The “use of unstable library feature 'rustc_private'” error is very repetitive

[jakubadamw]

rustc 1.22.0-nightly (0e6f4cf51 2017-09-27)

Compiling this:

#[macro_use] extern crate log;

pub fn main() {
    info!("This is a log message.");
}

while not including log as a dependency in Cargo.toml, produces the following output:

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:1:14
  |
1 | #[macro_use] extern crate log;
  |              ^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:4:2
  |
4 |  info!("This is a log message.");
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:4:2
  |
4 |  info!("This is a log message.");
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:4:2
  |
4 |  info!("This is a log message.");
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:4:2
  |
4 |  info!("This is a log message.");
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:4:2
  |
4 |  info!("This is a log message.");
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:4:2
  |
4 |  info!("This is a log message.");
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:4:2
  |
4 |  info!("This is a log message.");
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:4:2
  |
4 |  info!("This is a log message.");
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate

error: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, and unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> main.rs:4:2
  |
4 |  info!("This is a log message.");
  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate

error: aborting due to 10 previous errors

[estebank]

Mentoring instructions

Modify leveled_feature_err to accept an extra flag stating wether the help should be outputted or not (it's already checking wether the current compiler is nightly) so that TyCtxt::stability can keep track of wether a diagnostic of this type has already been emitted, and if it has, the help isn't shown.

[zackmdavis]

so that TyCtxt::stability can keep track of wether a diagnostic of this type has already been emitted

But TyCxt has access to the Session, which means we should be able to use the existing one_time_diagnostics field (if we slightly generalize it to take an Option<LintId> rather than a LintId to accomodate non-lint errors like the matter at hand).

However, the reported code isn't reproducing the issue for me with a stage 1 compiler, perhaps hinting at something more subtle going on with this report.

[virgil-palanciuc]

That actually works and is a pretty elegant/quick fix. The only caveat is that you shouldn't pass the span in the tuple... i'm guessing, it messes up the hashcode, and the error ends up being different each time. I can't yet figure out why (complete newbie to the codebase) - so if you want to take up this issue & fix it, please feel free to do so, it'll take me a little while (I'm slow/don't have much time available to work on it :) ).

[virgil-palanciuc]

Update - i had some time to inspect it. Basically, the error is reported in two places. Here:

1 | #[macro_use] extern crate log
  |              ^^^^^^^^^^^^^^^^^

and here:

 4 |     info!("This is a log message.");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The second one is the one that is reported many times - and the primary span seems to be different each time (which is why if you include the span in the "key", it sees the messages as different and emits them all). I'm planning to go from the primary span to file/line/column string, and include that in the message - basically, the key for one_time_diagnostics would be (None, None, modified_message) instead of (None, Some(span), message); this way, we would preserve the first two error messages, but remove the subsequent ones. Still feels a bit hackish, but I don't have a better idea.

[zackmdavis]

the primary span seems to be different each time

But they look like they're all pointing to the same code-snippet; what could be making them different? It looks like Spans have a mysterious ctxt field; if that's it, I'd rather manipulate that before setting/getting the one_time_diagnostics entry than pollute the error message string itself with file-position data.

[virgil-palanciuc]

So, I printed the location from the span - most of the "repeated" ones are within the macros, e.g.

    Loc {
        file: FileMap(<log macros>),
        line: 5,
        col: CharPos(
            36
        )
    }

I'm pretty sure I need to invoke EmitterWritter::fix_multispan_in_std_macros() on the primary span, in order to retrieve the true location (main.rs, line 4) - only problem is, that method is private. And now it's late :)

I have another idea, I'll try it tomorrow - anyway, this looks to be genuinely trickier than it first appeared to be

[virgil-palanciuc]

Ok guys, I've created PR #45540 - let me know if you think it's a reasonable solution.