Unhelpful error message when putting URL in docstring

[DavidVorick]

I wrote a docstring that had a url in it. The code looked something like this:

#![forbid(unsafe_code)]
#![deny(missing_docs)]
#![deny(unused_must_use)]
#![deny(unused_mut)]

//! seed implements functions for moving between a seed and a seed phrase. This blog post provides
//! a full specification for the code presented here:
//!
//!     https://blog.sia.tech/a-technical-breakdown-of-mysky-seeds-ba9964505978

use anyhow::{bail, Error, Result};
use sha2::{Digest, Sha256};
// ...

When I ran cargo test, the output looked something like this:

   Doc-tests seed_manager

running 1 test
test src/seed.rs - seed (line 9) ... FAILED

failures:

---- src/seed.rs - seed (line 9) stdout ----
error: expected expression, found `}`
 --> src/seed.rs:11:1
  |
2 | fn main() { #[allow(non_snake_case)] fn _doctest_main_src_seed_rs_9_0() {
  |                                                                         - while parsing this struct
3 | https://blog.sia.tech/a-technical-breakdown-of-mysky-seeds-ba9964505978
4 | } _doctest_main_src_seed_rs_9_0() }
  | ^ expected expression

error: struct literal body without path
 --> src/seed.rs:9:73
  |
2 |   fn main() { #[allow(non_snake_case)] fn _doctest_main_src_seed_rs_9_0() {
  |  _________________________________________________________________________^
3 | | https://blog.sia.tech/a-technical-breakdown-of-mysky-seeds-ba9964505978
4 | | } _doctest_main_src_seed_rs_9_0() }
  | |_^
  |
help: you might have forgotten to add the struct literal inside the block
  |
2 ~ fn main() { #[allow(non_snake_case)] fn _doctest_main_src_seed_rs_9_0() { SomeStruct {
3 | https://blog.sia.tech/a-technical-breakdown-of-mysky-seeds-ba9964505978
4 ~ } } _doctest_main_src_seed_rs_9_0() }
  |

error: aborting due to 2 previous errors

Couldn't compile the test.

failures:
    src/seed.rs - seed (line 9)

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.02s

error: test failed, to rerun pass '--doc'

With some help, I was able to determine that the problem was my URL was being interpreted as code due to having 4 preceding spaces. The compiler error presented here was not remotely helpful to me (I'm relatively new to rust) in determining what the problem was. I think this issue has two potential resolutions:

1. don't interpret something as code if it's clearly not code
2. present a more helpful error message to suggest that my docstring is being interpreted as code and that I should remove the 4 spaces

Meta

rustc --version --verbose:

rustc 1.64.0 (a55dd71d5 2022-09-19)
binary: rustc
commit-hash: a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52
commit-date: 2022-09-19
host: x86_64-unknown-linux-gnu
release: 1.64.0
LLVM version: 14.0.6

[ehuss]

Thanks for the report! I understand it can be confusing.

I'm going to close since this has been reported before. Closing as a duplicate of #64162, #59867, #88590, #100225.