diff --git a/src/bootstrap/sanity.rs b/src/bootstrap/sanity.rs
index 962d0666f69f1..9429af565258b 100644
--- a/src/bootstrap/sanity.rs
+++ b/src/bootstrap/sanity.rs
@@ -104,6 +104,14 @@ pub fn check(build: &mut Build) {
         need_cmd(build.cxx(host).as_ref());
     }
 
+    // The msvc hosts don't use jemalloc, turn it off globally to
+    // avoid packaging the dummy liballoc_jemalloc on that platform.
+    for host in build.config.host.iter() {
+        if host.contains("msvc") {
+            build.config.use_jemalloc = false;
+        }
+    }
+
     // Externally configured LLVM requires FileCheck to exist
     let filecheck = build.llvm_filecheck(&build.config.build);
     if !filecheck.starts_with(&build.out) && !filecheck.exists() && build.config.codegen_tests {
@@ -111,15 +119,6 @@ pub fn check(build: &mut Build) {
     }
 
     for target in build.config.target.iter() {
-        // Either can't build or don't want to run jemalloc on these targets
-        if target.contains("rumprun") ||
-           target.contains("bitrig") ||
-           target.contains("openbsd") ||
-           target.contains("msvc") ||
-           target.contains("emscripten") {
-            build.config.use_jemalloc = false;
-        }
-
         // Can't compile for iOS unless we're on OSX
         if target.contains("apple-ios") &&
            !build.config.build.contains("apple-darwin") {
diff --git a/src/liballoc_jemalloc/build.rs b/src/liballoc_jemalloc/build.rs
index 8b31c5a557747..028d742cc832e 100644
--- a/src/liballoc_jemalloc/build.rs
+++ b/src/liballoc_jemalloc/build.rs
@@ -27,6 +27,24 @@ fn main() {
     let build_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
     let src_dir = env::current_dir().unwrap();
 
+    // FIXME: This is a hack to support building targets that don't
+    // support jemalloc alongside hosts that do. The jemalloc build is
+    // controlled by a feature of the std crate, and if that feature
+    // changes between targets, it invalidates the fingerprint of
+    // std's build script (this is a cargo bug); so we must ensure
+    // that the feature set used by std is the same across all
+    // targets, which means we have to build the alloc_jemalloc crate
+    // for targets like emscripten, even if we don't use it.
+    if target.contains("rumprun") ||
+        target.contains("bitrig") ||
+        target.contains("openbsd") ||
+        target.contains("msvc") ||
+        target.contains("emscripten")
+    {
+        println!("cargo:rustc-cfg=dummy_jemalloc");
+        return;
+    }
+
     if let Some(jemalloc) = env::var_os("JEMALLOC_OVERRIDE") {
         let jemalloc = PathBuf::from(jemalloc);
         println!("cargo:rustc-link-search=native={}",
diff --git a/src/liballoc_jemalloc/lib.rs b/src/liballoc_jemalloc/lib.rs
index 5bbf1c35e0dd4..21e45f9c4b20c 100644
--- a/src/liballoc_jemalloc/lib.rs
+++ b/src/liballoc_jemalloc/lib.rs
@@ -23,124 +23,170 @@
 
 extern crate libc;
 
-use libc::{c_int, c_void, size_t};
+pub use imp::*;
 
-// Linkage directives to pull in jemalloc and its dependencies.
-//
-// On some platforms we need to be sure to link in `pthread` which jemalloc
-// depends on, and specifically on android we need to also link to libgcc.
-// Currently jemalloc is compiled with gcc which will generate calls to
-// intrinsics that are libgcc specific (e.g. those intrinsics aren't present in
-// libcompiler-rt), so link that in to get that support.
-#[link(name = "jemalloc", kind = "static")]
-#[cfg_attr(target_os = "android", link(name = "gcc"))]
-#[cfg_attr(all(not(windows),
-               not(target_os = "android"),
-               not(target_env = "musl")),
-           link(name = "pthread"))]
-#[cfg(not(cargobuild))]
-extern "C" {}
-
-// Note that the symbols here are prefixed by default on OSX and Windows (we
-// don't explicitly request it), and on Android and DragonFly we explicitly
-// request it as unprefixing cause segfaults (mismatches in allocators).
-extern "C" {
-    #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
-                   target_os = "dragonfly", target_os = "windows"),
-               link_name = "je_mallocx")]
-    fn mallocx(size: size_t, flags: c_int) -> *mut c_void;
-    #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
-                   target_os = "dragonfly", target_os = "windows"),
-               link_name = "je_rallocx")]
-    fn rallocx(ptr: *mut c_void, size: size_t, flags: c_int) -> *mut c_void;
-    #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
-                   target_os = "dragonfly", target_os = "windows"),
-               link_name = "je_xallocx")]
-    fn xallocx(ptr: *mut c_void, size: size_t, extra: size_t, flags: c_int) -> size_t;
-    #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
-                   target_os = "dragonfly", target_os = "windows"),
-               link_name = "je_sdallocx")]
-    fn sdallocx(ptr: *mut c_void, size: size_t, flags: c_int);
-    #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
-                   target_os = "dragonfly", target_os = "windows"),
-               link_name = "je_nallocx")]
-    fn nallocx(size: size_t, flags: c_int) -> size_t;
-}
+// See comments in build.rs for why we sometimes build a crate that does nothing
+#[cfg(not(dummy_jemalloc))]
+mod imp {
+    use libc::{c_int, c_void, size_t};
 
-// The minimum alignment guaranteed by the architecture. This value is used to
-// add fast paths for low alignment values. In practice, the alignment is a
-// constant at the call site and the branch will be optimized out.
-#[cfg(all(any(target_arch = "arm",
-              target_arch = "mips",
-              target_arch = "powerpc")))]
-const MIN_ALIGN: usize = 8;
-#[cfg(all(any(target_arch = "x86",
-              target_arch = "x86_64",
-              target_arch = "aarch64",
-              target_arch = "powerpc64",
-              target_arch = "mips64",
-              target_arch = "s390x")))]
-const MIN_ALIGN: usize = 16;
-
-// MALLOCX_ALIGN(a) macro
-fn mallocx_align(a: usize) -> c_int {
-    a.trailing_zeros() as c_int
-}
+    // Linkage directives to pull in jemalloc and its dependencies.
+    //
+    // On some platforms we need to be sure to link in `pthread` which jemalloc
+    // depends on, and specifically on android we need to also link to libgcc.
+    // Currently jemalloc is compiled with gcc which will generate calls to
+    // intrinsics that are libgcc specific (e.g. those intrinsics aren't present in
+    // libcompiler-rt), so link that in to get that support.
+    #[link(name = "jemalloc", kind = "static")]
+    #[cfg_attr(target_os = "android", link(name = "gcc"))]
+    #[cfg_attr(all(not(windows),
+                   not(target_os = "android"),
+                   not(target_env = "musl")),
+               link(name = "pthread"))]
+    #[cfg(not(cargobuild))]
+    extern "C" {}
+
+    // Note that the symbols here are prefixed by default on OSX and Windows (we
+    // don't explicitly request it), and on Android and DragonFly we explicitly
+    // request it as unprefixing cause segfaults (mismatches in allocators).
+    extern "C" {
+        #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
+                       target_os = "dragonfly", target_os = "windows"),
+                   link_name = "je_mallocx")]
+        fn mallocx(size: size_t, flags: c_int) -> *mut c_void;
+        #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
+                       target_os = "dragonfly", target_os = "windows"),
+                   link_name = "je_rallocx")]
+        fn rallocx(ptr: *mut c_void, size: size_t, flags: c_int) -> *mut c_void;
+        #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
+                       target_os = "dragonfly", target_os = "windows"),
+                   link_name = "je_xallocx")]
+        fn xallocx(ptr: *mut c_void, size: size_t, extra: size_t, flags: c_int) -> size_t;
+        #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
+                       target_os = "dragonfly", target_os = "windows"),
+                   link_name = "je_sdallocx")]
+        fn sdallocx(ptr: *mut c_void, size: size_t, flags: c_int);
+        #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
+                       target_os = "dragonfly", target_os = "windows"),
+                   link_name = "je_nallocx")]
+        fn nallocx(size: size_t, flags: c_int) -> size_t;
+    }
+
+    // The minimum alignment guaranteed by the architecture. This value is used to
+    // add fast paths for low alignment values. In practice, the alignment is a
+    // constant at the call site and the branch will be optimized out.
+    #[cfg(all(any(target_arch = "arm",
+                  target_arch = "mips",
+                  target_arch = "powerpc")))]
+    const MIN_ALIGN: usize = 8;
+    #[cfg(all(any(target_arch = "x86",
+                  target_arch = "x86_64",
+                  target_arch = "aarch64",
+                  target_arch = "powerpc64",
+                  target_arch = "mips64",
+                  target_arch = "s390x")))]
+    const MIN_ALIGN: usize = 16;
+
+    // MALLOCX_ALIGN(a) macro
+    fn mallocx_align(a: usize) -> c_int {
+        a.trailing_zeros() as c_int
+    }
+
+    fn align_to_flags(align: usize) -> c_int {
+        if align <= MIN_ALIGN {
+            0
+        } else {
+            mallocx_align(align)
+        }
+    }
+
+    #[no_mangle]
+    pub extern "C" fn __rust_allocate(size: usize, align: usize) -> *mut u8 {
+        let flags = align_to_flags(align);
+        unsafe { mallocx(size as size_t, flags) as *mut u8 }
+    }
+
+    #[no_mangle]
+    pub extern "C" fn __rust_reallocate(ptr: *mut u8,
+                                        _old_size: usize,
+                                        size: usize,
+                                        align: usize)
+                                        -> *mut u8 {
+        let flags = align_to_flags(align);
+        unsafe { rallocx(ptr as *mut c_void, size as size_t, flags) as *mut u8 }
+    }
+
+    #[no_mangle]
+    pub extern "C" fn __rust_reallocate_inplace(ptr: *mut u8,
+                                                _old_size: usize,
+                                                size: usize,
+                                                align: usize)
+                                                -> usize {
+        let flags = align_to_flags(align);
+        unsafe { xallocx(ptr as *mut c_void, size as size_t, 0, flags) as usize }
+    }
 
-fn align_to_flags(align: usize) -> c_int {
-    if align <= MIN_ALIGN {
+    #[no_mangle]
+    pub extern "C" fn __rust_deallocate(ptr: *mut u8, old_size: usize, align: usize) {
+        let flags = align_to_flags(align);
+        unsafe { sdallocx(ptr as *mut c_void, old_size as size_t, flags) }
+    }
+
+    #[no_mangle]
+    pub extern "C" fn __rust_usable_size(size: usize, align: usize) -> usize {
+        let flags = align_to_flags(align);
+        unsafe { nallocx(size as size_t, flags) as usize }
+    }
+
+    // These symbols are used by jemalloc on android but the really old android
+    // we're building on doesn't have them defined, so just make sure the symbols
+    // are available.
+    #[no_mangle]
+    #[cfg(target_os = "android")]
+    pub extern "C" fn pthread_atfork(_prefork: *mut u8,
+                                     _postfork_parent: *mut u8,
+                                     _postfork_child: *mut u8)
+                                     -> i32 {
         0
-    } else {
-        mallocx_align(align)
     }
 }
 
-#[no_mangle]
-pub extern "C" fn __rust_allocate(size: usize, align: usize) -> *mut u8 {
-    let flags = align_to_flags(align);
-    unsafe { mallocx(size as size_t, flags) as *mut u8 }
-}
+#[cfg(dummy_jemalloc)]
+mod imp {
+    fn bogus() -> ! {
+        panic!("jemalloc is not implemented for this platform");
+    }
 
-#[no_mangle]
-pub extern "C" fn __rust_reallocate(ptr: *mut u8,
-                                    _old_size: usize,
-                                    size: usize,
-                                    align: usize)
-                                    -> *mut u8 {
-    let flags = align_to_flags(align);
-    unsafe { rallocx(ptr as *mut c_void, size as size_t, flags) as *mut u8 }
-}
+    #[no_mangle]
+    pub extern "C" fn __rust_allocate(_size: usize, _align: usize) -> *mut u8 {
+        bogus()
+    }
 
-#[no_mangle]
-pub extern "C" fn __rust_reallocate_inplace(ptr: *mut u8,
-                                            _old_size: usize,
-                                            size: usize,
-                                            align: usize)
-                                            -> usize {
-    let flags = align_to_flags(align);
-    unsafe { xallocx(ptr as *mut c_void, size as size_t, 0, flags) as usize }
-}
+    #[no_mangle]
+    pub extern "C" fn __rust_reallocate(_ptr: *mut u8,
+                                        _old_size: usize,
+                                        _size: usize,
+                                        _align: usize)
+                                        -> *mut u8 {
+        bogus()
+    }
 
-#[no_mangle]
-pub extern "C" fn __rust_deallocate(ptr: *mut u8, old_size: usize, align: usize) {
-    let flags = align_to_flags(align);
-    unsafe { sdallocx(ptr as *mut c_void, old_size as size_t, flags) }
-}
+    #[no_mangle]
+    pub extern "C" fn __rust_reallocate_inplace(_ptr: *mut u8,
+                                                _old_size: usize,
+                                                _size: usize,
+                                                _align: usize)
+                                                -> usize {
+        bogus()
+    }
 
-#[no_mangle]
-pub extern "C" fn __rust_usable_size(size: usize, align: usize) -> usize {
-    let flags = align_to_flags(align);
-    unsafe { nallocx(size as size_t, flags) as usize }
-}
+    #[no_mangle]
+    pub extern "C" fn __rust_deallocate(_ptr: *mut u8, _old_size: usize, _align: usize) {
+        bogus()
+    }
 
-// These symbols are used by jemalloc on android but the really old android
-// we're building on doesn't have them defined, so just make sure the symbols
-// are available.
-#[no_mangle]
-#[cfg(target_os = "android")]
-pub extern "C" fn pthread_atfork(_prefork: *mut u8,
-                                 _postfork_parent: *mut u8,
-                                 _postfork_child: *mut u8)
-                                 -> i32 {
-    0
+    #[no_mangle]
+    pub extern "C" fn __rust_usable_size(_size: usize, _align: usize) -> usize {
+        bogus()
+    }
 }