bootstrap: Allow for build libstd to have its own codegen-unit setting.

config.toml.example

@@ -277,6 +277,10 @@
 # compiler.
 #codegen-units = 1
 
+# Sets the number of codegen units to build the standard library with,
+# regardless of what the codegen-unit setting for the rest of the compiler is.
+#codegen-units-std = 1
+
 # Whether or not debug assertions are enabled for the compiler and standard
 # library. Also enables compilation of debug! and trace! logging macros.
 #debug-assertions = false

src/bootstrap/builder.rs

@@ -1119,10 +1119,15 @@ impl<'a> Builder<'a> {
             cargo.arg("-v");
         }
 
-        // This must be kept before the thinlto check, as we set codegen units
-        // to 1 forcibly there.
-        if let Some(n) = self.config.rust_codegen_units {
-            cargo.env("RUSTC_CODEGEN_UNITS", n.to_string());
+        match (mode, self.config.rust_codegen_units_std, self.config.rust_codegen_units) {
+            (Mode::Std, Some(n), _) |
+            (Mode::Test, Some(n), _) |
+            (_, _, Some(n)) => {
+                cargo.env("RUSTC_CODEGEN_UNITS", n.to_string());
+            }
+            _ => {
+                // Don't set anything
+            }
         }
 
         if self.config.rust_optimize {

src/bootstrap/config.rs

@@ -95,6 +95,7 @@ pub struct Config {
     // rust codegen options
     pub rust_optimize: bool,
     pub rust_codegen_units: Option<u32>,
+    pub rust_codegen_units_std: Option<u32>,
     pub rust_debug_assertions: bool,
     pub rust_debuginfo: bool,
     pub rust_debuginfo_lines: bool,
@@ -294,6 +295,7 @@ impl Default for StringOrBool {
 struct Rust {
     optimize: Option<bool>,
     codegen_units: Option<u32>,
+    codegen_units_std: Option<u32>,
     debug_assertions: Option<bool>,
     debuginfo: Option<bool>,
     debuginfo_lines: Option<bool>,
@@ -580,6 +582,8 @@ impl Config {
                 Some(n) => config.rust_codegen_units = Some(n),
                 None => {}
             }
+
+            config.rust_codegen_units_std = rust.codegen_units_std;
         }
 
         if let Some(ref t) = toml.target {