Add `StepMetadata` to describe steps

This is used to replace the previous downcasting of executed steps, which wasn't very scalable. In addition to tests, we could also use the metadata e.g. for tracing.

5 files changed, 83 insertions, 45 deletions

diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index 560925abba6..3b4c7ec2b5f 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -24,7 +24,8 @@ use crate::core::build_steps::tool::SourceType;
 use crate::core::build_steps::{dist, llvm};
 use crate::core::builder;
 use crate::core::builder::{
-    Builder, Cargo, Kind, PathSet, RunConfig, ShouldRun, Step, TaskPath, crate_description,
+    Builder, Cargo, Kind, PathSet, RunConfig, ShouldRun, Step, StepMetadata, TaskPath,
+    crate_description,
 };
 use crate::core::config::{DebuginfoLevel, LlvmLibunwind, RustcLto, TargetSelection};
 use crate::utils::build_stamp;
@@ -305,6 +306,14 @@ impl Step for Std {
             builder.compiler(compiler.stage, builder.config.host_target),
         ));
     }
+
+    fn metadata(&self) -> Option<StepMetadata> {
+        Some(
+            StepMetadata::build("std", self.target)
+                .built_by(self.compiler)
+                .stage(self.compiler.stage),
+        )
+    }
 }
 
 fn copy_and_stamp(
@@ -1171,6 +1180,14 @@ impl Step for Rustc {
 
         build_compiler.stage
     }
+
+    fn metadata(&self) -> Option<StepMetadata> {
+        Some(
+            StepMetadata::build("rustc", self.target)
+                .built_by(self.build_compiler)
+                .stage(self.build_compiler.stage + 1),
+        )
+    }
 }
 
 pub fn rustc_cargo(
diff --git a/src/bootstrap/src/core/build_steps/llvm.rs b/src/bootstrap/src/core/build_steps/llvm.rs
index 8f2f143c352..de67a5f77e6 100644
--- a/src/bootstrap/src/core/build_steps/llvm.rs
+++ b/src/bootstrap/src/core/build_steps/llvm.rs
@@ -18,7 +18,7 @@ use build_helper::git::PathFreshness;
 #[cfg(feature = "tracing")]
 use tracing::instrument;
 
-use crate::core::builder::{Builder, RunConfig, ShouldRun, Step};
+use crate::core::builder::{Builder, RunConfig, ShouldRun, Step, StepMetadata};
 use crate::core::config::{Config, TargetSelection};
 use crate::utils::build_stamp::{BuildStamp, generate_smart_stamp_hash};
 use crate::utils::exec::command;
@@ -582,6 +582,10 @@ impl Step for Llvm {
 
         res
     }
+
+    fn metadata(&self) -> Option<StepMetadata> {
+        Some(StepMetadata::build("llvm", self.target))
+    }
 }
 
 pub fn get_llvm_version(builder: &Builder<'_>, llvm_config: &Path) -> String {
diff --git a/src/bootstrap/src/core/builder/mod.rs b/src/bootstrap/src/core/builder/mod.rs
index 7433f0b0f3b..b26f47a3171 100644
--- a/src/bootstrap/src/core/builder/mod.rs
+++ b/src/bootstrap/src/core/builder/mod.rs
@@ -130,6 +130,38 @@ pub trait Step: 'static + Clone + Debug + PartialEq + Eq + Hash {
         // as such calling them from ./x.py isn't logical.
         unimplemented!()
     }
+
+    /// Returns metadata of the step, for tests
+    fn metadata(&self) -> Option<StepMetadata> {
+        None
+    }
+}
+
+/// Metadata that describes an executed step, mostly for testing and tracing.
+#[allow(unused)]
+#[derive(Debug)]
+pub struct StepMetadata {
+    name: &'static str,
+    kind: Kind,
+    target: TargetSelection,
+    built_by: Option<Compiler>,
+    stage: Option<u32>,
+}
+
+impl StepMetadata {
+    pub fn build(name: &'static str, target: TargetSelection) -> Self {
+        Self { name, kind: Kind::Build, target, built_by: None, stage: None }
+    }
+
+    pub fn built_by(mut self, compiler: Compiler) -> Self {
+        self.built_by = Some(compiler);
+        self
+    }
+
+    pub fn stage(mut self, stage: u32) -> Self {
+        self.stage = Some(stage);
+        self
+    }
 }
 
 pub struct RunConfig<'a> {
diff --git a/src/bootstrap/src/core/builder/tests.rs b/src/bootstrap/src/core/builder/tests.rs
index d07df7f4a84..6268a2b59d6 100644
--- a/src/bootstrap/src/core/builder/tests.rs
+++ b/src/bootstrap/src/core/builder/tests.rs
@@ -8,6 +8,7 @@ use super::*;
 use crate::Flags;
 use crate::core::build_steps::doc::DocumentationFormat;
 use crate::core::config::Config;
+use crate::utils::cache::ExecutedStep;
 use crate::utils::tests::git::{GitCtx, git_test};
 
 static TEST_TRIPLE_1: &str = "i686-unknown-haiku";
@@ -1258,31 +1259,24 @@ mod staging {
 /// Renders the executed bootstrap steps for usage in snapshot tests with insta.
 /// Only renders certain important steps.
 /// Each value in `steps` should be a tuple of (Step, step output).
-fn render_steps(steps: &[(Box<dyn Any>, Box<dyn Any>)]) -> String {
+fn render_steps(steps: &[ExecutedStep]) -> String {
     steps
         .iter()
-        .filter_map(|(step, output)| {
-            // FIXME: implement an optional method on Step to produce metadata for test, instead
-            // of this downcasting
-            if let Some((rustc, output)) = downcast_step::<compile::Rustc>(step, output) {
-                Some(format!(
-                    "[build] {} -> {}",
-                    render_compiler(rustc.build_compiler),
-                    // FIXME: return the correct stage from the `Rustc` step, now it behaves weirdly
-                    render_compiler(Compiler::new(rustc.build_compiler.stage + 1, rustc.target)),
-                ))
-            } else if let Some((std, output)) = downcast_step::<compile::Std>(step, output) {
-                Some(format!(
-                    "[build] {} -> std {} <{}>",
-                    render_compiler(std.compiler),
-                    std.compiler.stage,
-                    std.target
-                ))
-            } else if let Some((llvm, output)) = downcast_step::<llvm::Llvm>(step, output) {
-                Some(format!("[build] llvm <{}>", llvm.target))
-            } else {
-                None
+        .filter_map(|step| {
+            use std::fmt::Write;
+
+            let Some(metadata) = &step.metadata else {
+                return None;
+            };
+
+            let mut record = format!("[{}] ", metadata.kind.as_str());
+            if let Some(compiler) = metadata.built_by {
+                write!(record, "{} -> ", render_compiler(compiler));
             }
+            let stage =
+                if let Some(stage) = metadata.stage { format!("{stage} ") } else { "".to_string() };
+            write!(record, "{} {stage}<{}>", metadata.name, metadata.target);
+            Some(record)
         })
         .map(|line| {
             line.replace(TEST_TRIPLE_1, "target1")
@@ -1293,19 +1287,6 @@ fn render_steps(steps: &[(Box<dyn Any>, Box<dyn Any>)]) -> String {
         .join("\n")
 }
 
-fn downcast_step<'a, S: Step>(
-    step: &'a Box<dyn Any>,
-    output: &'a Box<dyn Any>,
-) -> Option<(&'a S, &'a S::Output)> {
-    let Some(step) = step.downcast_ref::<S>() else {
-        return None;
-    };
-    let Some(output) = output.downcast_ref::<S::Output>() else {
-        return None;
-    };
-    Some((step, output))
-}
-
 fn render_compiler(compiler: Compiler) -> String {
     format!("rustc {} <{}>", compiler.stage, compiler.host)
 }
diff --git a/src/bootstrap/src/utils/cache.rs b/src/bootstrap/src/utils/cache.rs
index 0c737470958..5098e2f03c4 100644
--- a/src/bootstrap/src/utils/cache.rs
+++ b/src/bootstrap/src/utils/cache.rs
@@ -218,10 +218,15 @@ pub struct Cache {
         >,
     >,
     #[cfg(test)]
-    /// Contains steps in the same order in which they were executed
-    /// Useful for tests
-    /// Tuples (step, step output)
-    executed_steps: RefCell<Vec<(Box<dyn Any>, Box<dyn Any>)>>,
+    /// Contains step metadata of executed steps (in the same order in which they were executed).
+    /// Useful for tests.
+    executed_steps: RefCell<Vec<ExecutedStep>>,
+}
+
+#[cfg(test)]
+#[derive(Debug)]
+pub struct ExecutedStep {
+    pub metadata: Option<crate::core::builder::StepMetadata>,
 }
 
 impl Cache {
@@ -243,9 +248,8 @@ impl Cache {
 
         #[cfg(test)]
         {
-            let step: Box<dyn Any> = Box::new(step.clone());
-            let output: Box<dyn Any> = Box::new(value.clone());
-            self.executed_steps.borrow_mut().push((step, output));
+            let metadata = step.metadata();
+            self.executed_steps.borrow_mut().push(ExecutedStep { metadata });
         }
 
         stepcache.insert(step, value);
@@ -283,7 +287,7 @@ impl Cache {
     }
 
     #[cfg(test)]
-    pub fn into_executed_steps(mut self) -> Vec<(Box<dyn Any>, Box<dyn Any>)> {
+    pub fn into_executed_steps(mut self) -> Vec<ExecutedStep> {
         mem::take(&mut self.executed_steps.borrow_mut())
     }
 }