`anchor-syn` 0.31.0 is incompatible with `proc-macro2` >= 1.0.95

[niooss-ledger]

Hello,

When attempting to create a new Anchor project, anchor build fails with:

    Finished `release` profile [optimized] target(s) in 0.15s
   Compiling anchor-syn v0.31.0
error[E0599]: no method named `source_file` found for struct `proc_macro2::Span` in the current scope
   --> /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/anchor-syn-0.31.0/src/idl/defined.rs:499:66
    |
499 | ...an::call_site().source_file().path();
    |                    ^^^^^^^^^^^ method not found in `Span`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `anchor-syn` (lib) due to 1 previous error
Error: Building IDL failed. Run `ANCHOR_LOG=true anchor idl build` to see the logs.

TL;DR

anchor-syn has been using an unstable API from proc-macro2 where a recent release introduced a breaking change.

Root Cause Analysis

A few hours ago, proc-macro2 version 1.0.95 was released (https://github.com/dtolnay/proc-macro2/releases/tag/1.0.95) with a breaking change:

Update semver-exempt API under RUSTFLAGS=--cfg=procmacro2_semver_exempt to that of nightly-2025-04-16 (dtolnay/proc-macro2#497)

proc-macro2's documentation clearly warns about such breaking changes in minor versions in https://github.com/dtolnay/proc-macro2/blob/24bbf16d9df01d5f7d9ac39bdfbaea85f4c194fb/README.md#unstable-features :

To opt into the additional APIs available in the most recent nightly compiler, the procmacro2_semver_exempt config flag must be passed to rustc. [...] As these are unstable APIs that track the nightly compiler, minor versions of proc-macro2 may make breaking changes to them at any time.

Indeed, rust-lang/rust#139671 replaced

impl Span {
    // ...
    pub fn source_file(&self) -> SourceFile;
}
impl SourceFile {
    pub fn path(&self) -> PathBuf;
    pub fn is_real(&self) -> bool;
}

with:

impl Span {
    // ...
    pub fn file(&self) -> String; // Mapped file name, for display purposes.
    pub fn local_file(&self) -> Option<PathBuf>; // Real file name as it exists on disk.
}

anchor-syn has been using .source_file().path() in

anchor/lang/syn/src/idl/defined.rs

Line 499 in 649b9d6

let source_path = proc_macro2::Span::call_site().source_file().path();

Therefore it seems this code needs to be updated to the latest API. Could you please fix it?

System information

For information, I am building on x86_64 with:

$ anchor --version
anchor-cli 0.31.0
$ cargo --version
cargo 1.85.0 (d73d2caf9 2024-12-31)
$ rustc --version
rustc 1.85.0 (4d91de4e4 2025-02-17)
$ yarn --version
1.22.22

$ head -n 3 /etc/os-release 
PRETTY_NAME="Ubuntu 24.04.1 LTS"
NAME="Ubuntu"
VERSION_ID="24.04"

I reproduced this issue with anchor init test && cd test && anchor build.

[cryptopapi997]

Yep, you're correct. Opened a PR for it: #3663

[niooss-ledger]

For information, as a temporary workaround, I found that anchor build can use environment variable RUSTUP_TOOLCHAIN to override the version of the nightly Rust channel:

anchor/idl/src/build.rs

Lines 141 to 144 in 649b9d6

	// `nightly` toolchain is currently required for building the IDL.
	let toolchain = std::env::var("RUSTUP_TOOLCHAIN")
	.map(|toolchain| format!("+{}", toolchain))
	.unwrap_or_else(|_| "+nightly".to_string());

These commands worked for me, to make my current Anchor project build:

cargo update -p proc-macro2 --precise 1.0.94
RUSTUP_TOOLCHAIN=nightly-2025-04-14 anchor build

(This issue happens when using toolchain nightly-2025-04-16)

[jacobcreech]

Closing as this was fixed with #3663