cargo-metadata's dep_kinds field can have duplicate entries

[sfackler]

Using rustc 1.41.0-beta.1 (eb3f7c2d3 2019-12-17) and rustc 1.42.0-nightly (3a3f4a7cb 2019-12-28) the new dep_kinds field of cargo-metadata's output seems to behave weirdly in some cases.

For example, here's the resolve node for syn:

{
  "id": "syn 1.0.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "dependencies": [
    "proc-macro2 1.0.6 (registry+https://github.com/rust-lang/crates.io-index)",
    "quote 1.0.2 (registry+https://github.com/rust-lang/crates.io-index)",
    "unicode-xid 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)"
  ],
  "deps": [
    {
      "name": "proc_macro2",
      "pkg": "proc-macro2 1.0.6 (registry+https://github.com/rust-lang/crates.io-index)",
      "dep_kinds": [
        {
          "kind": null,
          "target": null
        },
        {
          "kind": null,
          "target": null
        },
        {
          "kind": null,
          "target": null
        }
      ]
    },
    {
      "name": "quote",
      "pkg": "quote 1.0.2 (registry+https://github.com/rust-lang/crates.io-index)",
      "dep_kinds": [
        {
          "kind": null,
          "target": null
        },
        {
          "kind": null,
          "target": null
        },
        {
          "kind": null,
          "target": null
        }
      ]
    },
    {
      "name": "unicode_xid",
      "pkg": "unicode-xid 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
      "dep_kinds": [
        {
          "kind": null,
          "target": null
        },
        {
          "kind": null,
          "target": null
        },
        {
          "kind": null,
          "target": null
        }
      ]
    }
  ],
  "features": [
    "clone-impls",
    "default",
    "derive",
    "full",
    "parsing",
    "printing",
    "proc-macro",
    "quote",
    "visit"
  ]
}

Note how the dependencies each have three dep_kinds entries, even though each dependency is only listed once in syn's Cargo.toml:

[package]
name = "syn"
version = "1.0.11" # don't forget to update html_root_url and syn.json
authors = ["David Tolnay <dtolnay@gmail.com>"]
license = "MIT OR Apache-2.0"
description = "Parser for Rust source code"
repository = "https://github.com/dtolnay/syn"
documentation = "https://docs.rs/syn"
categories = ["development-tools::procedural-macro-helpers"]
readme = "README.md"
include = [
    "/benches/**",
    "/build.rs",
    "/Cargo.toml",
    "/LICENSE-APACHE",
    "/LICENSE-MIT",
    "/README.md",
    "/src/**",
    "/tests/**",
]
edition = "2018"

[features]
default = ["derive", "parsing", "printing", "clone-impls", "proc-macro"]
derive = []
full = []
parsing = []
printing = ["quote"]
visit = []
visit-mut = []
fold = []
clone-impls = []
extra-traits = []
proc-macro = ["proc-macro2/proc-macro", "quote/proc-macro"]

[dependencies]
proc-macro2 = { version = "1.0", default-features = false }
quote = { version = "1.0", optional = true, default-features = false }
unicode-xid = "0.2"

[dev-dependencies]
anyhow = "1.0"
flate2 = "1.0"
insta = "0.12"
rayon = "1.0"
ref-cast = "1.0"
regex = "1.0"
reqwest = { version = "0.10.0-alpha.2", features = ["blocking"] }
tar = "0.4"
termcolor = "1.0"
walkdir = "2.1"

[[bench]]
name = "rust"
harness = false
required-features = ["full", "parsing"]

[[bench]]
name = "file"
required-features = ["full", "parsing"]

[package.metadata.docs.rs]
all-features = true

[package.metadata.playground]
all-features = true

[badges]
travis-ci = { repository = "dtolnay/syn" }

[workspace]
members = ["dev", "json"]

[ehuss]

Ah, a bit of an unexpected interaction in a workspace. Fix posted at #7756.