Fix missing messages when --message-format=json is deeply nested

This commit switches from serde_json::Value to RawValue, which can
process arbitrarily deeply nested JSON content without recursion.

Cargo.toml

@@ -48,7 +48,7 @@ semver = { version = "0.9.0", features = ["serde"] }
 serde = "1.0"
 serde_derive = "1.0"
 serde_ignored = "0.0.4"
-serde_json = "1.0.24"
+serde_json = { version = "1.0.30", features = ["raw_value"] }
 shell-escape = "0.1.4"
 tar = { version = "0.4.15", default-features = false }
 tempfile = "3.0"

src/cargo/util/machine_message.rs

@@ -1,5 +1,5 @@
 use serde::ser;
-use serde_json::{self, Value};
+use serde_json::{self, value::RawValue};
 
 use core::{PackageId, Target};
 
@@ -8,16 +8,17 @@ pub trait Message: ser::Serialize {
 }
 
 pub fn emit<T: Message>(t: &T) {
-    let mut json: Value = serde_json::to_value(t).unwrap();
-    json["reason"] = json!(t.reason());
-    println!("{}", json);
+    let json = serde_json::to_string(t).unwrap();
+    assert!(json.starts_with("{\""));
+    let reason = json!(t.reason());
+    println!("{{\"reason\":{},{}", reason, &json[1..]);
 }
 
 #[derive(Serialize)]
 pub struct FromCompiler<'a> {
     pub package_id: &'a PackageId,
     pub target: &'a Target,
-    pub message: serde_json::Value,
+    pub message: Box<RawValue>,
 }
 
 impl<'a> Message for FromCompiler<'a> {

tests/testsuite/check.rs

@@ -1,3 +1,5 @@
+use std::fmt::{self, Write};
+
 use glob::glob;
 use support::install::exe;
 use support::is_nightly;
@@ -690,3 +692,24 @@ fn does_not_use_empty_rustc_wrapper() {
     let p = project().file("src/lib.rs", "").build();
     p.cargo("check").env("RUSTC_WRAPPER", "").run();
 }
+
+#[test]
+fn error_from_deep_recursion() -> Result<(), fmt::Error> {
+    let mut big_macro = String::new();
+    writeln!(big_macro, "macro_rules! m {{")?;
+    for i in 0..130 {
+        writeln!(big_macro, "({}) => {{ m!({}); }};", i, i + 1)?;
+    }
+    writeln!(big_macro, "}}")?;
+    writeln!(big_macro, "m!(0);")?;
+
+    let p = project().file("src/lib.rs", &big_macro).build();
+    p.cargo("check --message-format=json")
+        .with_status(101)
+        .with_stdout_contains(
+            "[..]\"message\":\"recursion limit reached while expanding the macro `m`\"[..]",
+        )
+        .run();
+
+    Ok(())
+}