Print colored warnings when build script panics

Fixes #3672

src/cargo/core/compiler/custom_build.rs

@@ -11,7 +11,9 @@ use std::collections::hash_map::{Entry, HashMap};
 use std::collections::{BTreeSet, HashSet};
 use std::path::{Path, PathBuf};
 use std::str;
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
+
+const CARGO_WARNING: &str = "cargo:warning=";
 
 /// Contains the parsed output of a custom build script.
 #[derive(Clone, Debug, Hash, Default)]
@@ -343,9 +345,13 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> CargoRes
         state.running(&cmd);
         let timestamp = paths::set_invocation_time(&script_run_dir)?;
         let prefix = format!("[{} {}] ", id.name(), id.version());
+        let mut warnings_in_case_of_panic = Vec::new();
         let output = cmd
             .exec_with_streaming(
                 &mut |stdout| {
+                    if stdout.starts_with(CARGO_WARNING) {
+                        warnings_in_case_of_panic.push(stdout[CARGO_WARNING.len()..].to_owned());
+                    }
                     if extra_verbose {
                         state.stdout(format!("{}{}", prefix, stdout));
                     }
@@ -359,7 +365,19 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> CargoRes
                 },
                 true,
             )
-            .chain_err(|| format!("failed to run custom build command for `{}`", pkg_name))?;
+            .chain_err(|| format!("failed to run custom build command for `{}`", pkg_name));
+
+        if let Err(error) = output {
+            insert_warnings_in_build_outputs(
+                build_script_outputs,
+                id,
+                metadata_hash,
+                warnings_in_case_of_panic,
+            );
+            return Err(error);
+        }
+
+        let output = output.unwrap();
 
         // After the build command has finished running, we need to be sure to
         // remember all of its output so we can later discover precisely what it
@@ -429,6 +447,22 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> CargoRes
     Ok(job)
 }
 
+fn insert_warnings_in_build_outputs(
+    build_script_outputs: Arc<Mutex<BuildScriptOutputs>>,
+    id: PackageId,
+    metadata_hash: Metadata,
+    warnings: Vec<String>,
+) {
+    let build_output_with_only_warnings = BuildOutput {
+        warnings,
+        ..BuildOutput::default()
+    };
+    build_script_outputs
+        .lock()
+        .unwrap()
+        .insert(id, metadata_hash, build_output_with_only_warnings);
+}
+
 impl BuildOutput {
     pub fn parse_file(
         path: &Path,

tests/testsuite/build_script.rs

@@ -2980,6 +2980,39 @@ warning: bar
         .run();
 }
 
+#[cargo_test]
+fn warnings_emitted_when_build_script_panics() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [project]
+            name = "foo"
+            version = "0.5.0"
+            authors = []
+            build = "build.rs"
+        "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "build.rs",
+            r#"
+            fn main() {
+                println!("cargo:warning=foo");
+                println!("cargo:warning=bar");
+                panic!();
+            }
+        "#,
+        )
+        .build();
+
+    p.cargo("build")
+        .with_status(101)
+        .with_stdout("")
+        .with_stderr_contains("warning: foo\nwarning: bar")
+        .run();
+}
+
 #[cargo_test]
 fn warnings_hidden_for_upstream() {
     Package::new("bar", "0.1.0")