Build fails with rustc<=1.68.2

[H1rono]

Rocket Version

0.5.0

Operating System

macOS 14.1.2, ubuntu-latest in GitHub Actions

Rust Toolchain Version

rustc 1.68.2 (9eb3afe9e 2023-03-27)

What happened?

With rustc version <=1.68.2, compilation fails like this CI failure.

Test Case

// just include `rocket = "0.5.0"` in Cargo.toml
fn main() {
    println!("Hello, world!")
}

Log Output

error[E0061]: this function takes 1 argument but 0 arguments were supplied
  --> /Users/kh/.cargo/registry/src/github.com-1ecc6299db9ec823/rocket-0.5.0/src/server.rs:76:5
   |
76 |     debug!("Received request: {:#?}", hyp_req);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ an argument of type `UnsafeArg` is missing
   |
note: associated function defined here
  --> /Users/kh/.cargo/registry/src/github.com-1ecc6299db9ec823/yansi-1.0.0-rc.1/src/paint.rs:56:8
   |
56 |     fn new(self) -> Painted<Self> where Self: Sized {
   |        ^^^
   = note: this error originates in the macro `$crate::__private_api::format_args` which comes from the expansion of the macro `debug` (in Nightly builds, run with -Z macro-backtrace for more info)
help: provide the argument
  --> /Users/kh/.cargo/registry/src/github.com-1ecc6299db9ec823/log-0.4.20/src/macros.rs:51:62
   |
51 |                 $crate::__private_api::format_args!($($arg)+)(/* UnsafeArg */),
   |                                                              +++++++++++++++++

error[E0308]: mismatched types
  --> /Users/kh/.cargo/registry/src/github.com-1ecc6299db9ec823/rocket-0.5.0/src/server.rs:76:5
   |
76 |     debug!("Received request: {:#?}", hyp_req);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `UnsafeArg`, found struct `Painted`
   |
   = note: expected struct `UnsafeArg`
              found struct `Painted<UnsafeArg>`
   = note: this error originates in the macro `$crate::__private_api::format_args` which comes from the expansion of the macro `debug` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0061]: this function takes 1 argument but 0 arguments were supplied
   --> /Users/kh/.cargo/registry/src/github.com-1ecc6299db9ec823/rocket-0.5.0/src/server.rs:163:9
    |
163 |         debug!("sending response: {:#?}", hyp_response);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ an argument of type `UnsafeArg` is missing
    |
note: associated function defined here
   --> /Users/kh/.cargo/registry/src/github.com-1ecc6299db9ec823/yansi-1.0.0-rc.1/src/paint.rs:56:8
    |
56  |     fn new(self) -> Painted<Self> where Self: Sized {
    |        ^^^
    = note: this error originates in the macro `$crate::__private_api::format_args` which comes from the expansion of the macro `debug` (in Nightly builds, run with -Z macro-backtrace for more info)
help: provide the argument
   --> |/Users/kh/.cargo/registry/src/github.com-1ecc6299db9ec823/log-0.4.20/src/macros.rs:51:62
    |
51  |                 $crate::__private_api::format_args!($($arg)+)(/* UnsafeArg */),
    |                                                              +++++++++++++++++

error[E0308]: mismatched types
   --> /Users/kh/.cargo/registry/src/github.com-1ecc6299db9ec823/rocket-0.5.0/src/server.rs:163:9
    |
163 |         debug!("sending response: {:#?}", hyp_response);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `UnsafeArg`, found struct `Painted`
    |
    = note: expected struct `UnsafeArg`
               found struct `Painted<UnsafeArg>`
    = note: this error originates in the macro `$crate::__private_api::format_args` which comes from the expansion of the macro `debug` (in Nightly builds, run with -Z macro-backtrace for more info)

Some errors have detailed explanations: E0061, E0308.
For more information about an error, try `rustc --explain E0061`.
error: could not compile `rocket` due to 4 previous errors

Additional Context

I guessed that some changes in 1.69.0 introduced this bug, so I digged 1.69.0 changes but couldn't find any obvious one. :cry

System Checks

• My bug report relates to functionality.
• I have tested against the latest Rocket release or a recent git commit.
• I have tested against the latest stable rustc toolchain.
• I was unable to find this issue previously reported.

[SergioBenitez]

It's possible to get Rocket to compile on 1.66 with the following Cargo.toml and cargo update -Z minimal-versions:

[dependencies]
rocket = "0.5.0"
pear = "0.2.7"
tokio-util = "0.6.10"
log = "0.4.20"
multer = "2.1"
percent-encoding = "2.3.1"
mime = "0.3.17"
serde = "1.0.193"
tempfile = "3.7"
tokio = "1.35"
either = "1.9"

This means that the breakage is caused by some dependency and not Rocket itself. I really don't know what is causing this issue, but given the above, it should be possible to perform a bisect of sorts to find the culprit. My best guess is that this was "fixed" (i.e., masked) by rust-lang/rust#106745, though I don't know how.

Given the above, I don't believe there's anything we can do in Rocket, short of downgrading whatever dependency is causing the issue. It would be nice, however, for us to go through all of the declared dependency versions in our Cargo.toml and ensure that the version requirement is minimally correct. It seems that for many dependencies, we're using some feature that was introduced after the stated dependency version, though this isn't what's causing the issue here.

[H1rono]

Then, how about upgrading MSRV? The current MSRV is 1.56, as below.

Rocket/core/lib/Cargo.toml

Line 17 in 92a2559

rust-version = "1.56"

But now it is obvious that rocket is incompatible to rustc==1.56.

[SergioBenitez]

Then, how about upgrading MSRV? The current MSRV is 1.56, as below.

Rocket/core/lib/Cargo.toml

Line 17 in 92a2559

rust-version = "1.56"

But now it is obvious that rocket is incompatible to rustc==1.56.

That MSRV was correct until our dependencies went and upgraded their MSRV under us. If they were to downgrade their MSRV, Rocket would compile on 1.56, at least that's the theory. A different way to say that is that Rocket itself should compile with 1.56, but its dependencies appear to need at least 1.66, whereas at some point that wasn't the case. This is an issue with the MSRV Cargo.toml field in general: rust-lang/cargo#9930.

I'm not really sure what the best course of action is. If we choose the min MSRV of all of our dependencies, we miss out on a lower MSRV if a dependency were to make that possible in an update. If we choose the min MSRV for Rocket itself, then the MSRV may be incorrect when dependencies are taken into consideration.

There is no correct answer, it seems, at least not today. Even MSRV-aware dependency resolution is fraught with issues: you'd need to hold back updating dependencies, even when those updates are security sensitive. Perhaps the most practical answer is to choose the min MSRV across all dependencies at a point in time, or perhaps it's to eschew setting an MSRV altogether.