Auto merge of #44979 - hinaria:master, r=dtolnay

make `backtrace = false` compile for windows targets.

when building for windows with `backtrace = false`, `libstd` fails to compile because some modules that use items from `sys_common::backtrace::*` are still included, even though those modules aren't used or referenced by anything.

`sys_common::backtrace` doesn't exist when the backtrace feature is turned off.

--

i've also added `#[cfg(feature = "backtrace")]` to various items that exist exclusively to support `mod backtrace` since the compilation would fail since they would be unused in a configuration with backtraces turned off.

src/libstd/sys/windows/c.rs

@@ -279,10 +279,13 @@ pub const WAIT_TIMEOUT: DWORD = 258;
 pub const WAIT_FAILED: DWORD = 0xFFFFFFFF;
 
 #[cfg(target_env = "msvc")]
+#[cfg(feature = "backtrace")]
 pub const MAX_SYM_NAME: usize = 2000;
 #[cfg(target_arch = "x86")]
+#[cfg(feature = "backtrace")]
 pub const IMAGE_FILE_MACHINE_I386: DWORD = 0x014c;
 #[cfg(target_arch = "x86_64")]
+#[cfg(feature = "backtrace")]
 pub const IMAGE_FILE_MACHINE_AMD64: DWORD = 0x8664;
 
 pub const PROV_RSA_FULL: DWORD = 1;
@@ -575,6 +578,7 @@ pub struct OVERLAPPED {
 
 #[repr(C)]
 #[cfg(target_env = "msvc")]
+#[cfg(feature = "backtrace")]
 pub struct SYMBOL_INFO {
     pub SizeOfStruct: c_ulong,
     pub TypeIndex: c_ulong,
@@ -598,6 +602,7 @@ pub struct SYMBOL_INFO {
 
 #[repr(C)]
 #[cfg(target_env = "msvc")]
+#[cfg(feature = "backtrace")]
 pub struct IMAGEHLP_LINE64 {
     pub SizeOfStruct: u32,
     pub Key: *const c_void,
@@ -616,13 +621,15 @@ pub enum ADDRESS_MODE {
 }
 
 #[repr(C)]
+#[cfg(feature = "backtrace")]
 pub struct ADDRESS64 {
     pub Offset: u64,
     pub Segment: u16,
     pub Mode: ADDRESS_MODE,
 }
 
 #[repr(C)]
+#[cfg(feature = "backtrace")]
 pub struct STACKFRAME64 {
     pub AddrPC: ADDRESS64,
     pub AddrReturn: ADDRESS64,
@@ -638,6 +645,7 @@ pub struct STACKFRAME64 {
 }
 
 #[repr(C)]
+#[cfg(feature = "backtrace")]
 pub struct KDHELP64 {
     pub Thread: u64,
     pub ThCallbackStack: DWORD,
@@ -1089,6 +1097,7 @@ extern "system" {
     pub fn FindNextFileW(findFile: HANDLE, findFileData: LPWIN32_FIND_DATAW)
                          -> BOOL;
     pub fn FindClose(findFile: HANDLE) -> BOOL;
+    #[cfg(feature = "backtrace")]
     pub fn RtlCaptureContext(ctx: *mut CONTEXT);
     pub fn getsockopt(s: SOCKET,
                       level: c_int,
@@ -1120,7 +1129,9 @@ extern "system" {
                        res: *mut *mut ADDRINFOA) -> c_int;
     pub fn freeaddrinfo(res: *mut ADDRINFOA);
 
+    #[cfg(feature = "backtrace")]
     pub fn LoadLibraryW(name: LPCWSTR) -> HMODULE;
+    #[cfg(feature = "backtrace")]
     pub fn FreeLibrary(handle: HMODULE) -> BOOL;
     pub fn GetProcAddress(handle: HMODULE,
                           name: LPCSTR) -> *mut c_void;

src/libstd/sys/windows/mod.rs

@@ -20,9 +20,11 @@ use time::Duration;
 #[macro_use] pub mod compat;
 
 pub mod args;
+#[cfg(feature = "backtrace")]
 pub mod backtrace;
 pub mod c;
 pub mod condvar;
+#[cfg(feature = "backtrace")]
 pub mod dynamic_lib;
 pub mod env;
 pub mod ext;