testsuite: Support anyhow error chains in error messages.

This is intended to help with adding more usage of anyhow in the
testsuite, which can help show context for errors.

This also includes some small improvements to the error messages to
provide more information.

crates/cargo-test-support/src/lib.rs

@@ -8,6 +8,7 @@
 
 use std::env;
 use std::ffi::OsStr;
+use std::fmt::Write;
 use std::fs;
 use std::os;
 use std::path::{Path, PathBuf};
@@ -27,11 +28,26 @@ macro_rules! t {
     ($e:expr) => {
         match $e {
             Ok(e) => e,
-            Err(e) => panic!("{} failed with {}", stringify!($e), e),
+            Err(e) => $crate::panic_error(&format!("failed running {}", stringify!($e)), e),
         }
     };
 }
 
+#[track_caller]
+pub fn panic_error(what: &str, err: impl Into<anyhow::Error>) -> ! {
+    let err = err.into();
+    pe(what, err);
+    #[track_caller]
+    fn pe(what: &str, err: anyhow::Error) -> ! {
+        let mut result = format!("{}\nerror: {}", what, err);
+        for cause in err.chain().skip(1) {
+            drop(writeln!(result, "\nCaused by:"));
+            drop(write!(result, "{}", cause));
+        }
+        panic!("\n{}", result);
+    }
+}
+
 pub use cargo_test_macro::cargo_test;
 
 pub mod cross_compile;
@@ -737,7 +753,7 @@ impl Execs {
         self.ran = true;
         let p = (&self.process_builder).clone().unwrap();
         if let Err(e) = self.match_process(&p) {
-            panic!("\n{}", e)
+            panic_error(&format!("test failed running {}", p), e);
         }
     }
 
@@ -748,7 +764,7 @@ impl Execs {
         self.ran = true;
         let p = (&self.process_builder).clone().unwrap();
         match self.match_process(&p) {
-            Err(e) => panic!("\n{}", e),
+            Err(e) => panic_error(&format!("test failed running {}", p), e),
             Ok(output) => serde_json::from_slice(&output.stdout).unwrap_or_else(|e| {
                 panic!(
                     "\nfailed to parse JSON: {}\n\
@@ -764,7 +780,7 @@ impl Execs {
     pub fn run_output(&mut self, output: &Output) {
         self.ran = true;
         if let Err(e) = self.match_output(output) {
-            panic!("\n{}", e)
+            panic_error("process did not return the expected result", e)
         }
     }
 
@@ -858,9 +874,10 @@ impl Execs {
         match self.expect_exit_code {
             None => Ok(()),
             Some(expected) if code == Some(expected) => Ok(()),
-            Some(_) => bail!(
-                "exited with {:?}\n--- stdout\n{}\n--- stderr\n{}",
-                code,
+            Some(expected) => bail!(
+                "process exited with code {} (expected {})\n--- stdout\n{}\n--- stderr\n{}",
+                code.unwrap_or(-1),
+                expected,
                 String::from_utf8_lossy(stdout),
                 String::from_utf8_lossy(stderr)
             ),