Improve compiletest expected/not found formatting

compiletest, oh compiletest, you are truly one of the tools in this
repository. You're the omnipresent gatekeeper, ensuring that every new
change works, doesn't break the world, and is nice. We thank you for
your work, for your tests, for your test runs, for your features that
help writing tests, for all the stability and and good you have caused.
Without you, Rust wouldn't exist as it does, without you, nothing would
work, without you, we would all go insane from having changes break and
having to test them all by hand. Thank you, compiletest.

but holy shit i fucking hate your stupid debug output so much i simply
cannot take this anymore aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

By changing a few magic lines in this file called "runtest.rs", we can
cause compiletest to emit nicer messages. This is widely regarded as a
good thing. We stop wasting vertical space, allowing more errors to be
displayed at once. Additionally, we add colors, which make it so much
more pretty *and* gay, both of which are very good and useful.

There's a bit of fuckery needed to get the colors to work. `colored`
checks whether stdout is a terminal. We also print to stdout, so that
works well.
But.... for some stupid reason that I absolutely refuse to even attempt
to debug, stdout is *not* a terminal when executing tests *in a
terminal*.
But stderr is >:).
So this just checks whether stderr is a terminal.
If you have a use case where you dump compiletest stdout into a place
where colors are not supported while having stderr be a terminal, then
I'm sorry for you, but you are gonna get colors and you're gonna like
it. Stop it with the usual environment variable, which `colored` also
respects by default.

src/tools/compiletest/src/errors.rs

@@ -57,6 +57,18 @@ pub struct Error {
     pub msg: String,
 }
 
+impl Error {
+    pub fn render_for_expected(&self) -> String {
+        use colored::Colorize;
+        format!(
+            "{: <10}line {: >3}: {}",
+            self.kind.map(|kind| kind.to_string()).unwrap_or_default().to_uppercase(),
+            self.line_num,
+            self.msg.cyan(),
+        )
+    }
+}
+
 #[derive(PartialEq, Debug)]
 enum WhichLine {
     ThisLine,

src/tools/compiletest/src/main.rs

@@ -1,10 +1,16 @@
-use std::{env, sync::Arc};
+use std::{env, io::IsTerminal, sync::Arc};
 
 use compiletest::{common::Mode, log_config, parse_config, run_tests};
 
 fn main() {
     tracing_subscriber::fmt::init();
 
+    // colored checks stdout by default, but for some reason only stderr is a terminal.
+    // compiletest *does* print many things to stdout, but it doesn't really matter.
+    if std::io::stderr().is_terminal() && std::env::var("NO_COLOR") != Ok("1".to_owned()) {
+        colored::control::set_override(true);
+    }
+
     let config = Arc::new(parse_config(env::args().collect()));
 
     if config.valgrind_path.is_none() && config.force_valgrind {

src/tools/compiletest/src/runtest.rs

@@ -17,10 +17,10 @@ use crate::json;
 use crate::read2::{read2_abbreviated, Truncated};
 use crate::util::{add_dylib_path, dylib_env_var, logv, PathBufExt};
 use crate::ColorConfig;
+use colored::Colorize;
 use miropt_test_tools::{files_for_miropt_test, MiroptTest, MiroptTestFile};
 use regex::{Captures, Regex};
 use rustfix::{apply_suggestions, get_suggestions_from_json, Filter};
-
 use std::collections::{HashMap, HashSet};
 use std::env;
 use std::ffi::{OsStr, OsString};
@@ -1493,14 +1493,22 @@ impl<'test> TestCx<'test> {
                 unexpected.len(),
                 not_found.len()
             ));
-            println!("status: {}\ncommand: {}", proc_res.status, proc_res.cmdline);
+            println!("status: {}\ncommand: {}\n", proc_res.status, proc_res.cmdline);
             if !unexpected.is_empty() {
-                println!("unexpected errors (from JSON output): {:#?}\n", unexpected);
+                println!("{}", "--- unexpected errors (from JSON output) ---".green());
+                for error in &unexpected {
+                    println!("{}", error.render_for_expected());
+                }
+                println!("{}", "---".green());
             }
             if !not_found.is_empty() {
-                println!("not found errors (from test file): {:#?}\n", not_found);
+                println!("{}", "--- not found errors (from test file) ---".red());
+                for error in &not_found {
+                    println!("{}", error.render_for_expected());
+                }
+                println!("{}", "---".red());
             }
-            panic!();
+            panic!("errors differ from expected");
         }
     }