std: Improve downstream codegen in `Command::env`

This commit rejiggers the generics used in the implementation of
`Command::env` with the purpose of reducing the amount of codegen that
needs to happen in consumer crates, instead preferring to generate code
into libstd.

This was found when profiling the compile times of the `cc` crate where
the binary rlib produced had a lot of `BTreeMap` code compiled into it
but the crate doesn't actually use `BTreeMap`. It turns out that
`Command::env` is generic enough to codegen the entire implementation in
calling crates, but in this case there's no performance concern so it's
fine to compile the code into the standard library.

This change is done by removing the generic on the `CommandEnv` map
which is intended to handle case-insensitive variables on Windows.
Instead now a generic isn't used but rather a `use` statement defined
per-platform is used.

With this commit a debug build of `Command::new("foo").env("a", "b")`
drops from 21k lines of LLVM IR to 10k.

9 files changed, 56 insertions, 70 deletions

diff --git a/src/libstd/sys/cloudabi/shims/process.rs b/src/libstd/sys/cloudabi/shims/process.rs
index e719b362cbf..03a59d6d7c8 100644
--- a/src/libstd/sys/cloudabi/shims/process.rs
+++ b/src/libstd/sys/cloudabi/shims/process.rs
@@ -4,14 +4,16 @@ use crate::io;
 use crate::sys::fs::File;
 use crate::sys::pipe::AnonPipe;
 use crate::sys::{unsupported, Void};
-use crate::sys_common::process::{CommandEnv, DefaultEnvKey};
+use crate::sys_common::process::CommandEnv;
+
+pub use crate::ffi::OsString as EnvKey;
 
 ////////////////////////////////////////////////////////////////////////////////
 // Command
 ////////////////////////////////////////////////////////////////////////////////
 
 pub struct Command {
-    env: CommandEnv<DefaultEnvKey>,
+    env: CommandEnv,
 }
 
 // passed back to std::process with the pipes connected to the child, if any
@@ -37,7 +39,7 @@ impl Command {
 
     pub fn arg(&mut self, _arg: &OsStr) {}
 
-    pub fn env_mut(&mut self) -> &mut CommandEnv<DefaultEnvKey> {
+    pub fn env_mut(&mut self) -> &mut CommandEnv {
         &mut self.env
     }
 
diff --git a/src/libstd/sys/sgx/process.rs b/src/libstd/sys/sgx/process.rs
index a02e009d953..edf933d10e0 100644
--- a/src/libstd/sys/sgx/process.rs
+++ b/src/libstd/sys/sgx/process.rs
@@ -4,14 +4,16 @@ use crate::io;
 use crate::sys::fs::File;
 use crate::sys::pipe::AnonPipe;
 use crate::sys::{unsupported, Void};
-use crate::sys_common::process::{CommandEnv, DefaultEnvKey};
+use crate::sys_common::process::CommandEnv;
+
+pub use crate::ffi::OsString as EnvKey;
 
 ////////////////////////////////////////////////////////////////////////////////
 // Command
 ////////////////////////////////////////////////////////////////////////////////
 
 pub struct Command {
-    env: CommandEnv<DefaultEnvKey>
+    env: CommandEnv,
 }
 
 // passed back to std::process with the pipes connected to the child, if any
@@ -38,7 +40,7 @@ impl Command {
     pub fn arg(&mut self, _arg: &OsStr) {
     }
 
-    pub fn env_mut(&mut self) -> &mut CommandEnv<DefaultEnvKey> {
+    pub fn env_mut(&mut self) -> &mut CommandEnv {
         &mut self.env
     }
 
diff --git a/src/libstd/sys/unix/process/mod.rs b/src/libstd/sys/unix/process/mod.rs
index bba4b21c462..056a20345f4 100644
--- a/src/libstd/sys/unix/process/mod.rs
+++ b/src/libstd/sys/unix/process/mod.rs
@@ -1,5 +1,6 @@
 pub use self::process_common::{Command, ExitStatus, ExitCode, Stdio, StdioPipes};
 pub use self::process_inner::Process;
+pub use crate::ffi::OsString as EnvKey;
 
 mod process_common;
 #[cfg(not(target_os = "fuchsia"))]
diff --git a/src/libstd/sys/unix/process/process_common.rs b/src/libstd/sys/unix/process/process_common.rs
index 21fca23a8fe..72e66cc8e72 100644
--- a/src/libstd/sys/unix/process/process_common.rs
+++ b/src/libstd/sys/unix/process/process_common.rs
@@ -7,7 +7,7 @@ use crate::ptr;
 use crate::sys::fd::FileDesc;
 use crate::sys::fs::{File, OpenOptions};
 use crate::sys::pipe::{self, AnonPipe};
-use crate::sys_common::process::{CommandEnv, DefaultEnvKey};
+use crate::sys_common::process::CommandEnv;
 use crate::collections::BTreeMap;
 
 use libc::{c_int, gid_t, uid_t, c_char, EXIT_SUCCESS, EXIT_FAILURE};
@@ -69,7 +69,7 @@ pub struct Command {
     program: CString,
     args: Vec<CString>,
     argv: Argv,
-    env: CommandEnv<DefaultEnvKey>,
+    env: CommandEnv,
 
     cwd: Option<CString>,
     uid: Option<uid_t>,
@@ -201,7 +201,7 @@ impl Command {
         self.stderr = Some(stderr);
     }
 
-    pub fn env_mut(&mut self) -> &mut CommandEnv<DefaultEnvKey> {
+    pub fn env_mut(&mut self) -> &mut CommandEnv {
         &mut self.env
     }
 
@@ -271,7 +271,7 @@ impl CStringArray {
     }
 }
 
-fn construct_envp(env: BTreeMap<DefaultEnvKey, OsString>, saw_nul: &mut bool) -> CStringArray {
+fn construct_envp(env: BTreeMap<OsString, OsString>, saw_nul: &mut bool) -> CStringArray {
     let mut result = CStringArray::with_capacity(env.len());
     for (k, v) in env {
         let mut k: OsString = k.into();
diff --git a/src/libstd/sys/vxworks/process/process_common.rs b/src/libstd/sys/vxworks/process/process_common.rs
index ba797354a73..509140229fd 100644
--- a/src/libstd/sys/vxworks/process/process_common.rs
+++ b/src/libstd/sys/vxworks/process/process_common.rs
@@ -7,11 +7,13 @@ use crate::ptr;
 use crate::sys::fd::FileDesc;
 use crate::sys::fs::{File, OpenOptions};
 use crate::sys::pipe::{self, AnonPipe};
-use crate::sys_common::process::{CommandEnv, DefaultEnvKey};
+use crate::sys_common::process::CommandEnv;
 use crate::collections::BTreeMap;
 
 use libc::{c_int, gid_t, uid_t, c_char, EXIT_SUCCESS, EXIT_FAILURE};
 
+pub use crate::ffi::OsString as EnvKey;
+
 ////////////////////////////////////////////////////////////////////////////////
 // Command
 ////////////////////////////////////////////////////////////////////////////////
@@ -37,7 +39,7 @@ pub struct Command {
     program: CString,
     args: Vec<CString>,
     argv: Argv,
-    env: CommandEnv<DefaultEnvKey>,
+    env: CommandEnv,
 
     cwd: Option<CString>,
     uid: Option<uid_t>,
@@ -170,7 +172,7 @@ impl Command {
         self.stderr = Some(stderr);
     }
 
-    pub fn env_mut(&mut self) -> &mut CommandEnv<DefaultEnvKey> {
+    pub fn env_mut(&mut self) -> &mut CommandEnv {
         &mut self.env
     }
 
@@ -240,7 +242,7 @@ impl CStringArray {
     }
 }
 
-fn construct_envp(env: BTreeMap<DefaultEnvKey, OsString>, saw_nul: &mut bool) -> CStringArray {
+fn construct_envp(env: BTreeMap<OsString, OsString>, saw_nul: &mut bool) -> CStringArray {
     let mut result = CStringArray::with_capacity(env.len());
     for (k, v) in env {
         let mut k: OsString = k.into();
diff --git a/src/libstd/sys/wasi/process.rs b/src/libstd/sys/wasi/process.rs
index 788b829f4ba..1c4d028b761 100644
--- a/src/libstd/sys/wasi/process.rs
+++ b/src/libstd/sys/wasi/process.rs
@@ -4,14 +4,16 @@ use crate::io;
 use crate::sys::fs::File;
 use crate::sys::pipe::AnonPipe;
 use crate::sys::{unsupported, Void};
-use crate::sys_common::process::{CommandEnv, DefaultEnvKey};
+use crate::sys_common::process::CommandEnv;
+
+pub use crate::ffi::OsString as EnvKey;
 
 ////////////////////////////////////////////////////////////////////////////////
 // Command
 ////////////////////////////////////////////////////////////////////////////////
 
 pub struct Command {
-    env: CommandEnv<DefaultEnvKey>
+    env: CommandEnv
 }
 
 // passed back to std::process with the pipes connected to the child, if any
@@ -38,7 +40,7 @@ impl Command {
     pub fn arg(&mut self, _arg: &OsStr) {
     }
 
-    pub fn env_mut(&mut self) -> &mut CommandEnv<DefaultEnvKey> {
+    pub fn env_mut(&mut self) -> &mut CommandEnv {
         &mut self.env
     }
 
diff --git a/src/libstd/sys/wasm/process.rs b/src/libstd/sys/wasm/process.rs
index a02e009d953..edf933d10e0 100644
--- a/src/libstd/sys/wasm/process.rs
+++ b/src/libstd/sys/wasm/process.rs
@@ -4,14 +4,16 @@ use crate::io;
 use crate::sys::fs::File;
 use crate::sys::pipe::AnonPipe;
 use crate::sys::{unsupported, Void};
-use crate::sys_common::process::{CommandEnv, DefaultEnvKey};
+use crate::sys_common::process::CommandEnv;
+
+pub use crate::ffi::OsString as EnvKey;
 
 ////////////////////////////////////////////////////////////////////////////////
 // Command
 ////////////////////////////////////////////////////////////////////////////////
 
 pub struct Command {
-    env: CommandEnv<DefaultEnvKey>
+    env: CommandEnv,
 }
 
 // passed back to std::process with the pipes connected to the child, if any
@@ -38,7 +40,7 @@ impl Command {
     pub fn arg(&mut self, _arg: &OsStr) {
     }
 
-    pub fn env_mut(&mut self) -> &mut CommandEnv<DefaultEnvKey> {
+    pub fn env_mut(&mut self) -> &mut CommandEnv {
         &mut self.env
     }
 
diff --git a/src/libstd/sys/windows/process.rs b/src/libstd/sys/windows/process.rs
index 05e0ca67064..8658deb8546 100644
--- a/src/libstd/sys/windows/process.rs
+++ b/src/libstd/sys/windows/process.rs
@@ -19,7 +19,7 @@ use crate::sys::pipe::{self, AnonPipe};
 use crate::sys::stdio;
 use crate::sys::cvt;
 use crate::sys_common::{AsInner, FromInner, IntoInner};
-use crate::sys_common::process::{CommandEnv, EnvKey};
+use crate::sys_common::process::CommandEnv;
 use crate::borrow::Borrow;
 
 use libc::{c_void, EXIT_SUCCESS, EXIT_FAILURE};
@@ -30,30 +30,28 @@ use libc::{c_void, EXIT_SUCCESS, EXIT_FAILURE};
 
 #[derive(Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
 #[doc(hidden)]
-pub struct WindowsEnvKey(OsString);
+pub struct EnvKey(OsString);
 
-impl From<OsString> for WindowsEnvKey {
+impl From<OsString> for EnvKey {
     fn from(k: OsString) -> Self {
         let mut buf = k.into_inner().into_inner();
         buf.make_ascii_uppercase();
-        WindowsEnvKey(FromInner::from_inner(FromInner::from_inner(buf)))
+        EnvKey(FromInner::from_inner(FromInner::from_inner(buf)))
     }
 }
 
-impl From<WindowsEnvKey> for OsString {
-    fn from(k: WindowsEnvKey) -> Self { k.0 }
+impl From<EnvKey> for OsString {
+    fn from(k: EnvKey) -> Self { k.0 }
 }
 
-impl Borrow<OsStr> for WindowsEnvKey {
+impl Borrow<OsStr> for EnvKey {
     fn borrow(&self) -> &OsStr { &self.0 }
 }
 
-impl AsRef<OsStr> for WindowsEnvKey {
+impl AsRef<OsStr> for EnvKey {
     fn as_ref(&self) -> &OsStr { &self.0 }
 }
 
-impl EnvKey for WindowsEnvKey {}
-
 
 fn ensure_no_nuls<T: AsRef<OsStr>>(str: T) -> io::Result<T> {
     if str.as_ref().encode_wide().any(|b| b == 0) {
@@ -66,7 +64,7 @@ fn ensure_no_nuls<T: AsRef<OsStr>>(str: T) -> io::Result<T> {
 pub struct Command {
     program: OsString,
     args: Vec<OsString>,
-    env: CommandEnv<WindowsEnvKey>,
+    env: CommandEnv,
     cwd: Option<OsString>,
     flags: u32,
     detach: bool, // not currently exposed in std::process
@@ -110,7 +108,7 @@ impl Command {
     pub fn arg(&mut self, arg: &OsStr) {
         self.args.push(arg.to_os_string())
     }
-    pub fn env_mut(&mut self) -> &mut CommandEnv<WindowsEnvKey> {
+    pub fn env_mut(&mut self) -> &mut CommandEnv {
         &mut self.env
     }
     pub fn cwd(&mut self, dir: &OsStr) {
@@ -498,7 +496,7 @@ fn make_command_line(prog: &OsStr, args: &[OsString]) -> io::Result<Vec<u16>> {
     }
 }
 
-fn make_envp(maybe_env: Option<BTreeMap<WindowsEnvKey, OsString>>)
+fn make_envp(maybe_env: Option<BTreeMap<EnvKey, OsString>>)
              -> io::Result<(*mut c_void, Vec<u16>)> {
     // On Windows we pass an "environment block" which is not a char**, but
     // rather a concatenation of null-terminated k=v\0 sequences, with a final
diff --git a/src/libstd/sys_common/process.rs b/src/libstd/sys_common/process.rs
index 4d40dec9724..bdf66fca359 100644
--- a/src/libstd/sys_common/process.rs
+++ b/src/libstd/sys_common/process.rs
@@ -1,47 +1,20 @@
 #![allow(dead_code)]
 #![unstable(feature = "process_internals", issue = "0")]
 
-use crate::ffi::{OsStr, OsString};
-use crate::env;
 use crate::collections::BTreeMap;
-use crate::borrow::Borrow;
-
-pub trait EnvKey:
-    From<OsString> + Into<OsString> +
-    Borrow<OsStr> + Borrow<Self> + AsRef<OsStr> +
-    Ord + Clone {}
-
-// Implement a case-sensitive environment variable key
-#[derive(Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
-pub struct DefaultEnvKey(OsString);
-
-impl From<OsString> for DefaultEnvKey {
-    fn from(k: OsString) -> Self { DefaultEnvKey(k) }
-}
-
-impl From<DefaultEnvKey> for OsString {
-    fn from(k: DefaultEnvKey) -> Self { k.0 }
-}
-
-impl Borrow<OsStr> for DefaultEnvKey {
-    fn borrow(&self) -> &OsStr { &self.0 }
-}
-
-impl AsRef<OsStr> for DefaultEnvKey {
-    fn as_ref(&self) -> &OsStr { &self.0 }
-}
-
-impl EnvKey for DefaultEnvKey {}
+use crate::env;
+use crate::ffi::{OsStr, OsString};
+use crate::sys::process::EnvKey;
 
 // Stores a set of changes to an environment
 #[derive(Clone, Debug)]
-pub struct CommandEnv<K> {
+pub struct CommandEnv {
     clear: bool,
     saw_path: bool,
-    vars: BTreeMap<K, Option<OsString>>
+    vars: BTreeMap<EnvKey, Option<OsString>>
 }
 
-impl<K: EnvKey> Default for CommandEnv<K> {
+impl Default for CommandEnv {
     fn default() -> Self {
         CommandEnv {
             clear: false,
@@ -51,10 +24,10 @@ impl<K: EnvKey> Default for CommandEnv<K> {
     }
 }
 
-impl<K: EnvKey> CommandEnv<K> {
+impl CommandEnv {
     // Capture the current environment with these changes applied
-    pub fn capture(&self) -> BTreeMap<K, OsString> {
-        let mut result = BTreeMap::<K, OsString>::new();
+    pub fn capture(&self) -> BTreeMap<EnvKey, OsString> {
+        let mut result = BTreeMap::<EnvKey, OsString>::new();
         if !self.clear {
             for (k, v) in env::vars_os() {
                 result.insert(k.into(), v);
@@ -90,7 +63,7 @@ impl<K: EnvKey> CommandEnv<K> {
         !self.clear && self.vars.is_empty()
     }
 
-    pub fn capture_if_changed(&self) -> Option<BTreeMap<K, OsString>> {
+    pub fn capture_if_changed(&self) -> Option<BTreeMap<EnvKey, OsString>> {
         if self.is_unchanged() {
             None
         } else {
@@ -103,6 +76,7 @@ impl<K: EnvKey> CommandEnv<K> {
         self.maybe_saw_path(&key);
         self.vars.insert(key.to_owned().into(), Some(value.to_owned()));
     }
+
     pub fn remove(&mut self, key: &OsStr) {
         self.maybe_saw_path(&key);
         if self.clear {
@@ -111,13 +85,16 @@ impl<K: EnvKey> CommandEnv<K> {
             self.vars.insert(key.to_owned().into(), None);
         }
     }
+
     pub fn clear(&mut self) {
         self.clear = true;
         self.vars.clear();
     }
+
     pub fn have_changed_path(&self) -> bool {
         self.saw_path || self.clear
     }
+
     fn maybe_saw_path(&mut self, key: &OsStr) {
         if !self.saw_path && key == "PATH" {
             self.saw_path = true;