diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index be28975c3ea..92a57630ca5 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -170,6 +170,11 @@ pub struct Build {
     fail_fast: bool,
     verbosity: usize,
 
+    // Targets for which to build.
+    build: String,
+    hosts: Vec<String>,
+    targets: Vec<String>,
+
     // Stage 0 (downloaded) compiler and cargo or their local rust equivalents.
     initial_rustc: PathBuf,
     initial_cargo: PathBuf,
@@ -243,6 +248,27 @@ impl Build {
         let cargo_info = channel::GitInfo::new(&src.join("src/tools/cargo"));
         let rls_info = channel::GitInfo::new(&src.join("src/tools/rls"));
 
+        let hosts = if !flags.host.is_empty() {
+            for host in flags.host.iter() {
+                if !config.host.contains(host) {
+                    panic!("specified host `{}` is not in configuration", host);
+                }
+            }
+            flags.host.clone()
+        } else {
+            config.host.clone()
+        };
+        let targets = if !flags.target.is_empty() {
+            for target in flags.target.iter() {
+                if !config.target.contains(target) {
+                    panic!("specified target `{}` is not in configuration", target);
+                }
+            }
+            flags.target.clone()
+        } else {
+            config.target.clone()
+        };
+
         Build {
             initial_rustc: config.initial_rustc.clone(),
             initial_cargo: config.initial_cargo.clone(),
@@ -250,6 +276,10 @@ impl Build {
             fail_fast: flags.cmd.fail_fast(),
             verbosity: cmp::max(flags.verbose, config.verbose),
 
+            build: config.host[0].clone(),
+            hosts: hosts,
+            targets: targets,
+
             flags: flags,
             config: config,
             src: src,
@@ -269,6 +299,12 @@ impl Build {
         }
     }
 
+    fn build_slice(&self) -> &[String] {
+        unsafe {
+            std::slice::from_raw_parts(&self.build, 1)
+        }
+    }
+
     /// Executes the entire build, as configured by the flags and configuration.
     pub fn build(&mut self) {
         unsafe {
@@ -798,7 +834,7 @@ impl Build {
     /// Returns the number of parallel jobs that have been configured for this
     /// build.
     fn jobs(&self) -> u32 {
-        self.flags.jobs.unwrap_or(num_cpus::get() as u32)
+        self.flags.jobs.unwrap_or_else(|| num_cpus::get() as u32)
     }
 
     /// Returns the path to the C compiler for the target specified.
diff --git a/src/bootstrap/sanity.rs b/src/bootstrap/sanity.rs
index 433ac3dedfd..e2c955c4f4f 100644
--- a/src/bootstrap/sanity.rs
+++ b/src/bootstrap/sanity.rs
@@ -194,17 +194,6 @@ $ pacman -R cmake && pacman -S mingw-w64-x86_64-cmake
         }
     }
 
-    for host in build.flags.host.iter() {
-        if !build.config.host.contains(host) {
-            panic!("specified host `{}` is not in configuration", host);
-        }
-    }
-    for target in build.flags.target.iter() {
-        if !build.config.target.contains(target) {
-            panic!("specified target `{}` is not in configuration", target);
-        }
-    }
-
     let run = |cmd: &mut Command| {
         cmd.output().map(|output| {
             String::from_utf8_lossy(&output.stdout)
diff --git a/src/bootstrap/step.rs b/src/bootstrap/step.rs
index 61e9a0d16da..bcfa004ac3c 100644
--- a/src/bootstrap/step.rs
+++ b/src/bootstrap/step.rs
@@ -1218,16 +1218,9 @@ invalid rule dependency graph detected, was a rule added and maybe typo'd?
 
         rules.into_iter().flat_map(|(rule, _)| {
             let hosts = if rule.only_host_build || rule.only_build {
-                &self.build.config.host[..1]
-            } else if self.build.flags.host.len() > 0 {
-                &self.build.flags.host
+                self.build.build_slice()
             } else {
-                &self.build.config.host
-            };
-            let targets = if self.build.flags.target.len() > 0 {
-                &self.build.flags.target
-            } else {
-                &self.build.config.target
+                &self.build.hosts
             };
             // Determine the actual targets participating in this rule.
             // NOTE: We should keep the full projection from build triple to
@@ -1236,19 +1229,18 @@ invalid rule dependency graph detected, was a rule added and maybe typo'd?
             // the original non-shadowed hosts array is used below.
             let arr = if rule.host {
                 // If --target was specified but --host wasn't specified,
-                // don't run any host-only tests. Also, respect any `--host`
-                // overrides as done for `hosts`.
+                // don't run any host-only tests.
                 if self.build.flags.host.len() > 0 {
-                    &self.build.flags.host[..]
+                    &self.build.hosts
                 } else if self.build.flags.target.len() > 0 {
                     &[]
                 } else if rule.only_build {
-                    &self.build.config.host[..1]
+                    self.build.build_slice()
                 } else {
-                    &self.build.config.host[..]
+                    &self.build.hosts
                 }
             } else {
-                targets
+                &self.build.targets
             };
 
             hosts.iter().flat_map(move |host| {