Rollup merge of rust-lang#87282 - pietroalbini:refactor-extended, r=Mark-Simulacrum

Ensure `./x.py dist` adheres to `build.tools`

According to `config.toml.example`, the way to produce dist artifacts for both the compiler and a *subset* of tools would be to enable the extended build and manually specify the list of tools to build:

```toml
[build]
extended = true
tools = ["cargo", "rustfmt"]
```

This works as expected for `./x.py build` and `./x.py install`, but *not* for `./x.py dist`. Before this PR `./x.py dist` simply ignored the contents of `build.tools`, building just rustc/rustdoc if `build.extended = false` and all of the tools otherwise. This PR does two things:

* Changes `./x.py dist extended` to only build the tools defined in `build.tools`, if `build.tools` is not empty. The rest of the extended step was refactored to simplify the code.
* Changes how dist jobs for tools are gated: instead of `assert!(builder.config.extended)` to prevent tools from being built with `build.extended = false`, tools are simply built by default depending on `build.extended` and `build.tools`. This also enables to **explicitly** dist tools even with `build.extended = false`.

This PR is best reviewed commit-by-commit.

Fixes rust-lang#86436

Author: JohnTitor

src/bootstrap/builder.rs

@@ -163,14 +163,8 @@ impl StepDescription {
     }
 
     fn maybe_run(&self, builder: &Builder<'_>, pathset: &PathSet) {
-        if builder.config.exclude.iter().any(|e| pathset.has(e)) {
-            eprintln!("Skipping {:?} because it is excluded", pathset);
+        if self.is_excluded(builder, pathset) {
             return;
-        } else if !builder.config.exclude.is_empty() {
-            eprintln!(
-                "{:?} not skipped for {:?} -- not in {:?}",
-                pathset, self.name, builder.config.exclude
-            );
         }
 
         // Determine the targets participating in this rule.
@@ -182,6 +176,21 @@ impl StepDescription {
         }
     }
 
+    fn is_excluded(&self, builder: &Builder<'_>, pathset: &PathSet) -> bool {
+        if builder.config.exclude.iter().any(|e| pathset.has(e)) {
+            eprintln!("Skipping {:?} because it is excluded", pathset);
+            return true;
+        }
+
+        if !builder.config.exclude.is_empty() {
+            eprintln!(
+                "{:?} not skipped for {:?} -- not in {:?}",
+                pathset, self.name, builder.config.exclude
+            );
+        }
+        false
+    }
+
     fn run(v: &[StepDescription], builder: &Builder<'_>, paths: &[PathBuf]) {
         let should_runs =
             v.iter().map(|desc| (desc.should_run)(ShouldRun::new(builder))).collect::<Vec<_>>();
@@ -1579,6 +1588,27 @@ impl<'a> Builder<'a> {
         self.cache.put(step, out.clone());
         out
     }
+
+    /// Ensure that a given step is built *only if it's supposed to be built by default*, returning
+    /// its output. This will cache the step, so it's safe (and good!) to call this as often as
+    /// needed to ensure that all dependencies are build.
+    pub(crate) fn ensure_if_default<T, S: Step<Output = Option<T>>>(
+        &'a self,
+        step: S,
+    ) -> S::Output {
+        let desc = StepDescription::from::<S>();
+        let should_run = (desc.should_run)(ShouldRun::new(self));
+
+        // Avoid running steps contained in --exclude
+        for pathset in &should_run.paths {
+            if desc.is_excluded(self, pathset) {
+                return None;
+            }
+        }
+
+        // Only execute if it's supposed to run as default
+        if desc.default && should_run.is_really_default() { self.ensure(step) } else { None }
+    }
 }
 
 #[cfg(test)]