Rollup merge of #74033 - ehuss:std-compile-all-platforms, r=Mark-Simulacrum

Add build support for Cargo's build-std feature.

This makes some changes to the standard library to make it easier to use with Cargo's build-std feature. The primary goal is to make it so that Cargo and its users do not need to know which crates to build and which features to use for every platform.

Conditional cfgs are adjusted so that there is usually a fall-through for unsupported platforms. Additionally, there is a "restricted-std" feature to mark `std` as unstable when used with build-std on no_std platforms. There is no intent to stabilize this feature for the foreseeable future.

This borrows some of the implementation for wasm which already does what this needs. More code sharing can be done with some other platforms (there is a lot of duplication with cloudabi, hermit, and sgx), but I figure that can be done in a future PR.

There are some small changes to stable behavior in this PR:
- `std::env::consts::ARCH` on asmjs now reports "wasm32", to match its actual architecture.
- Some of the wasm error messages for unsupported features report a slightly different error message so that the code can be reused.

There should otherwise not be any changes to how std is built for distribution via bootstrap.

This does not yet support all platforms when used with build-std.

- It doesn't work with 16-bit targets (hashbrown does not support that).
- It does not work with JSON spec targets.
    - In particular, all target triple snooping will need to be replaced with appropriate target option checking.
- Switching to gimli (#73441) will make cross-building *much* easier.
- There are still a ton of issues on the Cargo side to resolve. A big one is panic strategy support.

Future PRs are intended to address some of these issues.

2 files changed, 5 insertions, 7 deletions

diff --git a/src/libstd/sys_common/mod.rs b/src/libstd/sys_common/mod.rs
index e57bb267cbd..840f9093e00 100644
--- a/src/libstd/sys_common/mod.rs
+++ b/src/libstd/sys_common/mod.rs
@@ -51,13 +51,9 @@ pub mod condvar;
 pub mod fs;
 pub mod io;
 pub mod mutex;
-#[cfg(any(doc, // see `mod os`, docs are generated for multiple platforms
-          unix,
-          target_os = "redox",
-          target_os = "cloudabi",
-          target_os = "hermit",
-          target_arch = "wasm32",
-          all(target_vendor = "fortanix", target_env = "sgx")))]
+// `doc` is required because `sys/mod.rs` imports `unix/ext/mod.rs` on Windows
+// when generating documentation.
+#[cfg(any(doc, not(windows)))]
 pub mod os_str_bytes;
 pub mod poison;
 pub mod process;
@@ -74,6 +70,7 @@ cfg_if::cfg_if! {
     if #[cfg(any(target_os = "cloudabi",
                  target_os = "l4re",
                  target_os = "hermit",
+                 feature = "restricted-std",
                  all(target_arch = "wasm32", not(target_os = "emscripten")),
                  all(target_vendor = "fortanix", target_env = "sgx")))] {
         pub use crate::sys::net;
diff --git a/src/libstd/sys_common/mutex.rs b/src/libstd/sys_common/mutex.rs
index 899fc6a7235..e66d8994147 100644
--- a/src/libstd/sys_common/mutex.rs
+++ b/src/libstd/sys_common/mutex.rs
@@ -17,6 +17,7 @@ impl Mutex {
     /// Also, until `init` is called, behavior is undefined if this
     /// mutex is ever used reentrantly, i.e., `raw_lock` or `try_lock`
     /// are called by the thread currently holding the lock.
+    #[rustc_const_stable(feature = "const_sys_mutex_new", since = "1.0.0")]
     pub const fn new() -> Mutex {
         Mutex(imp::Mutex::new())
     }