Print errors to stderr (#173)

buildpacks/dotnet/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Error messages are now printed to stderr. [#173](https://github.com/heroku/buildpacks-dotnet/pull/173)
+
 ## [0.1.9] - 2024-12-04
 
 ### Added

buildpacks/dotnet/src/errors.rs

@@ -5,7 +5,7 @@ use crate::layers::sdk::SdkLayerError;
 use crate::DotnetBuildpackError;
 use bullet_stream::{style, Print};
 use indoc::formatdoc;
-use std::io::{self, stdout};
+use std::io::{self, stderr};
 
 pub(crate) fn on_error(error: libcnb::Error<DotnetBuildpackError>) {
     match error {
@@ -308,7 +308,7 @@ fn log_io_error(header: &str, occurred_while: &str, io_error: &io::Error) {
 }
 
 fn log_error(header: impl AsRef<str>, body: impl AsRef<str>, error: Option<String>) {
-    let mut log = Print::new(stdout()).without_header();
+    let mut log = Print::new(stderr()).without_header();
     if let Some(error) = error {
         let bullet = log.bullet(style::important("Debug info"));
         log = bullet.sub_bullet(error).done();

buildpacks/dotnet/tests/dotnet_publish_test.rs

@@ -1,6 +1,6 @@
 use crate::tests::default_build_config;
-use indoc::formatdoc;
-use libcnb_test::{assert_contains, assert_empty, TestRunner};
+use indoc::{formatdoc, indoc};
+use libcnb_test::{assert_contains, assert_empty, PackResult, TestRunner};
 use regex::Regex;
 
 #[test]
@@ -30,6 +30,33 @@ fn test_dotnet_publish_multi_tfm_solution() {
     );
 }
 
+#[test]
+#[ignore = "integration test"]
+fn test_dotnet_publish_with_compilation_error() {
+    TestRunner::default().build(
+        default_build_config("tests/fixtures/console_with_compilation_error")
+            .expected_pack_result(PackResult::Failure),
+        |context| {
+            assert_contains!(
+                &context.pack_stderr,
+                &indoc! {r"
+                  ! Unable to publish
+                  !
+                  ! The `dotnet publish` command exited unsuccessfully (exit status: 1).
+                  !
+                  ! This error usually happens due to compilation errors. Use the command output
+                  ! above to troubleshoot and retry your build.
+                  !
+                  ! The publish process can also fail for a number of other reasons, such as
+                  ! intermittent network issues, unavailability of the NuGet package feed and/or
+                  ! other external dependencies, etc.
+                  !
+                  ! Try again to see if the error resolves itself."}
+            );
+        },
+    );
+}
+
 #[test]
 #[ignore = "integration test"]
 fn test_dotnet_publish_with_debug_configuration() {

buildpacks/dotnet/tests/fixtures/console_with_compilation_error/Program.cs

@@ -0,0 +1 @@
+Console.WriteFoo;

buildpacks/dotnet/tests/fixtures/console_with_compilation_error/consoleapp.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>