Add support so that Miri can be quiet when the sysroot is unchanged

src/lib.rs

@@ -153,6 +153,12 @@ pub struct SysrootBuilder {
     rustflags: Vec<OsString>,
     cargo: Option<Command>,
     rustc_version: Option<rustc_version::VersionMeta>,
+    when_build_required: Option<Box<dyn FnOnce()>>,
+}
+
+pub enum SysrootStatus {
+    AlreadyCached,
+    SysrootBuilt,
 }
 
 /// Hash file name (in target/lib directory).
@@ -185,6 +191,7 @@ impl SysrootBuilder {
             rustflags: default_flags.iter().map(Into::into).collect(),
             cargo: None,
             rustc_version: None,
+            when_build_required: None,
         }
     }
 
@@ -232,6 +239,11 @@ impl SysrootBuilder {
         self
     }
 
+    pub fn when_build_required(mut self, when_build_required: impl FnOnce() + 'static) -> Self {
+        self.when_build_required = Some(Box::new(when_build_required));
+        self
+    }
+
     /// Our configured target can be either a built-in target name, or a path to a target file.
     /// We use the same logic as rustc to tell which is which:
     /// https://github.com/rust-lang/rust/blob/8d39ec1825024f3014e1f847942ac5bbfcf055b0/compiler/rustc_session/src/config.rs#L2252-L2263
@@ -379,7 +391,7 @@ panic = 'unwind'
     /// Build the `self` sysroot from the given sources.
     ///
     /// `src_dir` must be the `library` source folder, i.e., the one that contains `std/Cargo.toml`.
-    pub fn build_from_source(mut self, src_dir: &Path) -> Result<()> {
+    pub fn build_from_source(mut self, src_dir: &Path) -> Result<SysrootStatus> {
         // A bit of preparation.
         if !src_dir.join("std").join("Cargo.toml").exists() {
             bail!(
@@ -401,7 +413,12 @@ panic = 'unwind'
         let cur_hash = self.sysroot_compute_hash(src_dir, &rustc_version)?;
         if self.sysroot_read_hash() == Some(cur_hash) {
             // Already done!
-            return Ok(());
+            return Ok(SysrootStatus::AlreadyCached);
+        }
+
+        // A build is required, so we run the when-build-required function if one was set.
+        if let Some(when_build_required) = self.when_build_required.take() {
+            when_build_required();
         }
 
         // Prepare a workspace for cargo
@@ -493,6 +510,6 @@ panic = 'unwind'
         .context("failed to create target directory")?;
         fs::rename(staging_dir.path(), sysroot_lib_dir).context("failed installing sysroot")?;
 
-        Ok(())
+        return Ok(SysrootStatus::SysrootBuilt);
     }
 }