diff --git a/config.toml.example b/config.toml.example
index c311c933e7edc..2d5b3136450b9 100644
--- a/config.toml.example
+++ b/config.toml.example
@@ -120,19 +120,18 @@
 # Defaults to host platform
 #build = "x86_64-unknown-linux-gnu"
 
-# In addition to the build triple, other triples to produce full compiler
-# toolchains for. Each of these triples will be bootstrapped from the build
-# triple and then will continue to bootstrap themselves. This platform must
-# currently be able to run all of the triples provided here.
+# Which triples to produce a compiler toolchain for. Each of these triples will
+# be bootstrapped from the build triple themselves.
 #
 # Defaults to just the build triple
 #host = ["x86_64-unknown-linux-gnu"]
 
-# In addition to all host triples, other triples to produce the standard library
-# for. Each host triple will be used to produce a copy of the standard library
-# for each target triple.
+# Which triples to build libraries (core/alloc/std/test/proc_macro) for. Each of
+# these triples will be bootstrapped from the build triple themselves.
 #
-# Defaults to just the build triple
+# Defaults to `host`. If you set this explicitly, you likely want to add all
+# host triples to this list as well in order for those host toolchains to be
+# able to compile programs for their native target.
 #target = ["x86_64-unknown-linux-gnu"]
 
 # Use this directory to store build artifacts.
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index ad2f48778674f..5a79d3db5c905 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -273,10 +273,8 @@ struct TomlConfig {
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 struct Build {
     build: Option<String>,
-    #[serde(default)]
-    host: Vec<String>,
-    #[serde(default)]
-    target: Vec<String>,
+    host: Option<Vec<String>>,
+    target: Option<Vec<String>>,
     // This is ignored, the rust code always gets the build directory from the `BUILD_DIR` env variable
     build_dir: Option<String>,
     cargo: Option<String>,
@@ -505,11 +503,6 @@ impl Config {
             config.out = dir;
         }
 
-        // If --target was specified but --host wasn't specified, don't run any host-only tests.
-        let has_hosts = !flags.host.is_empty();
-        let has_targets = !flags.target.is_empty();
-        config.skip_only_host_steps = !has_hosts && has_targets;
-
         let toml = file
             .map(|file| {
                 let contents = t!(fs::read_to_string(&file));
@@ -528,25 +521,28 @@ impl Config {
             .unwrap_or_else(TomlConfig::default);
 
         let build = toml.build.clone().unwrap_or_default();
-        // set by bootstrap.py
-        config.hosts.push(config.build);
-        for host in build.host.iter().map(|h| TargetSelection::from_user(h)) {
-            if !config.hosts.contains(&host) {
-                config.hosts.push(host);
-            }
-        }
-        for target in config
-            .hosts
-            .iter()
-            .copied()
-            .chain(build.target.iter().map(|h| TargetSelection::from_user(h)))
-        {
-            if !config.targets.contains(&target) {
-                config.targets.push(target);
-            }
-        }
-        config.hosts = if !flags.host.is_empty() { flags.host } else { config.hosts };
-        config.targets = if !flags.target.is_empty() { flags.target } else { config.targets };
+
+        // If --target was specified but --host wasn't specified, don't run any host-only tests.
+        let has_hosts = build.host.is_some() || flags.host.is_some();
+        let has_targets = build.target.is_some() || flags.target.is_some();
+        config.skip_only_host_steps = !has_hosts && has_targets;
+
+        config.hosts = if let Some(arg_host) = flags.host.clone() {
+            arg_host
+        } else if let Some(file_host) = build.host {
+            file_host.iter().map(|h| TargetSelection::from_user(h)).collect()
+        } else {
+            vec![config.build]
+        };
+        config.targets = if let Some(arg_target) = flags.target.clone() {
+            arg_target
+        } else if let Some(file_target) = build.target {
+            file_target.iter().map(|h| TargetSelection::from_user(h)).collect()
+        } else {
+            // If target is *not* configured, then default to the host
+            // toolchains.
+            config.hosts.clone()
+        };
 
         config.nodejs = build.nodejs.map(PathBuf::from);
         config.gdb = build.gdb.map(PathBuf::from);
diff --git a/src/bootstrap/flags.rs b/src/bootstrap/flags.rs
index 2db4bb07a9ff1..ff8468574469e 100644
--- a/src/bootstrap/flags.rs
+++ b/src/bootstrap/flags.rs
@@ -20,8 +20,8 @@ pub struct Flags {
     pub stage: Option<u32>,
     pub keep_stage: Vec<u32>,
 
-    pub host: Vec<TargetSelection>,
-    pub target: Vec<TargetSelection>,
+    pub host: Option<Vec<TargetSelection>>,
+    pub target: Option<Vec<TargetSelection>>,
     pub config: Option<PathBuf>,
     pub jobs: Option<u32>,
     pub cmd: Subcommand,
@@ -526,14 +526,26 @@ Arguments:
                 .into_iter()
                 .map(|j| j.parse().expect("`keep-stage` should be a number"))
                 .collect(),
-            host: split(&matches.opt_strs("host"))
-                .into_iter()
-                .map(|x| TargetSelection::from_user(&x))
-                .collect::<Vec<_>>(),
-            target: split(&matches.opt_strs("target"))
-                .into_iter()
-                .map(|x| TargetSelection::from_user(&x))
-                .collect::<Vec<_>>(),
+            host: if matches.opt_present("host") {
+                Some(
+                    split(&matches.opt_strs("host"))
+                        .into_iter()
+                        .map(|x| TargetSelection::from_user(&x))
+                        .collect::<Vec<_>>(),
+                )
+            } else {
+                None
+            },
+            target: if matches.opt_present("target") {
+                Some(
+                    split(&matches.opt_strs("target"))
+                        .into_iter()
+                        .map(|x| TargetSelection::from_user(&x))
+                        .collect::<Vec<_>>(),
+                )
+            } else {
+                None
+            },
             config: cfg_file,
             jobs: matches.opt_str("jobs").map(|j| j.parse().expect("`jobs` should be a number")),
             cmd,