add UI test for unpretty

compiler/rustc_driver/src/pretty.rs

@@ -1,5 +1,6 @@
 //! The various pretty-printing routines.
 
+use crate::session_diagnostics::UnprettyDumpFail;
 use rustc_ast as ast;
 use rustc_ast_pretty::pprust;
 use rustc_errors::ErrorGuaranteed;
@@ -362,11 +363,10 @@ fn write_or_print(out: &str, ofile: Option<&Path>, sess: &Session) {
         None => print!("{}", out),
         Some(p) => {
             if let Err(e) = std::fs::write(p, out) {
-                let mut err = sess.struct_fatal(&format!(
-                    "pretty-print failed to write {} due to error `{e}`",
-                    p.display()
-                ));
-                err.emit();
+                sess.emit_fatal(UnprettyDumpFail {
+                    path: p.display().to_string(),
+                    err: e.to_string(),
+                });
             }
         }
     }

compiler/rustc_driver/src/session_diagnostics.rs

@@ -31,3 +31,10 @@ pub(crate) struct RLinkRustcVersionMismatch<'a> {
 #[derive(SessionDiagnostic)]
 #[diag(driver::rlink_no_a_file)]
 pub(crate) struct RlinkNotAFile;
+
+#[derive(SessionDiagnostic)]
+#[diag(driver::unpretty_dump_fail)]
+pub(crate) struct UnprettyDumpFail {
+    pub path: String,
+    pub err: String,
+}

compiler/rustc_error_messages/locales/en-US/driver.ftl

@@ -9,3 +9,5 @@ driver_rlink_encoding_version_mismatch = .rlink file was produced with encoding
 driver_rlink_rustc_version_mismatch = .rlink file was produced by rustc version `{$rustc_version}`, but the current version is `{$current_version}`
 
 driver_rlink_no_a_file = rlink must be a file
+
+driver_unpretty_dump_fail = pretty-print failed to write `{$path}` due to error `{$err}`

src/test/ui/unpretty/avoid-crash.rs

@@ -0,0 +1,3 @@
+// compile-flags: -o/tmp/ -Zunpretty=ast-tree
+
+fn main() {}

src/test/ui/unpretty/avoid-crash.stderr

@@ -0,0 +1,4 @@
+error: pretty-print failed to write `/tmp/` due to error `Is a directory (os error 21)`
+
+error: aborting due to previous error
+