razeri-plugin-api 0.20.0

// Copyright (C) 2026 Gabriel Shea
// Licensed under either of the Apache License, Version 2.0 or the MIT
// license, at your option.

//! Loading plugins built as shared libraries.
//!
//! A plugin compiled as a shared library (`.so`, `.dylib`, `.dll`) is loaded
//! from a directory at start-up by [`load_plugins_from_dir`]. The library and
//! the razeri that loads it then share Rust types: the dialect traits, the
//! registry builder, and every value passed between them. Rust lays those
//! types out the same way on both sides only when both sides are the same
//! build — the same compiler, and the same source of `razeri-plugin-api` and
//! `razeri-plugin-contract`. Anything else reads memory the wrong way, and the
//! process crashes rather than failing.
//!
//! So a plugin says how it was built, in a [`BuildDeclaration`], and says it
//! through a door every compiler reads alike: a C function returning a
//! `#[repr(C)]` record of C strings and one number. The loader reads that
//! first. A library whose declaration differs from this razeri's own in any
//! part, or that declares nothing, is refused with a [`PluginRefusal`] that
//! names what it was built with and what it needs, and none of its Rust code
//! is run.
//!
//! A plugin exports its declaration and its registration with one line,
//! [`export_plugin!`](crate::export_plugin). The same line lets the crate be
//! linked into a razeri you build yourself instead, which needs no match at
//! all because there is only one build.
//!
//! ## What else the loader does
//!
//! - **Panics stay in the plugin**: registration runs inside `catch_unwind`.
//! - **Libraries stay loaded**: a library whose registration ran is never
//!   unloaded, because what it registered lives as long as the process. A
//!   refused library is unloaded at once; nothing of it was kept.
//! - **An open directory is named**: on Unix, a plugin directory anyone can
//!   write to is reported in the log.

use crate::builder::PluginRegistryBuilder;
use std::fmt;

/// Where a plugin author reads how to link a dialect into a razeri they build
/// — the other way to run a plugin, and the one every refusal offers. It names
/// the page inside the book each release carries, because the book has no
/// public address.
pub const LINKING_GUIDE: &str = "plugins/linking-a-dialect.html in the razeri book";

/// Symbol name for the plugin registration function.
///
/// The function signature is [`RegisterPluginFn`]. It is called only after
/// the library's [`BuildDeclaration`] matches this razeri's.
pub const PLUGIN_REGISTER_SYMBOL: &str = "razeri_plugin_register_v1";

/// Symbol name for the function that says how a plugin was built.
///
/// The function signature is [`BuildDeclarationFn`]. A library without it is
/// refused.
pub const PLUGIN_BUILD_SYMBOL: &str = "razeri_plugin_build";

/// Type signature for the plugin registration entry point.
///
/// # Safety
///
/// The `builder` pointer must be valid and outlive the call, and the library
/// must have been built exactly as the caller was.
pub type RegisterPluginFn = unsafe fn(&mut PluginRegistryBuilder);

/// Type signature for the function that says how a plugin was built.
///
/// It uses the C calling convention and returns a pointer to a `#[repr(C)]`
/// record, so a library built by any compiler answers it the same way.
pub type BuildDeclarationFn = extern "C" fn() -> *const export::ExportedBuildDeclaration;

/// How a plugin was built: the compiler, and the two plugin crates it was
/// compiled against.
///
/// A plugin loaded from a directory is accepted only when its declaration is
/// equal to [`BuildDeclaration::this_build`] in every part.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct BuildDeclaration {
    /// The compiler, as `rustc -V` names it, such as
    /// `rustc 1.93.1 (01f6ddf75 2026-02-11)`.
    pub compiler: String,
    /// The version of `razeri-plugin-api`.
    pub plugin_api: String,
    /// The version of `razeri-plugin-contract`.
    pub contract: String,
    /// A fingerprint of the source of both crates. Two builds carrying the
    /// same version numbers over different source have different
    /// fingerprints.
    pub source: u64,
}

/// One part of a [`BuildDeclaration`], named so a refusal can say which parts
/// differ.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum BuildPart {
    /// The compiler.
    Compiler,
    /// The version of `razeri-plugin-api`.
    PluginApi,
    /// The version of `razeri-plugin-contract`.
    Contract,
    /// The source of the two crates, when their versions agree.
    Source,
}

impl BuildDeclaration {
    /// How this build of `razeri-plugin-api` was made — the declaration a host
    /// compares plugins against, and the one [`export_plugin!`](crate::export_plugin)
    /// exports from a plugin.
    #[must_use]
    pub fn this_build() -> Self {
        Self {
            compiler: export::COMPILER.to_string_lossy().into_owned(),
            plugin_api: export::PLUGIN_API.to_string_lossy().into_owned(),
            contract: export::CONTRACT.to_string_lossy().into_owned(),
            source: export::SOURCE,
        }
    }

    /// The parts in which `self` differs from `needs`, in the order a reader
    /// meets them. Empty when the two are the same build.
    #[must_use]
    pub fn differences(&self, needs: &Self) -> Vec<BuildPart> {
        let mut parts = Vec::new();
        if self.compiler != needs.compiler {
            parts.push(BuildPart::Compiler);
        }
        if self.plugin_api != needs.plugin_api {
            parts.push(BuildPart::PluginApi);
        }
        if self.contract != needs.contract {
            parts.push(BuildPart::Contract);
        }
        // Another version is another source; the fingerprint is news only
        // when both versions agree.
        let versions_agree = self.plugin_api == needs.plugin_api && self.contract == needs.contract;
        if versions_agree && self.source != needs.source {
            parts.push(BuildPart::Source);
        }
        parts
    }
}

impl fmt::Display for BuildDeclaration {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "{} against razeri-plugin-api {} and razeri-plugin-contract {} (source {:016x})",
            self.compiler, self.plugin_api, self.contract, self.source
        )
    }
}

/// Why the loader would not load a plugin library.
///
/// A refusal is always this razeri's. Its message says what to rebuild the
/// plugin with, from [`BuildDeclaration::this_build`], unless the plugin was
/// built against newer plugin crates than this razeri: then it says to run the
/// razeri the plugin was built for, and names both builds.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum PluginRefusal {
    /// The library does not say how it was built: it has no
    /// [`PLUGIN_BUILD_SYMBOL`]. Every plugin exported without
    /// [`export_plugin!`](crate::export_plugin) is one of these.
    Undeclared,
    /// The library says how it was built in a form this razeri cannot read.
    Unreadable {
        /// The form number the library declared.
        form: u32,
    },
    /// The library was built differently from this razeri.
    BuiltDifferently {
        /// How the library was built.
        built_with: BuildDeclaration,
    },
}

impl PluginRefusal {
    /// The refusal a plugin declaring `built_with` receives from this razeri,
    /// or `None` when it was built exactly as this razeri was.
    #[must_use]
    pub fn for_declaration(built_with: BuildDeclaration) -> Option<Self> {
        if built_with.differences(&BuildDeclaration::this_build()).is_empty() {
            None
        } else {
            Some(Self::BuiltDifferently { built_with })
        }
    }
}

/// Which side of a refusal is the older build: the plugin, or the razeri
/// loading it.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum OlderSide {
    /// The plugin was built against older plugin crates than this razeri.
    Plugin,
    /// This razeri was built against older plugin crates than the plugin.
    Razeri,
}

/// **Which side is older**, read from the two crate versions: the plugin
/// where neither of its versions is newer than this razeri's and one is
/// older, this razeri where the reverse holds. `None` where the versions
/// agree, where one is newer and the other older, or where either is not a
/// version this can read: no side is plainly the older one then.
fn older_side(built_with: &BuildDeclaration, needs: &BuildDeclaration) -> Option<OlderSide> {
    let pairs =
        [(&built_with.plugin_api, &needs.plugin_api), (&built_with.contract, &needs.contract)];
    let mut orders = Vec::new();
    for (plugin, razeri) in pairs {
        orders.push(release_numbers(plugin)?.cmp(&release_numbers(razeri)?));
    }
    let any = |order| orders.contains(&order);
    match (any(std::cmp::Ordering::Less), any(std::cmp::Ordering::Greater)) {
        (true, false) => Some(OlderSide::Plugin),
        (false, true) => Some(OlderSide::Razeri),
        _ => None,
    }
}

/// `0.18.1` as `[0, 18, 1]`; a pre-release or build suffix (`-pre.1`,
/// `+local`) is left off. `None` for anything that is not dotted numbers.
fn release_numbers(version: &str) -> Option<Vec<u64>> {
    let release = version.split(['-', '+']).next()?;
    release.split('.').map(|part| part.parse().ok()).collect()
}

impl fmt::Display for PluginRefusal {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let needs = BuildDeclaration::this_build();
        let older = match self {
            Self::BuiltDifferently { built_with } => older_side(built_with, &needs),
            Self::Undeclared | Self::Unreadable { .. } => None,
        };

        // **A plugin newer than this razeri is not told to rebuild**
        // (`fr-1029-72`): rebuilding would move a current plugin back to an
        // older contract. The cure is the razeri it was built for.
        if let (Some(OlderSide::Razeri), Self::BuiltDifferently { built_with }) = (older, self) {
            return write!(
                f,
                "Run the razeri this plugin was built for, or link it into a razeri you build \
                 yourself ({LINKING_GUIDE}). This razeri is older than the plugin: it was built \
                 against razeri-plugin-api {} and razeri-plugin-contract {}, and the plugin \
                 against razeri-plugin-api {} and razeri-plugin-contract {}. A plugin loaded from \
                 a directory must be built exactly as the razeri that loads it.",
                needs.plugin_api, needs.contract, built_with.plugin_api, built_with.contract
            );
        }

        // What to do, first.
        let only_the_source_differs = matches!(
            self,
            Self::BuiltDifferently { built_with }
                if built_with.differences(&needs) == [BuildPart::Source]
        );
        if only_the_source_differs {
            write!(
                f,
                "Rebuild it with {} against the same source of razeri-plugin-api {} and \
                 razeri-plugin-contract {} this razeri was built from (source {:016x})",
                needs.compiler, needs.plugin_api, needs.contract, needs.source
            )?;
        } else {
            write!(
                f,
                "Rebuild it with {} against razeri-plugin-api {} and razeri-plugin-contract {}",
                needs.compiler, needs.plugin_api, needs.contract
            )?;
        }
        if matches!(self, Self::Undeclared) {
            f.write_str(", exporting it with razeri_plugin_api::export_plugin!")?;
        }
        write!(f, ", or link it into a razeri you build yourself ({LINKING_GUIDE}). ")?;

        // Then why.
        match self {
            Self::Undeclared => f.write_str("It does not say how it was built")?,
            Self::Unreadable { form, .. } => write!(
                f,
                "It says how it was built in a form this razeri does not read (form {form})"
            )?,
            Self::BuiltDifferently { built_with } => {
                if older == Some(OlderSide::Plugin) {
                    f.write_str("The plugin is older than this razeri: ")?;
                    write!(f, "it was built {}", built_differently(built_with, &needs))?;
                } else {
                    write!(f, "It was built {}", built_differently(built_with, &needs))?;
                }
            }
        }
        f.write_str(
            ", and a plugin loaded from a directory must be built exactly as the razeri that \
             loads it.",
        )
    }
}

impl std::error::Error for PluginRefusal {}

/// The parts of `built_with` that differ from `needs`, as the end of the
/// sentence "It was built …".
fn built_differently(built_with: &BuildDeclaration, needs: &BuildDeclaration) -> String {
    let parts = built_with.differences(needs);
    let mut clauses = Vec::new();
    if parts.contains(&BuildPart::Compiler) {
        clauses.push(format!("with {}", built_with.compiler));
    }
    let mut against = Vec::new();
    if parts.contains(&BuildPart::PluginApi) {
        against.push(format!("razeri-plugin-api {}", built_with.plugin_api));
    }
    if parts.contains(&BuildPart::Contract) {
        against.push(format!("razeri-plugin-contract {}", built_with.contract));
    }
    if !against.is_empty() {
        clauses.push(format!("against {}", against.join(" and ")));
    }
    if parts.contains(&BuildPart::Source) {
        clauses.push(format!(
            "against another source of those two versions (source {:016x})",
            built_with.source
        ));
    }
    clauses.join(" ")
}

/// Outcome of loading a single plugin library.
#[derive(Debug)]
pub enum PluginLoadResult {
    /// Plugin loaded and registered successfully.
    Loaded {
        /// Display name (file stem).
        name: String,
        /// Path to the shared library.
        path: std::path::PathBuf,
    },
    /// The library was found and not loaded, because it was not built as
    /// this razeri was. None of its Rust code ran.
    Refused {
        /// Path to the shared library.
        path: std::path::PathBuf,
        /// Why it was refused.
        refusal: PluginRefusal,
    },
    /// Library could not be loaded or registration failed.
    Failed {
        /// Path to the shared library.
        path: std::path::PathBuf,
        /// Error description.
        error: String,
    },
}

/// Read a declaration a library exported.
///
/// # Errors
///
/// A [`PluginRefusal::Unreadable`] when the record is in a form this razeri
/// does not read, or a string in it is missing.
///
/// # Safety
///
/// `record` must be null or point to a live [`export::ExportedBuildDeclaration`]
/// whose string pointers are null or point to NUL-terminated strings.
pub unsafe fn read_declaration(
    record: *const export::ExportedBuildDeclaration,
) -> Result<BuildDeclaration, PluginRefusal> {
    let unreadable = |form| PluginRefusal::Unreadable { form };
    if record.is_null() {
        return Err(unreadable(0));
    }
    // The form comes first and is read on its own: a record in another form
    // may be laid out differently after it.
    // SAFETY: the caller promises a live record, which begins with its form.
    let form = unsafe { record.cast::<u32>().read() };
    if form != export::FORM {
        return Err(unreadable(form));
    }
    // SAFETY: a live record in the form this crate writes.
    let record = unsafe { &*record };
    // SAFETY: the caller promises each string pointer is null or NUL-terminated.
    let text = |part: *const std::ffi::c_char| {
        (!part.is_null())
            .then(|| unsafe { std::ffi::CStr::from_ptr(part) }.to_string_lossy().into_owned())
    };
    match (text(record.compiler), text(record.plugin_api), text(record.contract)) {
        (Some(compiler), Some(plugin_api), Some(contract)) => {
            Ok(BuildDeclaration { compiler, plugin_api, contract, source: record.source })
        }
        _ => Err(unreadable(form)),
    }
}

/// What [`export_plugin!`](crate::export_plugin) expands to reaches through
/// here. Not for use by hand.
#[doc(hidden)]
pub mod export {
    use std::ffi::{CStr, c_char};

    pub use inventory;

    /// The form of [`ExportedBuildDeclaration`] this crate writes and reads.
    /// A record declaring any other form is refused as unreadable.
    pub const FORM: u32 = 1;

    /// A [`BuildDeclaration`](super::BuildDeclaration) as it crosses the
    /// library boundary: C strings and one number, laid out as C lays them
    /// out, with the form first so a reader knows how to read the rest.
    /// A later form may add fields after these; it never moves one.
    #[repr(C)]
    pub struct ExportedBuildDeclaration {
        /// Which form of this record follows.
        pub form: u32,
        /// The compiler, NUL-terminated.
        pub compiler: *const c_char,
        /// The version of `razeri-plugin-api`, NUL-terminated.
        pub plugin_api: *const c_char,
        /// The version of `razeri-plugin-contract`, NUL-terminated.
        pub contract: *const c_char,
        /// The fingerprint of both crates' source.
        pub source: u64,
    }

    // SAFETY: every pointer in the record points to a string that lives for
    // the life of the program and is never written.
    unsafe impl Sync for ExportedBuildDeclaration {}

    /// The compiler this crate was built with, as `rustc -V` names it. The
    /// build script asks the compiler cargo is building with.
    pub const COMPILER: &CStr = c_str(concat!(env!("RAZERI_PLUGIN_API_COMPILER"), "\0"));

    /// This crate's version.
    pub const PLUGIN_API: &CStr = c_str(concat!(env!("CARGO_PKG_VERSION"), "\0"));

    /// The version of `razeri-plugin-contract` this crate was built against.
    pub const CONTRACT: &CStr = c_str_from_bytes(&CONTRACT_WITH_NUL);

    /// The fingerprint of both crates' source: the contract's, then this
    /// crate's, folded into one number.
    pub const SOURCE: u64 = fold(razeri_plugin_contract::SOURCE_FINGERPRINT, API_SOURCE);

    /// This crate's own source fingerprint, from its build script.
    const API_SOURCE: u64 = match u64::from_str_radix(env!("RAZERI_PLUGIN_API_SOURCE"), 16) {
        Ok(fingerprint) => fingerprint,
        Err(_) => panic!("the build script writes the fingerprint in hexadecimal"),
    };

    /// The contract's version with the NUL a C string ends in.
    const CONTRACT_WITH_NUL: [u8; razeri_plugin_contract::VERSION.len() + 1] =
        with_nul(razeri_plugin_contract::VERSION);

    const fn with_nul<const N: usize>(text: &str) -> [u8; N] {
        let bytes = text.as_bytes();
        let mut out = [0; N];
        let mut i = 0;
        while i < bytes.len() {
            out[i] = bytes[i];
            i += 1;
        }
        out
    }

    const fn c_str(text: &'static str) -> &'static CStr {
        c_str_from_bytes(text.as_bytes())
    }

    const fn c_str_from_bytes(bytes: &'static [u8]) -> &'static CStr {
        match CStr::from_bytes_with_nul(bytes) {
            Ok(text) => text,
            Err(_) => panic!("a part of the build declaration holds a NUL"),
        }
    }

    /// FNV-1a over the two fingerprints' bytes, the contract's first.
    const fn fold(contract: u64, api: u64) -> u64 {
        let mut hash: u64 = 0xcbf2_9ce4_8422_2325;
        let bytes = [contract.to_le_bytes(), api.to_le_bytes()];
        let mut half = 0;
        while half < 2 {
            let mut i = 0;
            while i < 8 {
                hash ^= bytes[half][i] as u64;
                hash = hash.wrapping_mul(0x0000_0100_0000_01b3);
                i += 1;
            }
            half += 1;
        }
        hash
    }

    /// This build's declaration, the record a plugin exports.
    pub static THIS_BUILD: ExportedBuildDeclaration = ExportedBuildDeclaration {
        form: FORM,
        compiler: COMPILER.as_ptr(),
        plugin_api: PLUGIN_API.as_ptr(),
        contract: CONTRACT.as_ptr(),
        source: SOURCE,
    };
}

// ── Feature-gated loading implementation ────────────────────────────

// Dynamic library loading (`libloading`/dlopen) is native-only: there is no
// shared-library loading on wasm32-unknown-unknown, and `libloading` is not a
// dependency of the wasm build. The `dynamic` feature is a no-op on wasm.
#[cfg(all(feature = "dynamic", not(target_arch = "wasm32")))]
mod loader {
    use super::{
        BuildDeclarationFn, PLUGIN_BUILD_SYMBOL, PLUGIN_REGISTER_SYMBOL, PluginLoadResult,
        PluginRefusal, PluginRegistryBuilder, RegisterPluginFn, read_declaration,
    };
    use std::path::Path;

    /// Load all plugin shared libraries from `dir` and register them
    /// with `builder`.
    ///
    /// Scans `dir` for files with the platform-appropriate extension
    /// (`.so`, `.dylib`, `.dll`). Each library is loaded, its build
    /// declaration is compared with this razeri's, and its registration
    /// function is called only when the two match.
    ///
    /// Returns one [`PluginLoadResult`] per library file found.
    pub fn load_plugins_from_dir(
        dir: &Path,
        builder: &mut PluginRegistryBuilder,
    ) -> Vec<PluginLoadResult> {
        warn_if_world_writable(dir);

        let entries = match std::fs::read_dir(dir) {
            Ok(e) => e,
            Err(e) => {
                tracing::debug!("Cannot read plugin directory {}: {e}", dir.display());
                return Vec::new();
            }
        };

        let mut results = Vec::new();

        for entry in entries.flatten() {
            let path = entry.path();
            if !is_shared_library(&path) {
                continue;
            }
            results.push(load_single_plugin(&path, builder));
        }

        results
    }

    /// Load a single shared library plugin.
    fn load_single_plugin(path: &Path, builder: &mut PluginRegistryBuilder) -> PluginLoadResult {
        let name = path.file_stem().and_then(|s| s.to_str()).unwrap_or("unknown").to_string();

        // SAFETY: opening a library runs its initialisers, which is the one
        // thing done before its declaration is read. A file in a plugin
        // directory is trusted that far and no further.
        let lib = match unsafe { libloading::Library::new(path) } {
            Ok(l) => l,
            Err(e) => {
                return PluginLoadResult::Failed {
                    path: path.to_path_buf(),
                    error: format!("Failed to load library: {e}"),
                };
            }
        };

        // How the library was built, read through its C door. Nothing else
        // in it is called until this matches this razeri's own build.
        // SAFETY: the symbol, when present, has the C signature every
        // compiler agrees on, and returns a record `read_declaration` reads
        // through its C layout.
        let declared = unsafe {
            match lib.get::<BuildDeclarationFn>(PLUGIN_BUILD_SYMBOL.as_bytes()) {
                Ok(declare) => read_declaration(declare()),
                Err(_) => Err(PluginRefusal::Undeclared),
            }
        };
        let refusal = match declared {
            Ok(built_with) => PluginRefusal::for_declaration(built_with),
            Err(refusal) => Some(refusal),
        };
        if let Some(refusal) = refusal {
            // Nothing the library holds was handed to razeri, so it can go.
            drop(lib);
            return PluginLoadResult::Refused { path: path.to_path_buf(), refusal };
        }

        // SAFETY: the library was built exactly as this razeri was, so the
        // registration's Rust signature means the same thing on both sides.
        let register_fn = unsafe {
            match lib.get::<RegisterPluginFn>(PLUGIN_REGISTER_SYMBOL.as_bytes()) {
                Ok(f) => *f,
                Err(e) => {
                    return PluginLoadResult::Failed {
                        path: path.to_path_buf(),
                        error: format!("Missing symbol '{PLUGIN_REGISTER_SYMBOL}': {e}"),
                    };
                }
            }
        };

        // Call registration wrapped in catch_unwind for safety.
        let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            // SAFETY: the builder outlives the call, and the build matched.
            unsafe { register_fn(builder) };
        }));

        // Once the registration has run, the library stays loaded for the
        // life of the process, whatever it did: anything it registered before
        // a panic points into it, and unloading it would leave those behind.
        std::mem::forget(lib);
        match result {
            Ok(()) => {
                tracing::info!("Loaded plugin '{name}' from {}", path.display());
                PluginLoadResult::Loaded { name, path: path.to_path_buf() }
            }
            Err(_) => PluginLoadResult::Failed {
                path: path.to_path_buf(),
                error: "Plugin registration panicked".to_string(),
            },
        }
    }

    /// Check if a path has a shared-library extension.
    fn is_shared_library(path: &Path) -> bool {
        let ext = path.extension().and_then(|e| e.to_str()).unwrap_or("");
        matches!(ext, "so" | "dylib" | "dll")
    }

    /// Emit a warning if the directory is world-writable (Unix only).
    #[cfg(unix)]
    fn warn_if_world_writable(dir: &Path) {
        use std::os::unix::fs::PermissionsExt;
        if let Ok(meta) = std::fs::metadata(dir) {
            let mode = meta.permissions().mode();
            if mode & 0o002 != 0 {
                tracing::warn!(
                    "Plugin directory {} is world-writable — this is a security risk",
                    dir.display()
                );
            }
        }
    }

    #[cfg(not(unix))]
    fn warn_if_world_writable(_dir: &Path) {
        // No-op on non-Unix platforms.
    }
}

#[cfg(all(feature = "dynamic", not(target_arch = "wasm32")))]
pub use loader::load_plugins_from_dir;

// ── Tests ────────────────────────────────────────────────────────────

#[cfg(test)]
mod tests {
    use super::export::{ExportedBuildDeclaration, FORM, THIS_BUILD};
    use super::*;

    /// A declaration that differs from this build only where the test says.
    fn this_build_but(change: impl FnOnce(&mut BuildDeclaration)) -> BuildDeclaration {
        let mut declared = BuildDeclaration::this_build();
        change(&mut declared);
        declared
    }

    /// The message a plugin declaring `declared` is refused with.
    fn refusal_text(declared: BuildDeclaration) -> String {
        PluginRefusal::for_declaration(declared)
            .expect("a declaration that differs is refused")
            .to_string()
    }

    #[test]
    fn symbol_names_are_stable() {
        assert_eq!(PLUGIN_REGISTER_SYMBOL, "razeri_plugin_register_v1");
        assert_eq!(PLUGIN_BUILD_SYMBOL, "razeri_plugin_build");
    }

    /// **This build names its compiler, both crates and their source.** A
    /// declaration with an empty part would match any other with the same
    /// empty part, which is the silence the declaration exists to end.
    #[test]
    fn this_build_names_its_compiler_both_crate_versions_and_its_source() {
        let this = BuildDeclaration::this_build();
        assert!(this.compiler.starts_with("rustc "), "{this:?}");
        assert_eq!(this.plugin_api, env!("CARGO_PKG_VERSION"));
        assert_eq!(this.contract, razeri_plugin_contract::VERSION);
        assert_ne!(this.source, 0, "{this:?}");
    }

    /// **What a plugin exports reads back, through its C layout, as the build
    /// that made it.**
    #[test]
    fn the_exported_declaration_reads_back_as_this_build() {
        // SAFETY: THIS_BUILD is a live record of static C strings.
        let read = unsafe { read_declaration(&raw const THIS_BUILD) };
        assert_eq!(read, Ok(BuildDeclaration::this_build()));
    }

    #[test]
    fn a_declaration_identical_to_this_build_is_accepted() {
        assert_eq!(PluginRefusal::for_declaration(BuildDeclaration::this_build()), None);
        let this = BuildDeclaration::this_build();
        assert!(this.differences(&this).is_empty());
    }

    /// **Another compiler is refused, and the refusal names both.** A newer
    /// compiler over the same source is enough to crash a process that calls
    /// into the library, so the compiler is compared on its own.
    #[test]
    fn a_different_compiler_is_refused_naming_the_compiler() {
        let needs = BuildDeclaration::this_build();
        let declared = this_build_but(|d| d.compiler = "rustc 1.0.0 (elsewhere)".to_string());
        assert_eq!(declared.differences(&needs), vec![BuildPart::Compiler]);

        let text = refusal_text(declared);
        assert!(text.contains("rustc 1.0.0 (elsewhere)"), "{text}");
        assert!(text.contains(&needs.compiler), "{text}");
    }

    #[test]
    fn a_different_plugin_api_version_is_refused_naming_it() {
        let needs = BuildDeclaration::this_build();
        let declared = this_build_but(|d| d.plugin_api = "0.0.1".to_string());
        assert_eq!(declared.differences(&needs), vec![BuildPart::PluginApi]);

        let text = refusal_text(declared);
        assert!(text.contains("razeri-plugin-api 0.0.1"), "{text}");
        assert!(text.contains(&format!("razeri-plugin-api {}", needs.plugin_api)), "{text}");
    }

    #[test]
    fn a_different_contract_version_is_refused_naming_the_contract() {
        let needs = BuildDeclaration::this_build();
        let declared = this_build_but(|d| d.contract = "0.0.2".to_string());
        assert_eq!(declared.differences(&needs), vec![BuildPart::Contract]);

        let text = refusal_text(declared);
        assert!(text.contains("razeri-plugin-contract 0.0.2"), "{text}");
        assert!(text.contains(&format!("razeri-plugin-contract {}", needs.contract)), "{text}");
    }

    /// **The same version numbers over different source are refused.** A
    /// crate's version moves when it is published, not when its source does,
    /// so a plugin built against the published crates and a razeri built from
    /// newer source can carry the same numbers and different types.
    #[test]
    fn the_same_versions_over_different_source_are_refused() {
        let needs = BuildDeclaration::this_build();
        let declared = this_build_but(|d| d.source = needs.source ^ 1);
        assert_eq!(declared.differences(&needs), vec![BuildPart::Source]);

        let text = refusal_text(declared);
        assert!(text.contains(&format!("{:016x}", needs.source)), "{text}");
        assert!(text.contains(&format!("{:016x}", needs.source ^ 1)), "{text}");
    }

    /// Every part that differs is named, in the order a reader meets them.
    #[test]
    fn every_part_that_differs_is_named() {
        let needs = BuildDeclaration::this_build();
        let declared = BuildDeclaration {
            compiler: "rustc 1.0.0 (elsewhere)".to_string(),
            plugin_api: "0.0.1".to_string(),
            contract: "0.0.2".to_string(),
            source: needs.source ^ 1,
        };
        assert_eq!(
            declared.differences(&needs),
            vec![BuildPart::Compiler, BuildPart::PluginApi, BuildPart::Contract]
        );
    }

    /// **The fix comes first.** Every refusal opens with what to do, names
    /// both ways to do it, and never names a type or a symbol the reader did
    /// not write.
    #[test]
    fn every_refusal_says_what_to_do_first_and_names_both_ways() {
        let needs = BuildDeclaration::this_build();
        let refusals = [
            PluginRefusal::Undeclared,
            PluginRefusal::Unreadable { form: FORM + 1 },
            PluginRefusal::for_declaration(this_build_but(|d| d.compiler = "rustc 1.0.0".into()))
                .expect("refused"),
        ];
        for refusal in refusals {
            let text = refusal.to_string();
            assert!(text.starts_with("Rebuild it "), "the fix is not first: {text}");
            assert!(text.contains(&needs.compiler), "{text}");
            assert!(text.contains(&format!("razeri-plugin-api {}", needs.plugin_api)), "{text}");
            assert!(text.contains(&format!("razeri-plugin-contract {}", needs.contract)), "{text}");
            assert!(text.contains("link it into a razeri you build"), "{text}");
            assert!(text.contains(LINKING_GUIDE), "{text}");
            for internal in ["BuildDeclaration", "PluginRefusal", "razeri_plugin_build"] {
                assert!(!text.contains(internal), "names {internal}: {text}");
            }
        }
    }

    /// `0.18.2` moved `steps` minor versions: `1` is `0.19.0`, `-1` is
    /// `0.17.0`.
    fn minor_moved(version: &str, steps: i64) -> String {
        let mut parts = version.split('.').map(|part| part.parse::<i64>().expect("a number"));
        let (major, minor) = (parts.next().expect("major"), parts.next().expect("minor"));
        format!("{major}.{}.0", minor + steps)
    }

    /// **A plugin newer than this razeri is told to run the razeri it was
    /// built for**, not to rebuild against an older contract (`fr-1029-72`).
    /// Measured on `pre.11`: a template built against contract 0.17.1, loaded
    /// by a `pre.10` razeri, was told to rebuild against 0.16.0 — a downgrade
    /// of a current plugin to an old contract.
    #[test]
    fn a_plugin_newer_than_this_razeri_is_told_to_run_the_razeri_it_was_built_for() {
        let needs = BuildDeclaration::this_build();
        let declared = this_build_but(|d| {
            d.plugin_api = minor_moved(&needs.plugin_api, 1);
            d.contract = minor_moved(&needs.contract, 1);
        });
        let text = refusal_text(declared.clone());
        assert!(text.starts_with("Run the razeri this plugin was built for"), "{text}");
        assert!(text.contains("This razeri is older than the plugin"), "{text}");
        assert!(!text.contains("Rebuild it"), "no downgrade is offered: {text}");
        for version in
            [&needs.plugin_api, &needs.contract, &declared.plugin_api, &declared.contract]
        {
            assert!(text.contains(version.as_str()), "names {version}: {text}");
        }
        assert!(text.contains(LINKING_GUIDE), "{text}");

        // Newer on one part and the same on the other is still newer.
        let text = refusal_text(this_build_but(|d| d.contract = minor_moved(&needs.contract, 1)));
        assert!(text.starts_with("Run the razeri this plugin was built for"), "{text}");
    }

    /// **A plugin older than this razeri is told to rebuild**, and told it is
    /// the older side.
    #[test]
    fn a_plugin_older_than_this_razeri_is_told_to_rebuild_and_that_it_is_older() {
        let needs = BuildDeclaration::this_build();
        let declared = this_build_but(|d| {
            d.plugin_api = minor_moved(&needs.plugin_api, -1);
            d.contract = minor_moved(&needs.contract, -1);
        });
        let text = refusal_text(declared);
        assert!(text.starts_with("Rebuild it with"), "{text}");
        assert!(text.contains("The plugin is older than this razeri"), "{text}");

        // One part newer and one older says neither is older, and keeps the
        // rebuild: there is no razeri the plugin was built for to point at.
        let mixed = refusal_text(this_build_but(|d| {
            d.plugin_api = minor_moved(&needs.plugin_api, 1);
            d.contract = minor_moved(&needs.contract, -1);
        }));
        assert!(mixed.starts_with("Rebuild it with"), "{mixed}");
        assert!(!mixed.contains("older than"), "{mixed}");
    }

    #[test]
    fn a_plugin_that_declares_nothing_is_told_so_and_how_to_export_one() {
        let text = PluginRefusal::Undeclared.to_string();
        assert!(text.contains("does not say how it was built"), "{text}");
        assert!(text.contains("export_plugin!"), "{text}");
    }

    /// **A form this razeri does not read is refused, not guessed at.**
    #[test]
    fn a_declaration_in_a_form_this_razeri_does_not_read_is_refused() {
        let later = ExportedBuildDeclaration {
            form: FORM + 1,
            compiler: std::ptr::null(),
            plugin_api: std::ptr::null(),
            contract: std::ptr::null(),
            source: 0,
        };
        // SAFETY: a live record; its null strings are never read for an
        // unknown form.
        let read = unsafe { read_declaration(&raw const later) };
        assert_eq!(read, Err(PluginRefusal::Unreadable { form: FORM + 1 }));
        let text = read.unwrap_err().to_string();
        assert!(text.contains(&format!("form {}", FORM + 1)), "{text}");
    }

    #[test]
    fn a_record_with_a_missing_part_is_refused_as_unreadable() {
        let broken = ExportedBuildDeclaration {
            form: FORM,
            compiler: std::ptr::null(),
            plugin_api: c"0.0.1".as_ptr(),
            contract: c"0.0.2".as_ptr(),
            source: 1,
        };
        // SAFETY: a live record whose strings are null or static.
        let read = unsafe { read_declaration(&raw const broken) };
        assert!(matches!(read, Err(PluginRefusal::Unreadable { form: FORM, .. })), "{read:?}");
    }

    #[test]
    fn no_record_at_all_is_refused_as_unreadable() {
        // SAFETY: null is allowed and never dereferenced.
        let read = unsafe { read_declaration(std::ptr::null()) };
        assert!(matches!(read, Err(PluginRefusal::Unreadable { .. })), "{read:?}");
    }

    #[cfg(all(feature = "dynamic", not(target_arch = "wasm32")))]
    mod dynamic_tests {
        use super::*;
        use std::path::Path;

        #[test]
        fn load_from_nonexistent_dir_returns_empty() {
            let mut builder = PluginRegistryBuilder::new();
            let results = load_plugins_from_dir(
                Path::new("/tmp/razeri-nonexistent-plugin-dir-12345"),
                &mut builder,
            );
            assert!(results.is_empty());
        }

        #[test]
        fn load_from_empty_dir_returns_empty() {
            let dir = tempfile::tempdir().unwrap();
            let mut builder = PluginRegistryBuilder::new();
            let results = load_plugins_from_dir(dir.path(), &mut builder);
            assert!(results.is_empty());
        }

        #[test]
        fn load_skips_non_library_files() {
            let dir = tempfile::tempdir().unwrap();
            std::fs::write(dir.path().join("notes.txt"), "hello").unwrap();
            std::fs::write(dir.path().join("data.json"), "{}").unwrap();

            let mut builder = PluginRegistryBuilder::new();
            let results = load_plugins_from_dir(dir.path(), &mut builder);
            assert!(results.is_empty());
        }

        #[test]
        fn load_invalid_so_returns_failed() {
            let dir = tempfile::tempdir().unwrap();
            let lib_name = if cfg!(target_os = "macos") {
                "bad.dylib"
            } else if cfg!(target_os = "windows") {
                "bad.dll"
            } else {
                "bad.so"
            };
            std::fs::write(dir.path().join(lib_name), "not a real library").unwrap();

            let mut builder = PluginRegistryBuilder::new();
            let results = load_plugins_from_dir(dir.path(), &mut builder);
            assert_eq!(results.len(), 1);
            assert!(
                matches!(&results[0], PluginLoadResult::Failed { error, .. } if error.contains("Failed to load library"))
            );
        }
    }
}