Handle "invalid JSON" from the compiler

Just opportunistically consider lines which start with `{` as valid
JSON, otherwise forward everything else to normal stdout/stderr.

Author: alexcrichton

src/cargo/core/compiler/mod.rs

@@ -1099,8 +1099,17 @@ fn on_stderr_line(
         return Ok(());
     }
 
-    let compiler_message: Box<serde_json::value::RawValue> = serde_json::from_str(line)
-        .map_err(|_| internal(&format!("compiler produced invalid json: `{}`", line)))?;
+    let compiler_message: Box<serde_json::value::RawValue> = match serde_json::from_str(line) {
+        Ok(msg) => msg,
+
+        // If the compiler produced a line that started with `{` but it wasn't
+        // valid JSON, maybe it wasn't JSON in the first place! Forward it along
+        // to stderr.
+        Err(_) => {
+            state.stderr(line);
+            return Ok(());
+        }
+    };
 
     // In some modes of compilation Cargo switches the compiler to JSON mode
     // but the user didn't request that so we still want to print pretty rustc

tests/testsuite/build.rs

@@ -4632,3 +4632,61 @@ fn pipelining_works() {
 ")
         .run();
 }
+
+#[test]
+fn forward_rustc_output() {
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = '2018'
+                [dependencies]
+                bar = { path = "bar" }
+            "#,
+        )
+        .file("src/lib.rs", "bar::foo!();")
+        .file(
+            "bar/Cargo.toml",
+            r#"
+                [package]
+                name = "bar"
+                version = "0.1.0"
+                [lib]
+                proc-macro = true
+            "#,
+        )
+        .file(
+            "bar/src/lib.rs",
+            r#"
+                extern crate proc_macro;
+                use proc_macro::*;
+
+                #[proc_macro]
+                pub fn foo(input: TokenStream) -> TokenStream {
+                    println!("a");
+                    println!("b");
+                    println!("{{}}");
+                    eprintln!("c");
+                    eprintln!("d");
+                    eprintln!("{{a"); // "malformed json"
+                    input
+                }
+            "#,
+        )
+        .build();
+
+    foo.cargo("build")
+        .with_stdout("a\nb\n{}")
+        .with_stderr("\
+[COMPILING] [..]
+[COMPILING] [..]
+c
+d
+{a
+[FINISHED] [..]
+")
+        .run();
+}