static:MODIFIERS=LIBRARY_NAME` metadata line
     /// emitted for cargo if `cargo_metadata` is enabled.

src/tool.rs

@@ -5,15 +5,15 @@ use std::{
     ffi::{OsStr, OsString},
     io::Write,
     path::{Path, PathBuf},
-    process::{Command, Stdio},
+    process::Command,
     sync::RwLock,
 };
 
 use crate::{
     command_helpers::{run_output, CargoOutput},
     run,
     tempfile::NamedTempfile,
-    Error, ErrorKind,
+    Error, ErrorKind, OutputKind,
 };
 
 /// Configuration used to represent an invocation of a C compiler.
@@ -158,14 +158,14 @@ impl Tool {
             cargo_output.print_debug(&stdout);
 
             // https://gitlab.kitware.com/cmake/cmake/-/blob/69a2eeb9dff5b60f2f1e5b425002a0fd45b7cadb/Modules/CMakeDetermineCompilerId.cmake#L267-271
-            let accepts_cl_style_flags =
-                run(Command::new(path).arg("-?").stdout(Stdio::null()), path, &{
-                    // the errors are not errors!
-                    let mut cargo_output = cargo_output.clone();
-                    cargo_output.warnings = cargo_output.debug;
-                    cargo_output
-                })
-                .is_ok();
+            let accepts_cl_style_flags = run(Command::new(path).arg("-?"), path, &{
+                // the errors are not errors!
+                let mut cargo_output = cargo_output.clone();
+                cargo_output.warnings = cargo_output.debug;
+                cargo_output.output = OutputKind::Discard;
+                cargo_output
+            })
+            .is_ok();
 
             let clang = stdout.contains(r#""clang""#);
             let gcc = stdout.contains(r#""gcc""#);
@@ -283,7 +283,7 @@ impl Tool {
     /// Don't push optimization arg if it conflicts with existing args.
     pub(crate) fn push_opt_unless_duplicate(&mut self, flag: OsString) {
         if self.is_duplicate_opt_arg(&flag) {
-            println!("Info: Ignoring duplicate arg {:?}", &flag);
+            eprintln!("Info: Ignoring duplicate arg {:?}", &flag);
         } else {
             self.push_cc_arg(flag);
         }

src/windows/find_tools.rs

@@ -257,7 +257,7 @@ mod impl_ {
     impl LibraryHandle {
         fn new(name: &[u8]) -> Option<Self> {
             let handle = unsafe { LoadLibraryA(name.as_ptr() as _) };
-            (!handle.is_null()).then(|| Self(handle))
+            (!handle.is_null()).then_some(Self(handle))
         }
 
         /// Get a function pointer to a function in the library.