Regression in beta: failed to parse slash in a custom cfg name #7613
Description
According to https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#platform-specific-dependencies, custom target specification is allowed, e.g.
[target."x86_64/windows.json".dependencies]
winhttp = "0.4.0"cargo +stable metadata works on this toml, while cargo +beta metadata fails
❯ cargo +stable --version
cargo 1.39.0 (1c6ec66d5 2019-09-30)
❯ cargo +beta --version
cargo 1.40.0-beta (5da4b4d47 2019-10-28)
❯ cat Cargo.toml
[package]
name = "None"
version = "0.1.0"
[target."x86_64/windows.json"]
dependencies = { hello = "1.0" }
❯ cargo +stable metadata
<snip>
❯ cargo +beta metadata
error: failed to parse manifest at `/tmp/Cargo.toml`
Caused by:
failed to parse `x86_64/windows.json` as a cfg expression: invalid target specifier: unexpected character / in target name