Cargo: `opt-level = s` and `opt-level = z` don't seem to work

[michaelwoerister]

Problem

Specifying opt-level = s or opt-level = z for a profile fails with the following error:

$ cargo build
error: failed to parse manifest at `/xoxo/stuff/cargo-opt-level-size-bug/Cargo.toml`

Caused by:
  could not parse input as TOML

Caused by:
  invalid number at line 10

Steps

1. Create a new project with cargo new repro
2. Add
   toml [profile.dev] opt-level = s
   to Cargo.toml.
3. Run cargo build

Possible Solution(s)

Fix the Cargo.toml parser?

Notes

Output of cargo version:

cargo 1.36.0 (c4fcfb725 2019-05-15)
cargo 1.38.0-nightly (26092da33 2019-07-31)

[ehuss]

They need to be specified as a string, like:

[profile.dev]
opt-level = 's'

The TOML language does not allow bare words as values.

[michaelwoerister]

Ah, interesting! Considers this a bug report about an unhelpful error message then :)

[aleksator]

Looks like the error message is coming from https://github.com/alexcrichton/toml-rs
cc @alexcrichton

[ehuss]

@aleksator I recently opened https://github.com/alexcrichton/toml-rs/issues/379 regarding the confusing error message here (it has cropped up in a few other places). I left some very rough guidance there, where number_or_date needs to check if the "keylike" value looks like it starts like a number or date, and if not should provide a better error message.

[aleksator]

Fixed in toml-rs 0.5.7 (thank you @ehuss and @alexcrichton!).
We can rename this issue and close it next time we update dependencies.

[ehuss]

Yep, just updated via #8772.