Could not compile percent-encoding

[danawoodman]

I've just attempted to install reqwest 0.9.21 but it is failing with Could not compile 'percent-encoding'.

cargo build output:

cargo build
   Compiling nodrop v0.1.13
   Compiling lazy_static v1.4.0
   Compiling smallvec v0.6.10
   Compiling either v1.5.3
   Compiling cfg-if v0.1.10
   Compiling scopeguard v1.0.0
   Compiling slab v0.4.2
   Compiling fnv v1.0.6
   Compiling ppv-lite86 v0.2.5
   Compiling futures v0.1.29
   Compiling matches v0.1.8
   Compiling itoa v0.4.4
   Compiling rustc-demangle v0.1.16
   Compiling rand_core v0.4.2
   Compiling percent-encoding v1.0.1
   Compiling remove_dir_all v0.5.2
   Compiling percent-encoding v2.1.0
   Compiling regex-syntax v0.6.12
   Compiling indexmap v1.2.0
   Compiling try-lock v0.2.2
error: local variables in const fn are unstable
  --> /Users/danawoodman/.cargo/registry/src/github.com-1ecc6299db9ec823/percent-encoding-2.1.0/lib.rs:72:13
   |
72 |         let chunk = self.mask[byte as usize / BITS_PER_CHUNK];
   |             ^^^^^

   Compiling adler32 v1.0.4
error: aborting due to previous error

error: Could not compile `percent-encoding`.
warning: build failed, waiting for other jobs to finish...
error: build failed

My Cargo.toml:

[package]
name = "cli"
version = "0.1.0"
authors = ["Dana Woodman <...>"]
edition = "2018"

[dependencies]
reqwest = "0.9.21"

• cargo -V: cargo 1.31.0 (339d9f9c8 2018-11-16)
• rustc -V: rustc 1.31.0 (abe02cefd 2018-12-04)

I'm a new Rust user so not sure I'm missing anything but I think I followed the README exactly 🤷🏻‍♂️

[seanmonstar]

The error message might seem cryptic, but it's really just saying that your compiler is too old :) With a newer compiler, you'll be able to compile percent-encoding.

[danawoodman]

@seanmonstar isn't 1.31 the latest version of Rust/Cargo?

[seanmonstar]

No, notice in the version string it was release last year. Rust has new versions every 6 weeks. The easiest way to manage Rust versions is using rustup.

[danawoodman]

@seanmonstar 🤦‍♂ man that is not obvious. I guess I had installed Rust a year ago and then using the installation via curl https://sh.rustup.rs -sSf | sh doesn't actually update you automatically so I just assumed I was using the latest version. Would be cool if they made that more obvious, but obviously not your concern! Thanks and sorry for the false report!

[seanmonstar]

Yea, the messaging is unclear. But there's hope! A new RFC for allowing a crate to specify it's required Rust version looks like it may be approved, which could see better error message in the future.