Don't print unnecessary sysroot messages

cargo-miri/Cargo.toml

@@ -18,7 +18,7 @@ directories = "5"
 rustc_version = "0.4"
 serde_json = "1.0.40"
 cargo_metadata = "0.18.0"
-rustc-build-sysroot = "0.4.6"
+rustc-build-sysroot = { git = "https://github.com/saethlin/rustc-build-sysroot", branch = "quiet-when-no-change" }
 
 # Enable some feature flags that dev-dependencies need but dependencies
 # do not.  This makes `./miri install` after `./miri build` faster.

cargo-miri/src/setup.rs

@@ -6,7 +6,7 @@ use std::fmt::Write;
 use std::path::PathBuf;
 use std::process::{self, Command};
 
-use rustc_build_sysroot::{BuildMode, SysrootBuilder, SysrootConfig};
+use rustc_build_sysroot::{BuildMode, SysrootBuilder, SysrootConfig, SysrootStatus};
 use rustc_version::VersionMeta;
 
 use crate::util::*;
@@ -137,32 +137,48 @@ pub fn setup(
     // not apply `RUSTFLAGS` to the sysroot either.
     let rustflags = &["-Cdebug-assertions=off", "-Coverflow-checks=on"];
 
-    // Do the build.
-    if print_sysroot || quiet {
-        // Be silent.
-    } else {
-        let mut msg = String::new();
-        write!(msg, "Preparing a sysroot for Miri (target: {target})").unwrap();
-        if verbose > 0 {
-            write!(msg, " in {}", sysroot_dir.display()).unwrap();
-        }
-        write!(msg, "...").unwrap();
-        if only_setup {
+    let mut msg = String::new();
+    write!(msg, "Preparing a sysroot for Miri (target: {target})").unwrap();
+    if verbose > 0 {
+        write!(msg, " in {}", sysroot_dir.display()).unwrap();
+    }
+    write!(msg, "...").unwrap();
+    let notify = move || {
+        if print_sysroot || quiet {
+            // Be silent.
+        } else if only_setup {
             // We want to be explicit.
             eprintln!("{msg}");
         } else {
             // We want to be quiet, but still let the user know that something is happening.
             eprint!("{msg} ");
         }
-    }
-    SysrootBuilder::new(&sysroot_dir, target)
+    };
+
+    // Do the build.
+    let status = SysrootBuilder::new(&sysroot_dir, target)
         .build_mode(BuildMode::Check)
         .rustc_version(rustc_version.clone())
         .sysroot_config(sysroot_config)
         .rustflags(rustflags)
         .cargo(cargo_cmd)
-        .build_from_source(&rust_src)
-        .unwrap_or_else(|err| {
+        .when_build_required(notify)
+        .build_from_source(&rust_src);
+    match status {
+        Ok(SysrootStatus::AlreadyCached) =>
+            if only_setup {
+                eprintln!("A sysroot for Miri is already available in `{}`.", sysroot_dir.display());
+            },
+        Ok(SysrootStatus::SysrootBuilt) => {
+            if print_sysroot || quiet {
+                // Be silent.
+            } else if only_setup {
+                eprintln!("A sysroot for Miri is now available in `{}`.", sysroot_dir.display());
+            } else {
+                eprintln!("done");
+            }
+        }
+        Err(err) =>
             if print_sysroot {
                 show_error!("failed to build sysroot")
             } else if only_setup {
@@ -171,15 +187,9 @@ pub fn setup(
                 show_error!(
                     "failed to build sysroot; run `cargo miri setup` to see the error details"
                 )
-            }
-        });
-    if print_sysroot || quiet {
-        // Be silent.
-    } else if only_setup {
-        eprintln!("A sysroot for Miri is now available in `{}`.", sysroot_dir.display());
-    } else {
-        eprintln!("done");
+            },
     }
+
     if print_sysroot {
         // Print just the sysroot and nothing else to stdout; this way we do not need any escaping.
         println!("{}", sysroot_dir.display());