'cargo leptos serve' does not play well with RUSTFLAGS="-Awarnings"

[mpalmer]

Describe the bug

This will probably take the award for "weirdest bug of the week" -- at least I hope so...

It appears that cargo leptos serve fails when RUSTFLAGS is set to -Awarnings. It's fine with -Dwarnings, and cargo leptos watch is cool with funky RUSTFLAGS, but the combination of RUSTFLAGS=-Awarnings and cargo leptos serve ends in Confusion and Delay, and also the following error:

error: output of --print=file-names missing when learning about target-specific information from rustc
command was: `/home/mpalmer/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/rustc - --crate-name ___ --print=file-names
 -Awarnings --target wasm32-unknown-unknown --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg`

--- stdout
___.wasm
lib___.rlib
___.wasm
lib___.a
/home/mpalmer/.rustup/toolchains/stable-x86_64-unknown-linux-gnu
off
___
debug_assertions
panic="abort"
proc_macro
target_abi=""
target_arch="wasm32"
target_endian="little"
target_env=""
target_family="wasm"
target_has_atomic="16"
target_has_atomic="32"
target_has_atomic="64"
target_has_atomic="8"
target_has_atomic="ptr"
target_os="unknown"
target_pointer_width="32"
target_vendor="unknown"

Leptos Dependencies

actix-files = { version = "0.6", optional = true }
actix-web = { version = "4", optional = true, features = ["macros"] }
console_error_panic_hook = "0.1"
http = { version = "1.0.0", optional = true }
leptos = { version = "0.6" }
leptos_meta = { version = "0.6" }
leptos_actix = { version = "0.6", optional = true }
leptos_router = { version = "0.6" }
wasm-bindgen = "=0.2.92"

To Reproduce
Steps to reproduce the behavior:

1. Create a new project with cargo leptos new --git leptos-rs/start (I chose not to use nightly features, if that turns out to matter)
2. Run cargo leptos serve, note that everything's fine.
3. Run RUSTFLAGS=-Awarnings cargo leptos serve, note that everything goes pear-shaped.

Expected behavior

Well... not that. 😆 It should probably fail on my code, not rustc...