Repl prints an error twice

[nicolo-ribaudo]

Version: Deno 2.1.7

1. Run deno to start the repl
2. Execute for (console.log("a") of [1]);

It reports the syntax error twice:

➜ deno            
Deno 2.1.7
exit using ctrl+d, ctrl+c, or close()
REPL is running with all permissions allowed.
To specify permissions, run `deno repl` with allow flags.
> for (console.log("a") of [1]);
error: The left-hand side of an assignment expression must be a variable or a property access. at file:///repl.tsx:1:6

  for (console.log("a") of [1]);
       ~~~~~~~~~~~~~~~~

The left-hand side of an assignment expression must be a variable or a property access. at file:///repl.tsx:1:6

  for (console.log("a") of [1]);
       ~~~~~~~~~~~~~~~~

This is not a SWC bug (in their playground they only report it once), and I couldn't find other syntax errors that are printed twice.

I cannot find any other error for which this happens.

[bartlomieju]

I can reproduce it, it's most likely related to how we are piping transpilation errors back into the REPL. I'll look into fixing this.

[bartlomieju]

Here's what I found:

1. The transpilation actually produces two parsing diagnostics:

FoldProgram(
    ParseDiagnostics(
        ParseDiagnosticsError(
            [
                ParseDiagnostic {
                    specifier: Url {
                        scheme: "file",
                        cannot_be_a_base: false,
                        username: "",
                        password: None,
                        host: None,
                        port: None,
                        path: "/repl.tsx",
                        query: None,
                        fragment: None,
                    },
                    range: SourceRange {
                        start: SourcePos(
                            5,
                        ),
                        end: SourcePos(
                            21,
                        ),
                    },
                    kind: TS2406,
                    source: SourceTextInfo {
                        start_pos: StartSourcePos(
                            0,
                        ),
                        text: "for (console.log(\"a\") of [1]);",
                    },
                },
                ParseDiagnostic {
                    specifier: Url {
                        scheme: "file",
                        cannot_be_a_base: false,
                        username: "",
                        password: None,
                        host: None,
                        port: None,
                        path: "/repl.tsx",
                        query: None,
                        fragment: None,
                    },
                    range: SourceRange {
                        start: SourcePos(
                            5,
                        ),
                        end: SourcePos(
                            21,
                        ),
                    },
                    kind: TS2406,
                    source: SourceTextInfo {
                        start_pos: StartSourcePos(
                            0,
                        ),
                        text: "for (console.log(\"a\") of [1]);",
                    },
                },
            ],
        ),
    ),
)

What is strange is that they're identical.

2. When trying to downcast the error here:
   

   deno/cli/tools/repl/session.rs

   Lines 412 to 415 in 1ef341c

   	match any_and_jserrorbox_downcast_ref::<ParseDiagnosticsError>(
   	&err,
   	) {
   	Some(diagnostics) => Ok(EvaluationOutput::Error(

   
   It's actually not getting downcasted and is treated as "other" error, which is just getting pretty-printed - so it prints both identical diagnostics.