feat: Introduce perma unstable --compile-time-deps option for cargo build

src/bin/cargo/commands/build.rs

@@ -38,6 +38,7 @@ pub fn cli() -> Command {
         .arg_build_plan()
         .arg_unit_graph()
         .arg_timings()
+        .arg_compile_time_deps()
         .arg_manifest_path()
         .arg_lockfile_path()
         .arg_ignore_rust_version()

src/bin/cargo/commands/check.rs

@@ -35,6 +35,7 @@ pub fn cli() -> Command {
         .arg_target_dir()
         .arg_unit_graph()
         .arg_timings()
+        .arg_compile_time_deps()
         .arg_manifest_path()
         .arg_lockfile_path()
         .arg_ignore_rust_version()

src/cargo/core/compiler/build_config.rs

@@ -50,6 +50,8 @@ pub struct BuildConfig {
     pub timing_outputs: Vec<TimingOutput>,
     /// Output SBOM precursor files.
     pub sbom: bool,
+    /// Build compile time dependencies only, e.g., build scripts and proc macros
+    pub compile_time_deps_only: bool,
 }
 
 fn default_parallelism() -> CargoResult<u32> {
@@ -129,6 +131,7 @@ impl BuildConfig {
             future_incompat_report: false,
             timing_outputs: Vec::new(),
             sbom,
+            compile_time_deps_only: false,
         })
     }
 

src/cargo/core/compiler/build_runner/compilation_files.rs

@@ -448,6 +448,10 @@ impl<'a, 'gctx: 'a> CompilationFiles<'a, 'gctx> {
         bcx: &BuildContext<'a, 'gctx>,
     ) -> CargoResult<Arc<Vec<OutputFile>>> {
         let ret = match unit.mode {
+            _ if unit.skip_non_compile_time_dep => {
+                // This skips compilations so no outputs
+                vec![]
+            }
             CompileMode::Doc => {
                 let path = if bcx.build_config.intent.wants_doc_json_output() {
                     self.out_dir(unit)

src/cargo/core/compiler/mod.rs

@@ -180,50 +180,55 @@ fn compile<'gctx>(
         return Ok(());
     }
 
-    // Build up the work to be done to compile this unit, enqueuing it once
-    // we've got everything constructed.
-    fingerprint::prepare_init(build_runner, unit)?;
-
-    let job = if unit.mode.is_run_custom_build() {
-        custom_build::prepare(build_runner, unit)?
-    } else if unit.mode.is_doc_test() {
-        // We run these targets later, so this is just a no-op for now.
-        Job::new_fresh()
-    } else if build_plan {
-        Job::new_dirty(
-            rustc(build_runner, unit, &exec.clone())?,
-            DirtyReason::FreshBuild,
-        )
-    } else {
-        let force = exec.force_rebuild(unit) || force_rebuild;
-        let mut job = fingerprint::prepare_target(build_runner, unit, force)?;
-        job.before(if job.freshness().is_dirty() {
-            let work = if unit.mode.is_doc() || unit.mode.is_doc_scrape() {
-                rustdoc(build_runner, unit)?
-            } else {
-                rustc(build_runner, unit, exec)?
-            };
-            work.then(link_targets(build_runner, unit, false)?)
+    // If we are in `--compile-time-deps` and the given unit is not a compile time
+    // dependency, skip compling the unit and jumps to dependencies, which still
+    // have chances to be compile time dependencies
+    if !unit.skip_non_compile_time_dep {
+        // Build up the work to be done to compile this unit, enqueuing it once
+        // we've got everything constructed.
+        fingerprint::prepare_init(build_runner, unit)?;
+
+        let job = if unit.mode.is_run_custom_build() {
+            custom_build::prepare(build_runner, unit)?
+        } else if unit.mode.is_doc_test() {
+            // We run these targets later, so this is just a no-op for now.
+            Job::new_fresh()
+        } else if build_plan {
+            Job::new_dirty(
+                rustc(build_runner, unit, &exec.clone())?,
+                DirtyReason::FreshBuild,
+            )
         } else {
-            // We always replay the output cache,
-            // since it might contain future-incompat-report messages
-            let show_diagnostics = unit.show_warnings(bcx.gctx)
-                && build_runner.bcx.gctx.warning_handling()? != WarningHandling::Allow;
-            let work = replay_output_cache(
-                unit.pkg.package_id(),
-                PathBuf::from(unit.pkg.manifest_path()),
-                &unit.target,
-                build_runner.files().message_cache_path(unit),
-                build_runner.bcx.build_config.message_format,
-                show_diagnostics,
-            );
-            // Need to link targets on both the dirty and fresh.
-            work.then(link_targets(build_runner, unit, true)?)
-        });
-
-        job
-    };
-    jobs.enqueue(build_runner, unit, job)?;
+            let force = exec.force_rebuild(unit) || force_rebuild;
+            let mut job = fingerprint::prepare_target(build_runner, unit, force)?;
+            job.before(if job.freshness().is_dirty() {
+                let work = if unit.mode.is_doc() || unit.mode.is_doc_scrape() {
+                    rustdoc(build_runner, unit)?
+                } else {
+                    rustc(build_runner, unit, exec)?
+                };
+                work.then(link_targets(build_runner, unit, false)?)
+            } else {
+                // We always replay the output cache,
+                // since it might contain future-incompat-report messages
+                let show_diagnostics = unit.show_warnings(bcx.gctx)
+                    && build_runner.bcx.gctx.warning_handling()? != WarningHandling::Allow;
+                let work = replay_output_cache(
+                    unit.pkg.package_id(),
+                    PathBuf::from(unit.pkg.manifest_path()),
+                    &unit.target,
+                    build_runner.files().message_cache_path(unit),
+                    build_runner.bcx.build_config.message_format,
+                    show_diagnostics,
+                );
+                // Need to link targets on both the dirty and fresh.
+                work.then(link_targets(build_runner, unit, true)?)
+            });
+
+            job
+        };
+        jobs.enqueue(build_runner, unit, job)?;
+    }
 
     // Be sure to compile all dependencies of this target as well.
     let deps = Vec::from(build_runner.unit_deps(unit)); // Create vec due to mutable borrow.

src/cargo/core/compiler/standard_lib.rs

@@ -210,6 +210,7 @@ fn generate_roots(
                 /*dep_hash*/ 0,
                 IsArtifact::No,
                 None,
+                false,
             ));
         }
     }

src/cargo/core/compiler/unit.rs

@@ -112,6 +112,13 @@ pub struct UnitInner {
     ///
     /// [`FeaturesFor::ArtifactDep`]: crate::core::resolver::features::FeaturesFor::ArtifactDep
     pub artifact_target_for_features: Option<CompileTarget>,
+
+    /// Skip compiling this unit because `--compile-time-deps` flag is set and
+    /// this is not a compile time dependency.
+    ///
+    /// Since dependencies of this unit might be compile time dependencies, we
+    /// set this field instead of completely dropping out this unit from unit graph.
+    pub skip_non_compile_time_dep: bool,
 }
 
 impl UnitInner {
@@ -245,6 +252,7 @@ impl UnitInterner {
         dep_hash: u64,
         artifact: IsArtifact,
         artifact_target_for_features: Option<CompileTarget>,
+        skip_non_compile_time_dep: bool,
     ) -> Unit {
         let target = match (is_std, target.kind()) {
             // This is a horrible hack to support build-std. `libstd` declares
@@ -281,6 +289,7 @@ impl UnitInterner {
             dep_hash,
             artifact,
             artifact_target_for_features,
+            skip_non_compile_time_dep,
         });
         Unit { inner }
     }

src/cargo/core/compiler/unit_dependencies.rs

@@ -866,6 +866,7 @@ fn new_unit_dep_with_profile(
         /*dep_hash*/ 0,
         artifact.map_or(IsArtifact::No, |_| IsArtifact::Yes),
         artifact_target,
+        false,
     );
     Ok(UnitDep {
         unit,

src/cargo/core/manifest.rs

@@ -1079,6 +1079,11 @@ impl Target {
         *self.kind() == TargetKind::CustomBuild
     }
 
+    /// Returns `true` if it is a compile time depencencies, e.g., build script or proc macro
+    pub fn is_compile_time_dependency(&self) -> bool {
+        self.is_custom_build() || self.proc_macro()
+    }
+
     /// Returns the arguments suitable for `--crate-type` to pass to rustc.
     pub fn rustc_crate_types(&self) -> Vec<CrateType> {
         self.kind().rustc_crate_types()

src/cargo/ops/cargo_compile/mod.rs

@@ -449,6 +449,7 @@ pub fn create_bcx<'a, 'gctx>(
         &units,
         &scrape_units,
         host_kind_requested.then_some(explicit_host_kind),
+        build_config.compile_time_deps_only,
     );
 
     let mut extra_compiler_args = HashMap::new();
@@ -582,12 +583,16 @@ where `<compatible-ver>` is the latest version supporting rustc {rustc_version}"
 /// This is also responsible for adjusting the `debug` setting for host
 /// dependencies, turning off debug if the user has not explicitly enabled it,
 /// and the unit is not shared with a target unit.
+///
+/// This is also responsible for adjusting whether each unit should be compiled
+/// or not regarding `--compile-time-deps` flag.
 fn rebuild_unit_graph_shared(
     interner: &UnitInterner,
     unit_graph: UnitGraph,
     roots: &[Unit],
     scrape_units: &[Unit],
     to_host: Option<CompileKind>,
+    compile_time_deps_only: bool,
 ) -> (Vec<Unit>, Vec<Unit>, UnitGraph) {
     let mut result = UnitGraph::new();
     // Map of the old unit to the new unit, used to avoid recursing into units
@@ -602,8 +607,10 @@ fn rebuild_unit_graph_shared(
                 &mut result,
                 &unit_graph,
                 root,
+                true,
                 false,
                 to_host,
+                compile_time_deps_only,
             )
         })
         .collect();
@@ -628,14 +635,21 @@ fn traverse_and_share(
     new_graph: &mut UnitGraph,
     unit_graph: &UnitGraph,
     unit: &Unit,
+    unit_is_root: bool,
     unit_is_for_host: bool,
     to_host: Option<CompileKind>,
+    compile_time_deps_only: bool,
 ) -> Unit {
     if let Some(new_unit) = memo.get(unit) {
         // Already computed, no need to recompute.
         return new_unit.clone();
     }
     let mut dep_hash = StableHasher::new();
+    let skip_non_compile_time_deps = compile_time_deps_only
+        && (!unit.target.is_compile_time_dependency() ||
+            // Root unit is not a dependency unless other units are dependant
+            // to it.
+            unit_is_root);
     let new_deps: Vec<_> = unit_graph[unit]
         .iter()
         .map(|dep| {
@@ -645,8 +659,13 @@ fn traverse_and_share(
                 new_graph,
                 unit_graph,
                 &dep.unit,
+                false,
                 dep.unit_for.is_for_host(),
                 to_host,
+                // If we should compile the current unit, we should also compile
+                // its dependencies. And if not, we should compile compile time
+                // dependencies only.
+                skip_non_compile_time_deps,
             );
             new_dep_unit.hash(&mut dep_hash);
             UnitDep {
@@ -712,6 +731,7 @@ fn traverse_and_share(
             unit.dep_hash,
             unit.artifact,
             unit.artifact_target_for_features,
+            unit.skip_non_compile_time_dep,
         );
 
         // We can now turn the deferred value into its actual final value.
@@ -742,8 +762,11 @@ fn traverse_and_share(
         // Since `dep_hash` is now filled in, there's no need to specify the artifact target
         // for target-dependent feature resolution
         None,
+        skip_non_compile_time_deps,
     );
-    assert!(memo.insert(unit.clone(), new_unit.clone()).is_none());
+    if !unit_is_root || !compile_time_deps_only {
+        assert!(memo.insert(unit.clone(), new_unit.clone()).is_none());
+    }
     new_graph.entry(new_unit.clone()).or_insert(new_deps);
     new_unit
 }
@@ -904,6 +927,7 @@ fn override_rustc_crate_types(
             unit.dep_hash,
             unit.artifact,
             unit.artifact_target_for_features,
+            unit.skip_non_compile_time_dep,
         )
     };
     units[0] = match unit.target.kind() {

src/cargo/ops/cargo_compile/unit_generator.rs

@@ -167,6 +167,7 @@ impl<'a> UnitGenerator<'a, '_> {
                     /*dep_hash*/ 0,
                     IsArtifact::No,
                     None,
+                    false,
                 )
             })
             .collect()

src/cargo/util/command_prelude.rs

@@ -521,6 +521,10 @@ pub trait CommandExt: Sized {
             .hide(true),
         )
     }
+
+    fn arg_compile_time_deps(self) -> Self {
+        self._arg(flag("compile-time-deps", "").hide(true))
+    }
 }
 
 impl CommandExt for Command {
@@ -806,6 +810,7 @@ Run `{cmd}` to see possible targets."
         build_config.build_plan = self.flag("build-plan");
         build_config.unit_graph = self.flag("unit-graph");
         build_config.future_incompat_report = self.flag("future-incompat-report");
+        build_config.compile_time_deps_only = self.flag("compile-time-deps");
 
         if self._contains("timings") {
             for timing_output in self._values_of("timings") {
@@ -840,6 +845,10 @@ Run `{cmd}` to see possible targets."
             gctx.cli_unstable()
                 .fail_if_stable_opt("--unit-graph", 8002)?;
         }
+        if build_config.compile_time_deps_only {
+            gctx.cli_unstable()
+                .fail_if_stable_opt("--compile-time-deps", 14434)?;
+        }
 
         let opts = CompileOptions {
             build_config,

src/doc/src/reference/unstable.md

@@ -2190,3 +2190,17 @@ More information can be found in the [config chapter](config.md#cache).
 ## doctest-xcompile
 
 Doctest cross-compiling is now unconditionally enabled starting in Rust 1.89. Running doctests with `cargo test` will now honor the `--target` flag.
+
+## compile-time-deps
+
+This permanently-unstable flag to only build proc-macros and build scripts (and their required dependencies),
+as well as run the build scripts.
+
+It is intended for use by tools like rust-analyzer and will never be stabilized.
+
+Example:
+
+```console
+cargo +nightly build --compile-time-deps -Z unstable-options
+cargo +nightly check --compile-time-deps --all-targets -Z unstable-options
+```