infer_schema! reaches the recursion limit easily

[Kazakuri]

Setup

Versions

• Rust: rustc 1.21.0-nightly (2bb8fca18 2017-08-23)
• Diesel: 0.16.0

Problem Description

What are you trying to accomplish?

Having diesel automatically infer the schema for a table with more than ~10 columns.

It looks like the default recursion limit for macros is 64 which can be quite easy to reach with 6 lines of code being generated for each database column.

What is the expected output?

Diesel compiles with no issues.

What is the actual output?

error: recursion limit reached while expanding the macro `table_body`
  --> src\database\schema.rs:1:1
   |
1  | / table! {
2  | |     /// Representation of the `users` table.
3  | |     ///
4  | |     /// (Automatically generated by Diesel.)
...  |
84 | |     }
85 | | }
   | |_^
   |
   = help: consider adding a `#![recursion_limit="128"]` attribute to your crate
   = note: this error originates in a macro outside of the current crate

Steps to reproduce

You can use pretty much any schema, just pad it with a bunch of extra blank documentation comments.

table! {
    /// Representation of the `users` table.
    ///
    /// (Automatically generated by Diesel.)
    users (id) {
        /// The `id` column of the `users` table.
        ///
        /// Its SQL type is `Int4`.
        ///
        /// (Automatically generated by Diesel.)
        id -> Int4,
    }
    ///
    /// etc...
    ///
}

Checklist

• I have already looked over the issue tracker for similar issues.

[killercup]

That is sadly a problem we can't easily solve in Diesel. For now, just follow the compiler's advice: Add the #![recursion_limit="128"] attribute.

Personally, I set this to 1024 at the first error just to not have to deal with this again. Also, IIRC, the default (64 apparently) was recently increased in rustc, so maybe you'll see this error less often in the future.

That said, we could solve this in Diesel: We'd need to replace the macro with a procedural macro. That is not an easy feat (the macro is quite complex and has a lot of edge cases), and not currently our focus. We'll absolutely look more deeply into this once the next batch of proc-macro features becomes stable in rustc, though.

[killercup]

Oh, or do you have a quick fix for this in mind? I've not looked into it more deeply, but if there was an easy change we could make to consume more doc attribute tokens per macro call we'd reduce the table macro's call stack drastically. (Still, not something I really want to spend much time on—there are more pressing matters—but I'd accept a PR for this for sure!)

[mattdeboard]

Hi, so the trouble I'm having is that any table with more than 24 columns (these are extant tables) throws this error. I can run diesel print-schema fine. It outputs the right schema. So since apparently !infer_schema("dotenv:DATABASE_URL", "foo") is equivalent to diesel print-schema -s foo I'm surprised this isn't working correctly.

I tried doing diesel print-schema -s foo > src/models.rs but even then, when I do cargo run, the compiler complains about the recursion limit. Can you explain what the problem is exactly?

edit: Ok apparently the answer is this is a limitation of Rust itself. Here is an explanation with pointers to more detailed explanations.

[Kazakuri]

So basically my understanding of it is that the recursion limit is hit when expanding the table! macro that is generated from the diesel print-schema/!infer_schema("dotenv:DATABASE_URL", "foo") commands.

Like killercup said, the way the table! macro expands is the older^(?) recursive style macros instead of a procedural style, so that every line inside the macro body is a recursively parsed.

So unless the macro gets re-written which has a lot of problems in itself, the only real solution is increasing the recursion limit of your program.

[mattdeboard]

@Kazakuri Ya I have added that to the top of my main.rs but it's still happening. Any idea?

Also I added the huge-tables feature to diesel, but STILL getting the error. There are less than 52 columns on the table :-\