Metabuild (RFC 2196)

Author: ehuss

Diff for: src/cargo/core/compiler/custom_build.rs

@@ -134,6 +134,10 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> CargoRes
     let build_plan = bcx.build_config.build_plan;
     let invocation_name = unit.buildkey();
 
+    if let Some(deps) = unit.pkg.manifest().metabuild() {
+        prepare_metabuild(cx, build_script_unit, deps)?;
+    }
+
     // Building the command to execute
     let to_exec = script_output.join(unit.target.name());
 
@@ -536,6 +540,38 @@ impl BuildOutput {
     }
 }
 
+fn prepare_metabuild<'a, 'cfg>(
+    cx: &Context<'a, 'cfg>,
+    unit: &Unit<'a>,
+    deps: &[String],
+) -> CargoResult<()> {
+    let mut output = Vec::new();
+    let available_deps = cx.dep_targets(unit);
+    // Filter out optional dependencies, and look up the actual lib name.
+    let meta_deps: Vec<_> = deps
+        .iter()
+        .filter_map(|name| {
+            available_deps
+                .iter()
+                .find(|u| u.pkg.name().as_str() == name.as_str())
+                .map(|dep| dep.target.crate_name())
+        })
+        .collect();
+    for dep in &meta_deps {
+        output.push(format!("extern crate {};\n", dep));
+    }
+    output.push("fn main() {\n".to_string());
+    for dep in &meta_deps {
+        output.push(format!("    {}::metabuild();\n", dep));
+    }
+    output.push("}\n".to_string());
+    let output = output.join("");
+    let path = unit.target.src_path();
+    fs::create_dir_all(path.parent().unwrap())?;
+    paths::write_if_changed(path, &output)?;
+    Ok(())
+}
+
 impl BuildDeps {
     pub fn new(output_file: &Path, output: Option<&BuildOutput>) -> BuildDeps {
         BuildDeps {

Diff for: src/cargo/core/features.rs

@@ -189,6 +189,9 @@ features! {
 
         // "default-run" manifest option,
         [unstable] default_run: bool,
+
+        // Declarative build scripts.
+        [unstable] metabuild: bool,
     }
 }
 

Diff for: src/cargo/core/manifest.rs

@@ -44,6 +44,7 @@ pub struct Manifest {
     edition: Edition,
     im_a_teapot: Option<bool>,
     default_run: Option<String>,
+    metabuild: Option<Vec<String>>,
 }
 
 /// When parsing `Cargo.toml`, some warnings should silenced
@@ -300,6 +301,7 @@ impl Manifest {
         im_a_teapot: Option<bool>,
         default_run: Option<String>,
         original: Rc<TomlManifest>,
+        metabuild: Option<Vec<String>>,
     ) -> Manifest {
         Manifest {
             summary,
@@ -321,6 +323,7 @@ impl Manifest {
             im_a_teapot,
             default_run,
             publish_lockfile,
+            metabuild,
         }
     }
 
@@ -348,6 +351,9 @@ impl Manifest {
     pub fn targets(&self) -> &[Target] {
         &self.targets
     }
+    pub fn targets_mut(&mut self) -> &mut[Target] {
+        &mut self.targets
+    }
     pub fn version(&self) -> &Version {
         self.package_id().version()
     }
@@ -443,6 +449,10 @@ impl Manifest {
     pub fn default_run(&self) -> Option<&str> {
         self.default_run.as_ref().map(|s| &s[..])
     }
+
+    pub fn metabuild(&self) -> Option<&Vec<String>> {
+        self.metabuild.as_ref()
+    }
 }
 
 impl VirtualManifest {
@@ -746,6 +756,11 @@ impl Target {
         self.doc = doc;
         self
     }
+    pub fn set_src_path(&mut self, src_path: PathBuf) -> &mut Target {
+        assert!(src_path.is_absolute());
+        self.src_path =  NonHashedPathBuf { path: src_path };
+        self
+    }
 }
 
 impl fmt::Display for Target {

Diff for: src/cargo/core/package.rs

@@ -105,6 +105,9 @@ impl Package {
     pub fn manifest(&self) -> &Manifest {
         &self.manifest
     }
+    pub fn manifest_mut(&mut self) -> &mut Manifest {
+        &mut self.manifest
+    }
     /// Get the path to the manifest
     pub fn manifest_path(&self) -> &Path {
         &self.manifest_path

Diff for: src/cargo/core/workspace.rs

@@ -1,6 +1,9 @@
+#![allow(deprecated)] // for SipHasher
+
 use std::cell::RefCell;
 use std::collections::hash_map::{Entry, HashMap};
 use std::collections::BTreeMap;
+use std::hash::{Hash, Hasher, SipHasher};
 use std::path::{Path, PathBuf};
 use std::slice;
 
@@ -150,6 +153,7 @@ impl<'cfg> Workspace<'cfg> {
         };
         ws.root_manifest = ws.find_root(manifest_path)?;
         ws.find_members()?;
+        ws.fixup()?;
         ws.validate()?;
         Ok(ws)
     }
@@ -513,6 +517,11 @@ impl<'cfg> Workspace<'cfg> {
         Ok(())
     }
 
+    fn fixup(&mut self) -> CargoResult<()> {
+        let target_dir = self.target_dir();
+        self.packages.fixup(target_dir)
+    }
+
     /// Validates a workspace, ensuring that a number of invariants are upheld:
     ///
     /// 1. A workspace only has one root.
@@ -773,6 +782,29 @@ impl<'cfg> Packages<'cfg> {
             }
         }
     }
+
+    fn fixup(&mut self, target_dir: Filesystem) -> CargoResult<()> {
+        for maybe_pkg in self.packages.values_mut() {
+            if let MaybePackage::Package(pkg) = maybe_pkg {
+                let mut hasher = SipHasher::new_with_keys(0, 0);
+                pkg.hash(&mut hasher);
+                let hash = hasher.finish();
+                let name = pkg.name();
+                for target in pkg.manifest_mut().targets_mut().iter_mut() {
+                    // TODO: Don't rely on magic name?
+                    if target.is_custom_build()
+                        && target.src_path().file_name().unwrap() == "metabuild.rs"
+                    {
+                        let path = target_dir
+                            .join(".metabuild")
+                            .join(format!("metabuild-{}-{:016x}.rs", name, hash));
+                        target.set_src_path(path.into_path_unlocked());
+                    }
+                }
+            }
+        }
+        Ok(())
+    }
 }
 
 impl<'a, 'cfg> Members<'a, 'cfg> {

Diff for: src/cargo/util/paths.rs

@@ -142,6 +142,27 @@ pub fn write(path: &Path, contents: &[u8]) -> CargoResult<()> {
     Ok(())
 }
 
+pub fn write_if_changed<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> CargoResult<()> {
+    (|| -> CargoResult<()> {
+        let contents = contents.as_ref();
+        let mut f = OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open(&path)?;
+        let mut orig = Vec::new();
+        f.read_to_end(&mut orig)?;
+        if orig != contents {
+            f.set_len(0)?;
+            f.seek(io::SeekFrom::Start(0))?;
+            f.write_all(contents)?;
+        }
+        Ok(())
+    })()
+        .chain_err(|| format!("failed to write `{}`", path.as_ref().display()))?;
+    Ok(())
+}
+
 pub fn append(path: &Path, contents: &[u8]) -> CargoResult<()> {
     (|| -> CargoResult<()> {
         let mut f = OpenOptions::new()

Diff for: src/cargo/util/toml/mod.rs

@@ -484,6 +484,44 @@ impl TomlProfile {
     }
 }
 
+#[derive(Clone, Debug, Serialize, Eq, PartialEq)]
+pub struct StringOrVec(Vec<String>);
+
+impl<'de> de::Deserialize<'de> for StringOrVec {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: de::Deserializer<'de>,
+    {
+        struct Visitor;
+
+        impl<'de> de::Visitor<'de> for Visitor {
+            type Value = StringOrVec;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("string or list of strings")
+            }
+
+            fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
+            where
+                E: de::Error,
+            {
+                Ok(StringOrVec(vec![s.to_string()]))
+            }
+
+            fn visit_seq<V>(self, v: V) -> Result<Self::Value, V::Error>
+            where
+                V: de::SeqAccess<'de>,
+            {
+                let seq = de::value::SeqAccessDeserializer::new(v);
+                Vec::deserialize(seq).map(StringOrVec)
+
+            }
+        }
+
+        deserializer.deserialize_any(Visitor)
+    }
+}
+
 #[derive(Clone, Debug, Serialize, Eq, PartialEq)]
 #[serde(untagged)]
 pub enum StringOrBool {
@@ -571,6 +609,7 @@ pub struct TomlProject {
     version: semver::Version,
     authors: Option<Vec<String>>,
     build: Option<StringOrBool>,
+    metabuild: Option<StringOrVec>,
     links: Option<String>,
     exclude: Option<Vec<String>>,
     include: Option<Vec<String>>,
@@ -775,6 +814,10 @@ impl TomlManifest {
             Edition::Edition2015
         };
 
+        if project.metabuild.is_some() {
+            features.require(Feature::metabuild())?;
+        }
+
         // If we have no lib at all, use the inferred lib if available
         // If we have a lib with a path, we're done
         // If we have a lib with no path, use the inferred lib or_else package name
@@ -784,6 +827,7 @@ impl TomlManifest {
             package_root,
             edition,
             &project.build,
+            &project.metabuild,
             &mut warnings,
             &mut errors,
         )?;
@@ -974,6 +1018,7 @@ impl TomlManifest {
             project.im_a_teapot,
             project.default_run.clone(),
             Rc::clone(me),
+            project.metabuild.clone().map(|sov| sov.0),
         );
         if project.license_file.is_some() && project.license.is_some() {
             manifest.warnings_mut().add_warning(

Diff for: src/cargo/util/toml/targets.rs

@@ -16,15 +16,16 @@ use std::collections::HashSet;
 
 use core::{compiler, Edition, Target};
 use util::errors::CargoResult;
-use super::{LibKind, PathValue, StringOrBool, TomlBenchTarget, TomlBinTarget, TomlExampleTarget,
-            TomlLibTarget, TomlManifest, TomlTarget, TomlTestTarget};
+use super::{LibKind, PathValue, StringOrBool, StringOrVec, TomlBenchTarget, TomlBinTarget,
+            TomlExampleTarget, TomlLibTarget, TomlManifest, TomlTarget, TomlTestTarget};
 
 pub fn targets(
     manifest: &TomlManifest,
     package_name: &str,
     package_root: &Path,
     edition: Edition,
     custom_build: &Option<StringOrBool>,
+    metabuild: &Option<StringOrVec>,
     warnings: &mut Vec<String>,
     errors: &mut Vec<String>,
 ) -> CargoResult<Vec<Target>> {
@@ -91,6 +92,9 @@ pub fn targets(
 
     // processing the custom build script
     if let Some(custom_build) = manifest.maybe_custom_build(custom_build, package_root) {
+        if metabuild.is_some() {
+            bail!("cannot specify both `metabuild` and `build`");
+        }
         let name = format!(
             "build-script-{}",
             custom_build
@@ -103,6 +107,24 @@ pub fn targets(
             package_root.join(custom_build),
         ));
     }
+    if let Some(metabuild) = metabuild {
+        // Verify names match available build deps.
+        let bdeps = manifest.build_dependencies.as_ref();
+        for name in &metabuild.0 {
+            if !bdeps.map_or(false, |bd| bd.contains_key(name)) {
+                bail!(
+                    "metabuild package `{}` must be specified in `build-dependencies`",
+                    name
+                );
+            }
+        }
+
+        targets.push(Target::custom_build_target(
+            &format!("metabuild-{}", package.name),
+            // TODO: This "magic" name is kinda weird.
+            package_root.join("metabuild.rs"),
+        ));
+    }
 
     Ok(targets)
 }

Diff for: src/doc/src/reference/unstable.md

@@ -320,3 +320,36 @@ both `src/bin/a.rs` and `src/bin/b.rs`:
 [project]
 default-run = "a"
 ```
+
+### Metabuild
+* Tracking Issue: [rust-lang/rust#49803](https://github.com/rust-lang/rust/issues/49803)
+* RFC: [#2196](https://github.com/rust-lang/rfcs/blob/master/text/2196-metabuild.md)
+
+Metabuild is a feature to have declarative build scripts.  Instead of writing
+a `build.rs` script, you specify a list of build dependencies in the
+`metabuild` key in `Cargo.toml`.  A build script is automatically generated
+that runs each build dependency in order.  Metabuild packages can then read
+metadata from `Cargo.toml` to specify their behavior.
+
+Include `cargo-features` at the top of `Cargo.toml`, a `metadata` key in the
+`package`, list the dependencies in `build-dependencies`, and add any metadata
+that the metabuild packages require.  Example:
+
+```toml
+cargo-features = ["metabuild"]
+
+[package]
+name = "mypackage"
+version = "0.0.1"
+metabuild = ["foo", "bar"]
+
+[build-dependencies]
+foo = "1.0"
+bar = "1.0"
+
+[package.metadata.foo]
+extra-info = "qwerty"
+```
+
+Metabuild packages should have a public function called `metabuild` that
+performs the same actions as a regular `build.rs` script would perform.

Diff for: tests/testsuite/main.rs

@@ -61,6 +61,7 @@ mod jobserver;
 mod local_registry;
 mod lockfile_compat;
 mod login;
+mod metabuild;
 mod metadata;
 mod net_config;
 mod new;