How to tell why an "unknown feature" feature is needed?

Question

I'm getting the following error:

    error[E0635]: unknown feature `proc_macro_span_shrink`
  --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.50/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0635`.
error: could not compile `proc-macro2` (lib) due to previous error

I found a fix for this by setting my rust version to a specific older nightly build, but that doesn't work with another project, and I really want a better solution. I'm assuming I'm using an out-of-date version of something, or maybe I have a dependency in my cargo.toml file I can get rid of, but that error doesn't tell me anything about which one.

After some searching I tried running cargo tree -e features -i proc-macro2, but while that output a lot of stuff referring to several dependencies, nothing referred specifically to proc_macro_span_shrink.

proc-macro2 v1.0.50
├── quote v1.0.17
│   └── syn v1.0.90
│       └── proc-macro-error v1.0.4
│           ├── proc-macro-error feature "default"
│           │   ├── clap_derive v4.1.8 (proc-macro)
│           │   │   └── clap_derive feature "default"
│           │   │       └── clap v4.1.8
│           │   │           ├── clap feature "color"
│           │   │           │   └── clap feature "default"
│           │   │           │       └── petersburg v0.0.1 (~/Github/petersburg)
│           │   │           │           └── petersburg feature "default" (command-line)
│           │   │           ├── clap feature "default" (*)
│           │   │           ├── clap feature "derive"
│           │   │           │   └── petersburg v0.0.1 (~/Github/petersburg) (*)
│           │   │           ├── clap feature "error-context"
│           │   │           │   ├── clap feature "default" (*)
│           │   │           │   └── clap feature "suggestions"
│           │   │           │       └── clap feature "default" (*)
│           │   │           ├── clap feature "help"
│           │   │           │   └── clap feature "default" (*)
│           │   │           ├── clap feature "std"
│           │   │           │   └── clap feature "default" (*)
│           │   │           ├── clap feature "suggestions" (*)
│           │   │           └── clap feature "usage"
│           │   │               └── clap feature "default" (*)
│           │   ├── glib-macros v0.15.10 (proc-macro)
│           │   │   └── glib-macros feature "default"
│           │   │       └── glib v0.15.10
│           │   │           └── glib feature "default"
│           │   │               ├── atk v0.15.1
│           │   │               │   └── atk feature "default"
│           │   │               │       └── gtk v0.15.4
│           │   │               │           └── gtk feature "default"
│           │   │               │               └── petersburg v0.0.1 ~/Github/petersburg) (*)
│           │   │               ├── cairo-rs v0.15.10
│           │   │               │   ├── cairo-rs feature "default"
│           │   │               │   │   ├── gdk v0.15.4
│           │   │               │   │   │   └── gdk feature "default"
│           │   │               │   │   │       ├── gtk v0.15.4 (*)
│           │   │               │   │   │       └── petersburg v0.0.1 (~/Github/petersburg) (*)
│           │   │               │   │   ├── gtk v0.15.4 (*)
│           │   │               │   │   └── petersburg v0.0.1 (~/Github/petersburg) (*)
│           │   │               │   ├── cairo-rs feature "glib"
│           │   │               │   │   └── cairo-rs feature "use_glib"
│           │   │               │   │       └── cairo-rs feature "default" (*)
│           │   │               │   ├── cairo-rs feature "png"
│           │   │               │   │   └── petersburg v0.0.1 (~/Github/petersburg) (*)
│           │   │               │   └── cairo-rs feature "use_glib" (*)
│           │   │               ├── gdk v0.15.4 (*)
│           │   │               ├── gdk-pixbuf v0.15.10
│           │   │               │   └── gdk-pixbuf feature "default"
│           │   │               │       ├── gdk v0.15.4 (*)
│           │   │               │       └── gtk v0.15.4 (*)
│           │   │               ├── gio v0.15.10
│           │   │               │   └── gio feature "default"
│           │   │               │       ├── gdk v0.15.4 (*)
│           │   │               │       ├── gdk-pixbuf v0.15.10 (*)
│           │   │               │       └── gtk v0.15.4 (*)
│           │   │               ├── gtk v0.15.4 (*)
│           │   │               ├── pango v0.15.10
│           │   │               │   └── pango feature "default"
│           │   │               │       ├── gdk v0.15.4 (*)
│           │   │               │       └── gtk v0.15.4 (*)
│           │   │               └── petersburg v0.0.1 (~/Github/petersburg) (*)
│           │   └── gtk3-macros v0.15.4 (proc-macro)
│           │       └── gtk3-macros feature "default"
│           │           └── gtk v0.15.4 (*)
│           ├── proc-macro-error feature "syn"
│           │   └── proc-macro-error feature "syn-error"
│           │       └── proc-macro-error feature "default" (*)
│           └── proc-macro-error feature "syn-error" (*)
│       ├── syn feature "clone-impls"
│       │   └── syn feature "default"
│       │       ├── clap_derive v4.1.8 (proc-macro) (*)
│       │       └── thiserror-impl v1.0.30 (proc-macro)
│       │           └── thiserror-impl feature "default"
│       │               └── thiserror v1.0.30
│       │                   └── thiserror feature "default"
│       │                       ├── cairo-rs v0.15.10 (*)
│       │                       ├── gio v0.15.10 (*)
│       │                       ├── glib v0.15.10 (*)
│       │                       └── proc-macro-crate v1.1.3
│       │                           └── proc-macro-crate feature "default"
│       │                               ├── glib-macros v0.15.10 (proc-macro) (*)
│       │                               └── gtk3-macros v0.15.4 (proc-macro) (*)
│       ├── syn feature "default" (*)
│       ├── syn feature "derive"
│       │   └── syn feature "default" (*)
│       ├── syn feature "full"
│       │   ├── clap_derive v4.1.8 (proc-macro) (*)
│       │   ├── glib-macros v0.15.10 (proc-macro) (*)
│       │   └── gtk3-macros v0.15.4 (proc-macro) (*)
│       ├── syn feature "parsing"
│       │   └── syn feature "default" (*)
│       ├── syn feature "printing"
│       │   └── syn feature "default" (*)
│       ├── syn feature "proc-macro"
│       │   └── syn feature "default" (*)
│       └── syn feature "quote"
│           ├── syn feature "printing" (*)
│           └── syn feature "proc-macro" (*)
│   ├── quote feature "default"
│   │   ├── clap_derive v4.1.8 (proc-macro) (*)
│   │   ├── glib-macros v0.15.10 (proc-macro) (*)
│   │   ├── gtk3-macros v0.15.4 (proc-macro) (*)
│   │   ├── proc-macro-error v1.0.4 (*)
│   │   ├── proc-macro-error-attr v1.0.4 (proc-macro)
│   │   │   └── proc-macro-error-attr feature "default"
│   │   │       └── proc-macro-error v1.0.4 (*)
│   │   └── thiserror-impl v1.0.30 (proc-macro) (*)
│   └── quote feature "proc-macro"
│       ├── quote feature "default" (*)
│       └── syn feature "proc-macro" (*)
└── syn v1.0.90 (*)
├── proc-macro2 feature "default"
│   ├── clap_derive v4.1.8 (proc-macro) (*)
│   ├── glib-macros v0.15.10 (proc-macro) (*)
│   ├── gtk3-macros v0.15.4 (proc-macro) (*)
│   ├── proc-macro-error v1.0.4 (*)
│   ├── proc-macro-error-attr v1.0.4 (proc-macro) (*)
│   └── thiserror-impl v1.0.30 (proc-macro) (*)
└── proc-macro2 feature "proc-macro"
    ├── proc-macro2 feature "default" (*)
    ├── quote feature "proc-macro" (*)
    └── syn feature "proc-macro" (*)

To be clear, while a "Package X is at fault" answer would be appreciated, I really want to know how to figure this out myself.

Answer 1

A "feature" as mentioned in the error is for unstable compiler features as you might see in The Unstable Book or any WIP RFCs used for language / std library feature flags. A "feature" as understood by cargo tree are Cargo features as documented in The Cargo Book for crate-specific feature flags. They are completely different things.

Since this is an error for an unknown compiler feature, there are two possibilities:

• The crate is too old and trying to use a feature that has since been stabilized. In this case you need to downgrade your Rust version (how to downgrade), or update the dependency (using cargo update -p proc-macro2) or updating/upgrading a parent dependency.
• The crate is too new and trying to use a feature that your compiler doesn't know about yet. In this case you would update your Rust version (rustup update), or potentially you could downgrade the dependency (using cargo update -p proc-macro2 --precise=<OLDER VERSION>).

The first is the solution in this case since the error is from proc-macro2 v1.0.50 and the error was fixed in v1.0.60 as per this issue. The core problem was actually neither of the above possibilities (kind of) since a feature was renamed during development as per this PR.

How do I find the offending dependency?

With a problem like this (an unknown compiler feature), the problem is always an incompatibility between the particular crate and your compiler. You can find the offending crate via its path:

--> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.50/src/lib.rs:92:30
                                                           ^^^^^^^^^^^

Answer 2

I am facing the same issue recently and tried to switch the rust from nightly to stable fixed this issue, just tried to switch to stable:

rustup default stable