feat(tvix/repl): Support multiline input

Transparently support multiline input in the Tvix REPL, by handling the
UnexpectedEOF error returned by the parser and using it to progressively
build up an input expr over multiple iterations of the REPL's outer
loop.

This works quite nicely:

❯ cargo r --bin tvix
   Compiling tvix-cli v0.1.0 (/home/aspen/code/depot/tvix/cli)
     Finished dev [unoptimized + debuginfo] target(s) in 1.72s
     Running `target/debug/tvix`
tvix-repl> { foo
         >   =
         >   1;
         > }
=> { foo = 1; } :: set
<press up arrow>
tvix-repl> { foo
  =
  1;
}
=> { foo = 1; } :: set

Change-Id: Ib0ed4766b13e8231d696cdc27281ac158e20a777
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11732
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/Cargo.lock

@@ -4119,6 +4119,7 @@ dependencies = [
  "clap",
  "dirs",
  "nix-compat",
+ "rnix",
  "rustyline",
  "thiserror",
  "tokio",

tvix/Cargo.nix

@@ -13042,6 +13042,10 @@ rec {
             name = "nix-compat";
             packageId = "nix-compat";
           }
+          {
+            name = "rnix";
+            packageId = "rnix";
+          }
           {
             name = "rustyline";
             packageId = "rustyline";

tvix/cli/Cargo.toml

@@ -18,6 +18,7 @@ bytes = "1.4.0"
 clap = { version = "4.0", features = ["derive", "env"] }
 dirs = "4.0.0"
 rustyline = "10.0.0"
+rnix = "0.11.0"
 thiserror = "1.0.38"
 tokio = "1.28.0"
 tracing = { version = "0.1.37", features = ["max_level_trace", "release_max_level_info"] }

tvix/cli/src/main.rs

@@ -9,7 +9,7 @@ use tracing_subscriber::{EnvFilter, Layer};
 use tvix_build::buildservice;
 use tvix_eval::builtins::impure_builtins;
 use tvix_eval::observer::{DisassemblingObserver, TracingObserver};
-use tvix_eval::{EvalIO, Value};
+use tvix_eval::{ErrorKind, EvalIO, Value};
 use tvix_glue::builtins::add_fetcher_builtins;
 use tvix_glue::builtins::add_import_builtins;
 use tvix_glue::tvix_io::TvixIO;
@@ -123,6 +123,22 @@ fn init_io_handle(tokio_runtime: &tokio::runtime::Runtime, args: &Args) -> Rc<Tv
     ))
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+enum AllowIncomplete {
+    Allow,
+    #[default]
+    RequireComplete,
+}
+
+impl AllowIncomplete {
+    fn allow(&self) -> bool {
+        matches!(self, Self::Allow)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+struct IncompleteInput;
+
 /// Interprets the given code snippet, printing out warnings, errors
 /// and the result itself. The return value indicates whether
 /// evaluation succeeded.
@@ -132,7 +148,8 @@ fn interpret(
     path: Option<PathBuf>,
     args: &Args,
     explain: bool,
-) -> bool {
+    allow_incomplete: AllowIncomplete,
+) -> Result<bool, IncompleteInput> {
     let mut eval = tvix_eval::Evaluation::new(
         Box::new(TvixIO::new(tvix_store_io.clone() as Rc<dyn EvalIO>)) as Box<dyn EvalIO>,
         true,
@@ -163,6 +180,18 @@ fn interpret(
         eval.evaluate(code, path)
     };
 
+    if allow_incomplete.allow()
+        && result.errors.iter().any(|err| {
+            matches!(
+                &err.kind,
+                ErrorKind::ParseErrors(pes)
+                    if pes.iter().any(|pe| matches!(pe, rnix::parser::ParseError::UnexpectedEOF))
+            )
+        })
+    {
+        return Err(IncompleteInput);
+    }
+
     if args.display_ast {
         if let Some(ref expr) = result.expr {
             eprintln!("AST: {}", tvix_eval::pretty_print_expr(expr));
@@ -188,7 +217,7 @@ fn interpret(
     }
 
     // inform the caller about any errors
-    result.errors.is_empty()
+    Ok(result.errors.is_empty())
 }
 
 /// Interpret the given code snippet, but only run the Tvix compiler
@@ -257,7 +286,16 @@ fn main() {
     if let Some(file) = &args.script {
         run_file(io_handle, file.clone(), &args)
     } else if let Some(expr) = &args.expr {
-        if !interpret(io_handle, expr, None, &args, false) {
+        if !interpret(
+            io_handle,
+            expr,
+            None,
+            &args,
+            false,
+            AllowIncomplete::RequireComplete,
+        )
+        .unwrap()
+        {
             std::process::exit(1);
         }
     } else {
@@ -274,7 +312,15 @@ fn run_file(io_handle: Rc<TvixStoreIO>, mut path: PathBuf, args: &Args) {
     let success = if args.compile_only {
         lint(&contents, Some(path), args)
     } else {
-        interpret(io_handle, &contents, Some(path), args, false)
+        interpret(
+            io_handle,
+            &contents,
+            Some(path),
+            args,
+            false,
+            AllowIncomplete::RequireComplete,
+        )
+        .unwrap()
     };
 
     if !success {
@@ -318,20 +364,59 @@ fn run_prompt(io_handle: Rc<TvixStoreIO>, args: &Args) {
         None => None,
     };
 
+    let mut multiline_input: Option<String> = None;
     loop {
-        let readline = rl.readline("tvix-repl> ");
+        let prompt = if multiline_input.is_some() {
+            "         > "
+        } else {
+            "tvix-repl> "
+        };
+
+        let readline = rl.readline(prompt);
         match readline {
             Ok(line) => {
                 if line.is_empty() {
                     continue;
                 }
 
-                rl.add_history_entry(&line);
-
-                if let Some(without_prefix) = line.strip_prefix(":d ") {
-                    interpret(Rc::clone(&io_handle), without_prefix, None, args, true);
+                let input = if let Some(mi) = &mut multiline_input {
+                    mi.push('\n');
+                    mi.push_str(&line);
+                    mi
                 } else {
-                    interpret(Rc::clone(&io_handle), &line, None, args, false);
+                    &line
+                };
+
+                let res = if let Some(without_prefix) = input.strip_prefix(":d ") {
+                    interpret(
+                        Rc::clone(&io_handle),
+                        without_prefix,
+                        None,
+                        args,
+                        true,
+                        AllowIncomplete::Allow,
+                    )
+                } else {
+                    interpret(
+                        Rc::clone(&io_handle),
+                        input,
+                        None,
+                        args,
+                        false,
+                        AllowIncomplete::Allow,
+                    )
+                };
+
+                match res {
+                    Ok(_) => {
+                        rl.add_history_entry(input);
+                        multiline_input = None;
+                    }
+                    Err(IncompleteInput) => {
+                        if multiline_input.is_none() {
+                            multiline_input = Some(line);
+                        }
+                    }
                 }
             }
             Err(ReadlineError::Interrupted) | Err(ReadlineError::Eof) => break,