diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index a1d3453377ab0..05b66f94727ad 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -1207,7 +1207,7 @@ impl<'a> Builder<'a> {
             assert_eq!(target, compiler.host);
         }
 
-        if self.config.rust_optimize {
+        if self.config.rust_optimize.is_release() {
             // FIXME: cargo bench/install do not accept `--release`
             if cmd != "bench" && cmd != "install" {
                 cargo.arg("--release");
@@ -1263,7 +1263,7 @@ impl<'a> Builder<'a> {
         }
 
         let profile_var = |name: &str| {
-            let profile = if self.config.rust_optimize { "RELEASE" } else { "DEV" };
+            let profile = if self.config.rust_optimize.is_release() { "RELEASE" } else { "DEV" };
             format!("CARGO_PROFILE_{}_{}", profile, name)
         };
 
@@ -1652,6 +1652,9 @@ impl<'a> Builder<'a> {
             }
         };
         cargo.env(profile_var("DEBUG"), debuginfo_level.to_string());
+        if let Some(opt_level) = &self.config.rust_optimize.get_opt_level() {
+            cargo.env(profile_var("OPT_LEVEL"), opt_level);
+        }
         if !self.config.dry_run() && self.cc.borrow()[&target].args().iter().any(|arg| arg == "-gz")
         {
             rustflags.arg("-Clink-arg=-gz");
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index b91275e73e9c4..5f5f7ea25fb95 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -202,7 +202,7 @@ pub struct Config {
     pub llvm_use_libcxx: bool,
 
     // rust codegen options
-    pub rust_optimize: bool,
+    pub rust_optimize: RustOptimize,
     pub rust_codegen_units: Option<u32>,
     pub rust_codegen_units_std: Option<u32>,
     pub rust_debug_assertions: bool,
@@ -875,17 +875,55 @@ impl Default for StringOrBool {
     }
 }
 
+#[derive(Clone, Debug, Deserialize, PartialEq, Eq)]
+#[serde(untagged)]
+pub enum RustOptimize {
+    #[serde(deserialize_with = "deserialize_and_validate_opt_level")]
+    String(String),
+    Bool(bool),
+}
+
+impl Default for RustOptimize {
+    fn default() -> RustOptimize {
+        RustOptimize::Bool(false)
+    }
+}
+
+fn deserialize_and_validate_opt_level<'de, D>(d: D) -> Result<String, D::Error>
+where
+    D: serde::de::Deserializer<'de>,
+{
+    let v = String::deserialize(d)?;
+    if ["0", "1", "2", "3", "s", "z"].iter().find(|x| **x == v).is_some() {
+        Ok(v)
+    } else {
+        Err(format!(r#"unrecognized option for rust optimize: "{}", expected one of "0", "1", "2", "3", "s", "z""#, v)).map_err(serde::de::Error::custom)
+    }
+}
+
+impl RustOptimize {
+    pub(crate) fn is_release(&self) -> bool {
+        if let RustOptimize::Bool(true) | RustOptimize::String(_) = &self { true } else { false }
+    }
+
+    pub(crate) fn get_opt_level(&self) -> Option<String> {
+        match &self {
+            RustOptimize::String(s) => Some(s.clone()),
+            RustOptimize::Bool(_) => None,
+        }
+    }
+}
+
 #[derive(Deserialize)]
 #[serde(untagged)]
 enum StringOrInt<'a> {
     String(&'a str),
     Int(i64),
 }
-
 define_config! {
     /// TOML representation of how the Rust build is configured.
     struct Rust {
-        optimize: Option<bool> = "optimize",
+        optimize: Option<RustOptimize> = "optimize",
         debug: Option<bool> = "debug",
         codegen_units: Option<u32> = "codegen-units",
         codegen_units_std: Option<u32> = "codegen-units-std",
@@ -971,7 +1009,7 @@ impl Config {
         config.ninja_in_file = true;
         config.llvm_static_stdcpp = false;
         config.backtrace = true;
-        config.rust_optimize = true;
+        config.rust_optimize = RustOptimize::Bool(true);
         config.rust_optimize_tests = true;
         config.submodules = None;
         config.docs = true;
@@ -1546,7 +1584,7 @@ impl Config {
         config.llvm_assertions = llvm_assertions.unwrap_or(false);
         config.llvm_tests = llvm_tests.unwrap_or(false);
         config.llvm_plugins = llvm_plugins.unwrap_or(false);
-        config.rust_optimize = optimize.unwrap_or(true);
+        config.rust_optimize = optimize.unwrap_or(RustOptimize::Bool(true));
 
         let default = debug == Some(true);
         config.rust_debug_assertions = debug_assertions.unwrap_or(default);
diff --git a/src/bootstrap/config/tests.rs b/src/bootstrap/config/tests.rs
index 3bee659abd1c5..732df54cdacbd 100644
--- a/src/bootstrap/config/tests.rs
+++ b/src/bootstrap/config/tests.rs
@@ -178,3 +178,18 @@ fn profile_user_dist() {
     }
     Config::parse_inner(&["check".to_owned()], get_toml);
 }
+
+#[test]
+fn rust_optimize() {
+    assert_eq!(parse("").rust_optimize.is_release(), true);
+    assert_eq!(parse("rust.optimize = false").rust_optimize.is_release(), false);
+    assert_eq!(parse("rust.optimize = true").rust_optimize.is_release(), true);
+    assert_eq!(parse("rust.optimize = \"1\"").rust_optimize.get_opt_level(), Some("1".to_string()));
+    assert_eq!(parse("rust.optimize = \"s\"").rust_optimize.get_opt_level(), Some("s".to_string()));
+}
+
+#[test]
+#[should_panic]
+fn invalid_rust_optimize() {
+    parse("rust.optimize = \"a\"");
+}
diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index c960053d7a0f2..6a51450a777e8 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -785,7 +785,7 @@ impl Build {
     /// Component directory that Cargo will produce output into (e.g.
     /// release/debug)
     fn cargo_dir(&self) -> &'static str {
-        if self.config.rust_optimize { "release" } else { "debug" }
+        if self.config.rust_optimize.is_release() { "release" } else { "debug" }
     }
 
     fn tools_dir(&self, compiler: Compiler) -> PathBuf {
diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index 2c1f612e39f52..a2938ab4caf37 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -801,7 +801,7 @@ impl Step for Clippy {
         cargo.arg("-p").arg("clippy_dev");
         // clippy_dev gets confused if it can't find `clippy/Cargo.toml`
         cargo.current_dir(&builder.src.join("src").join("tools").join("clippy"));
-        if builder.config.rust_optimize {
+        if builder.config.rust_optimize.is_release() {
             cargo.env("PROFILE", "release");
         } else {
             cargo.env("PROFILE", "debug");