compiletest: Don't panic when trying to print unknown JSON after failure

If we're printing process output after some other check has failed, and the
output is JSON-like, we shouldn't necessarily assume that it is valid JSON
produced by the compiler. If it isn't, printing it as-is is more helpful than
printing a less-relevant error message.

Author: Zalathar

src/tools/compiletest/src/json.rs

@@ -91,7 +91,12 @@ pub fn rustfix_diagnostics_only(output: &str) -> String {
         .collect()
 }
 
-pub fn extract_rendered(output: &str) -> String {
+pub enum OnUnknownJson {
+    Error,
+    Print,
+}
+
+pub fn extract_rendered(output: &str, on_unknown_json: OnUnknownJson) -> String {
     output
         .lines()
         .filter_map(|line| {
@@ -125,11 +130,16 @@ pub fn extract_rendered(output: &str) -> String {
                     // Ignore the notification.
                     None
                 } else {
-                    print!(
-                        "failed to decode compiler output as json: line: {}\noutput: {}",
-                        line, output
-                    );
-                    panic!()
+                    match on_unknown_json {
+                        OnUnknownJson::Error => {
+                            print!(
+                                "failed to decode compiler output as json: line: {}\noutput: {}",
+                                line, output
+                            );
+                            panic!()
+                        }
+                        OnUnknownJson::Print => Some(format!("{}\n", line)),
+                    }
                 }
             } else {
                 // preserve non-JSON lines, such as ICEs

src/tools/compiletest/src/runtest.rs

@@ -31,7 +31,7 @@ use std::io::{self, BufReader};
 use std::iter;
 use std::path::{Path, PathBuf};
 use std::process::{Child, Command, ExitStatus, Output, Stdio};
-use std::str;
+use std::str::{self, FromStr};
 use std::sync::Arc;
 
 use anyhow::Context;
@@ -3783,7 +3783,7 @@ impl<'test> TestCx<'test> {
         } else if explicit_format {
             proc_res.stderr.clone()
         } else {
-            json::extract_rendered(&proc_res.stderr)
+            json::extract_rendered(&proc_res.stderr, json::OnUnknownJson::Error)
         };
 
         let normalized_stderr = self.normalize_output(&stderr, &self.props.normalize_stderr);
@@ -4564,7 +4564,7 @@ pub struct ProcRes {
 impl ProcRes {
     pub fn print_info(&self) {
         fn render(name: &str, contents: &str) -> String {
-            let contents = json::extract_rendered(contents);
+            let contents = json::extract_rendered(contents, json::OnUnknownJson::Print);
             let contents = contents.trim_end();
             if contents.is_empty() {
                 format!("{name}: none")