Add a -Zbuild-std-features flag

src/cargo/core/compiler/standard_lib.rs

@@ -99,11 +99,18 @@ pub fn resolve_std<'cfg>(
     spec_pkgs.push("test".to_string());
     let spec = Packages::Packages(spec_pkgs);
     let specs = spec.to_package_id_specs(&std_ws)?;
-    let features = vec!["panic-unwind".to_string(), "backtrace".to_string()];
+    let features = match &config.cli_unstable().build_std_features {
+        Some(list) => list.clone(),
+        None => vec![
+            "panic-unwind".to_string(),
+            "backtrace".to_string(),
+            "default".to_string(),
+        ],
+    };
     // dev_deps setting shouldn't really matter here.
     let opts = ResolveOpts::new(
         /*dev_deps*/ false, &features, /*all_features*/ false,
-        /*uses_default_features*/ true,
+        /*uses_default_features*/ false,
     );
     let resolve = ops::resolve_ws_with_opts(
         &std_ws,

src/cargo/core/features.rs

@@ -350,6 +350,7 @@ pub struct CliUnstable {
     pub binary_dep_depinfo: bool,
     #[serde(deserialize_with = "deserialize_build_std")]
     pub build_std: Option<Vec<String>>,
+    pub build_std_features: Option<Vec<String>>,
     pub timings: Option<Vec<String>>,
     pub doctest_xcompile: bool,
     pub panic_abort_tests: bool,
@@ -455,6 +456,7 @@ impl CliUnstable {
             "build-std" => {
                 self.build_std = Some(crate::core::compiler::standard_lib::parse_unstable_flag(v))
             }
+            "build-std-features" => self.build_std_features = Some(parse_features(v)),
             "timings" => self.timings = Some(parse_timings(v)),
             "doctest-xcompile" => self.doctest_xcompile = parse_empty(k, v)?,
             "panic-abort-tests" => self.panic_abort_tests = parse_empty(k, v)?,

src/doc/src/reference/unstable.md

@@ -358,6 +358,15 @@ something doesn't quite work the way you'd like it to, feel free to check out
 the [issue tracker](https://github.com/rust-lang/wg-cargo-std-aware/issues) of
 the tracking repository, and if it's not there please file a new issue!
 
+### build-std-features
+* Tracking Repository: https://github.com/rust-lang/wg-cargo-std-aware
+
+This flag is a sibling to the `-Zbuild-std` feature flag. This will configure
+the features enabled for the standard library itself when building the standard
+library. The default enabled features, at this time, are `backtrace` and
+`panic_unwind`. This flag expects a comma-separated list and, if provided, will
+override the default list of features enabled.
+
 ### timings
 * Tracking Issue: [#7405](https://github.com/rust-lang/cargo/issues/7405)
 

tests/testsuite/mock-std/src/libstd/Cargo.toml

@@ -9,3 +9,6 @@ path = "lib.rs"
 
 [dependencies]
 registry-dep-using-alloc = { version = "*", features = ['mockbuild'] }
+
+[features]
+feature1 = []

tests/testsuite/mock-std/src/libstd/lib.rs

@@ -5,5 +5,8 @@
 pub use std::*;
 
 #[stable(since = "1.0.0", feature = "dummy")]
-pub fn custom_api() {
-}
+pub fn custom_api() {}
+
+#[cfg(feature = "feature1")]
+#[stable(since = "1.0.0", feature = "dummy")]
+pub fn conditional_function() {}

tests/testsuite/mock-std/src/libtest/Cargo.toml

@@ -9,10 +9,13 @@ path = "lib.rs"
 
 [dependencies]
 proc_macro = { path = "../libproc_macro" }
+std = { path = "../libstd" }
 panic_unwind = { path = "../libpanic_unwind" }
 compiler_builtins = { path = "../libcompiler_builtins" }
 registry-dep-using-std = { version = "*", features = ['mockbuild'] }
 
 [features]
 panic-unwind = []
 backtrace = []
+feature1 = ["std/feature1"]
+default = []

tests/testsuite/standard_lib.rs

@@ -632,3 +632,26 @@ fn cargo_config_injects_compiler_builtins() {
         .with_stderr_does_not_contain("[..]libstd[..]")
         .run();
 }
+
+#[cargo_test]
+fn different_features() {
+    let setup = match setup() {
+        Some(s) => s,
+        None => return,
+    };
+    let p = project()
+        .file(
+            "src/lib.rs",
+            "
+                pub fn foo() {
+                    std::conditional_function();
+                }
+            ",
+        )
+        .build();
+    p.cargo("build")
+        .build_std(&setup)
+        .arg("-Zbuild-std-features=feature1")
+        .target_host()
+        .run();
+}