Only emit ANSI escapes when process is terminal

scripts/ci.sh

@@ -9,6 +9,7 @@ echo "Starting build at: ${start} on ${host_name}"
 export RUST_BACKTRACE="full"
 
 cargo deny check
+cargo +nightly fmt --all -- --check
 cargo build --verbose
 cargo test --verbose --all-features
 cargo clippy --workspace --all-targets --all-features -- --deny warnings

src/macros_utils.rs

@@ -1,4 +1,5 @@
 use std::collections::BTreeMap;
+use std::io::IsTerminal as _;
 use std::ops::Range;
 
 use codespan_reporting::diagnostic::{Diagnostic, Label};
@@ -89,13 +90,20 @@ pub fn format_error<'a>(json: &'a Value, error: &Error<'a>) -> String {
         )
         .with_message(error.to_string())]);
 
-    let output = Vec::<u8>::new();
-    let mut writer = termcolor::Ansi::new(output);
     let config = term::Config::default();
+    let bytes = Vec::<u8>::new();
+
+    let bytes = if std::io::stderr().is_terminal() {
+        let mut writer = termcolor::Ansi::new(bytes);
+        term::emit(&mut writer, &config, &files, &diagnostic).unwrap();
+        writer.into_inner()
+    } else {
+        let mut writer = termcolor::NoColor::new(bytes);
+        term::emit(&mut writer, &config, &files, &diagnostic).unwrap();
+        writer.into_inner()
+    };
 
-    term::emit(&mut writer, &config, &files, &diagnostic).unwrap();
-
-    String::from_utf8(writer.into_inner()).unwrap()
+    String::from_utf8(bytes).unwrap()
 }
 
 /// Serialize a JSON [Value] and keeps the span information of each

tests/error_msg.rs

@@ -1,17 +1,34 @@
+use std::any::Any;
+use std::io::IsTerminal as _;
+
 use assert_json::{assert_json, validators};
 use indoc::indoc;
 
 macro_rules! assert_panic_output {
-    ($output:expr, $($assert:tt)+) => {{
+    ($expected_output:expr, $($assert:tt)+) => {{
         let out_result = std::panic::catch_unwind(|| $($assert)+);
-        let out_err = out_result.err().unwrap();
-        assert!(out_err.is::<String>());
-        let out = out_err.downcast_ref::<String>().unwrap();
-        let out = String::from_utf8(strip_ansi_escapes::strip(out.clone().into_bytes())).unwrap();
-        assert!(out.contains($output.trim()), "\n\texpected:\n{}\n\tgot:\n{}", $output.trim(), out)
+        let err = out_result_to_string(out_result);
+        let expected_output = $expected_output.trim();
+        assert!(err.contains(expected_output), "\n\texpected:\n{expected_output}\n\tgot:\n{err}")
     }};
 }
 
+#[expect(unsafe_code)]
+fn out_result_to_string(result: Result<(), Box<dyn Any + Send>>) -> String {
+    let err = result.unwrap_err();
+    let s = err
+        .downcast::<String>()
+        .expect("the assert output should be a String");
+
+    // ANSI escapes should only be written when `assert_json!` is called from a terminal
+    if std::io::stderr().is_terminal() {
+        let bytes = strip_ansi_escapes::strip(s.into_bytes());
+        unsafe { String::from_utf8_unchecked(bytes) }
+    } else {
+        *s
+    }
+}
+
 #[test]
 fn primitive_invalid_type() {
     let expected_output = indoc! {r"