Installation error with latest rustc-demangle

[ankane]

Hi, thanks for this project! I wanted to report an error during installation with rustc-demangle.

cargo +nightly-2019-12-05 install c2rust

Error

error[E0658]: use of unstable library feature 'matches_macro'
   --> /Users/andrew/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-demangle-0.1.21/src/v0.rs:700:20
    |
700 |                 if matches!((quote, c), ('\'', '"') | ('"', '\'')) {
    |                    ^^^^^^^
    |
    = note: for more information, see https:/rust-lang/rust/issues/65721
    = help: add `#![feature(matches_macro)]` to the crate attributes to enable

It appears to be due to this change in rustc-demangle 0.1.21, which uses the matches! macro, which isn't stable in nightly-2019-12-05.

[thedataking]

Have you tried cargo +nightly-2019-12-05 install c2rust --locked? If so, this is likely a duplicate of #323.

[ankane]

Thanks @thedataking, looks like that fixed it. A few suggestions to make it easier for new users:

1. Would be good to merge Add --locked Flag to Cargo Install Instructions #335 to add --locked to the readme instructions
2. --locked doesn't seem to work with the "Installing from Git" instructions (possibly cargo install --locked doesn't honour the lockfile for a git source rust-lang/cargo#7495). I think this can be updated to:

git clone https:/immunant/c2rust.git
cd c2rust
cargo +nightly-2019-12-05 install --path c2rust --locked

3. On Mac, brew install llvm installs LLVM 12, which isn't doesn't appear to be supported by the latest release. Would be good to either make a new release with dfa995c or update the Mac instructions to brew install llvm@9 and LLVM_CONFIG_PATH=/usr/local/opt/llvm@9/bin/llvm-config.