Macro with unknown variants in match block compiles

[gregorydemay]

Summary

Macro with a match block behaves unexpectedly: when a variant is not recognized, it seems that the first variant in the enum definition is taken instead of failing compilation.

Environment

Observed with askama version 0.12.1 and rustc 1.73.0 (stable-aarch64-apple-darwin unchanged)

Steps to reproduce

Based on the hello world starter template described here:

In templates/hello.html: consider a macro which based on an enum variant displays one link or another:

{% macro etherscan_link -%}
{% match network %}
  {%- when Sepolia -%}
  <a href="https://sepolia.etherscan.io">Link</a>
  {%- when Mainnet -%}
  <a href="https://etherscan.io/address">Link</a>
{% endmatch %}
{%- endmacro %}

Here is the {% call etherscan_link() %}

In main.rs, the file is adjusted to contain the enum definition with 2 variants and the template is instantiated with the second variant.

use askama::Template;

enum EthereumNetwork{
    Sepolia,
    Mainnet
}

#[derive(Template)]
#[template(path = "hello.html")]
struct HelloTemplate<'a> {
    network: EthereumNetwork,
    name: &'a str,
}

fn main() {
    let hello = HelloTemplate { name: "world", network: EthereumNetwork::Mainnet };
    println!("{}", hello.render().unwrap());
}

Expected Result

cargo runs should return

Here is the <a href="https://etherscan.io/address">Link</a>

or compilation should fail if the match pattern in the macro is not recognized.

Actual Result

cargo run wrongly returns

Here is the <a href="https://sepolia.etherscan.io">Link</a>

which would be the link for EthereumNetwork::Sepolia, even though the template is instantiated with the other variant EthereumNetwork::Mainnet.

The same behaviour can be observed if the match pattern in the macro is completely wrong

{% macro etherscan_link -%}
{% match network %}
  {%- when Wrong1 -%}
  <a href="https://sepolia.etherscan.io">Link</a>
  {%- when Wrong2 -%}
  <a href="https://etherscan.io/address">Link</a>
{% endmatch %}
{%- endmacro %}

Workaround

For the macro to function as expected it seems like the full path in each variant is required:

{% macro etherscan_link -%}
{% match network %}
  {%- when EthereumNetwork::Sepolia -%}
  <a href="https://sepolia.etherscan.io">Link</a>
  {%- when EthereumNetwork::Mainnet -%}
  <a href="https://etherscan.io/address">Link</a>
{% endmatch %}
{%- endmacro %}

In that case cargo run returns the expected link

Here is the <a href="https://etherscan.io/address">Link</a>

[Dirkjan Ochtman]

This is basically what happens in Rust code, too. You should be able to write analogous Rust code, which will fail the same way.

[gregorydemay]

This is basically what happens in Rust code, too. You should be able to write analogous Rust code, which will fail the same way.

I think here is depends exactly on how the match pattern is written. For example (see Rust playground):

#[derive(Debug)]
enum EthereumNetwork{
    Sepolia,
    Mainnet
}

fn main() {
   let network = EthereumNetwork::Mainnet;
   
   let url = match &network {
       Sepolia => "https://sepolia.etherscan.io",
       Mainnet => "https://etherscan.io/address"
   };
   
   println!("URL {} for network {:?}", url, network);
}

does not compile

Compiling playground v0.0.1 (/playground)
error[E0170]: pattern binding `Sepolia` is named the same as one of the variants of the type `EthereumNetwork`
  --> src/main.rs:11:8
   |
11 |        Sepolia => "https://sepolia.etherscan.io",
   |        ^^^^^^^ help: to match on the variant, qualify the path: `EthereumNetwork::Sepolia`
   |
   = note: `#[deny(bindings_with_variant_name)]` on by default

It's true that if the match pattern is totally wrong like in here

let url = match &network {
      Wrong1 => "https://sepolia.etherscan.io",
      Wrong2 => "https://etherscan.io/address"
 };

then it does compile, but with some useful warnings

Compiling playground v0.0.1 (/playground)
warning: unreachable pattern
  --> src/main.rs:17:7
   |
16 |       Wrong1 => "https://sepolia.etherscan.io",
   |       ------ matches any value
17 |       Wrong2 => "https://etherscan.io/address"
   |       ^^^^^^ unreachable pattern
   |
   = note: `#[warn(unreachable_patterns)]` on by default

[Dirkjan Ochtman]

Sure -- if you want to make a PR to improve on the current situation I'm open to that but I won't have time to work on this anytime soon.

[gregorydemay]

Sure -- if you want to make a PR to improve on the current situation

I don't really know the internals of Askama, could you point me in the right direction? I can then try to come up with something when I have time.

[Dirkjan Ochtman]

Code generation for match blocks is here:

https://github.com/djc/askama/blob/main/askama_derive/src/generator.rs#L544

Happy to answer other questions if you have them.

[René Kijewski]

Duplicate of #711. Adding #[warn(unreachable_patterns)] in generated code is suppressed.