diff --git a/src/cargo/core/compiler/build_context/mod.rs b/src/cargo/core/compiler/build_context/mod.rs
index 7eab9dea45f..d116b2e2e09 100644
--- a/src/cargo/core/compiler/build_context/mod.rs
+++ b/src/cargo/core/compiler/build_context/mod.rs
@@ -15,6 +15,8 @@ use crate::util::{profile, Cfg, Config, Rustc};
 mod target_info;
 pub use self::target_info::{FileFlavor, TargetInfo};
 
+pub const HOST_SANDBOX_TARGET: &str = "wasm32-unknown-unknown";
+
 /// The build context, containing all information about a build task.
 ///
 /// It is intended that this is mostly static information. Stuff that mutates
@@ -38,10 +40,13 @@ pub struct BuildContext<'a, 'cfg> {
     pub rustc: Rustc,
     /// Build information for the host arch.
     pub host_config: TargetConfig,
+    /// Build information for the host sandbox arch (wasm).
+    pub host_sandbox_config: TargetConfig,
     /// Build information for the target.
     pub target_config: TargetConfig,
     pub target_info: TargetInfo,
     pub host_info: TargetInfo,
+    pub host_sandbox_info: TargetInfo,
     pub units: &'a UnitInterner<'a>,
 }
 
@@ -59,18 +64,21 @@ impl<'a, 'cfg> BuildContext<'a, 'cfg> {
         let rustc = config.load_global_rustc(Some(ws))?;
 
         let host_config = TargetConfig::new(config, &rustc.host)?;
+        let host_sandbox_config = TargetConfig::new(config, HOST_SANDBOX_TARGET)?;
         let target_config = match build_config.requested_target.as_ref() {
             Some(triple) => TargetConfig::new(config, triple)?,
             None => host_config.clone(),
         };
-        let (host_info, target_info) = {
+        let (host_info, host_sandbox_info, target_info) = {
             let _p = profile::start("BuildContext::probe_target_info");
             debug!("probe_target_info");
             let host_info =
                 TargetInfo::new(config, &build_config.requested_target, &rustc, Kind::Host)?;
+            let host_sandbox_info =
+                TargetInfo::new(config, &build_config.requested_target, &rustc, Kind::HostSandbox)?;
             let target_info =
                 TargetInfo::new(config, &build_config.requested_target, &rustc, Kind::Target)?;
-            (host_info, target_info)
+            (host_info, host_sandbox_info, target_info)
         };
 
         Ok(BuildContext {
@@ -82,7 +90,9 @@ impl<'a, 'cfg> BuildContext<'a, 'cfg> {
             target_config,
             target_info,
             host_config,
+            host_sandbox_config,
             host_info,
+            host_sandbox_info,
             build_config,
             profiles,
             extra_compiler_args,
@@ -111,6 +121,7 @@ impl<'a, 'cfg> BuildContext<'a, 'cfg> {
         };
         let (name, info) = match kind {
             Kind::Host => (self.host_triple(), &self.host_info),
+            Kind::HostSandbox => (self.host_sandbox_triple(), &self.host_sandbox_info),
             Kind::Target => (self.target_triple(), &self.target_info),
         };
         platform.matches(name, info.cfg())
@@ -130,6 +141,7 @@ impl<'a, 'cfg> BuildContext<'a, 'cfg> {
     pub fn cfg(&self, kind: Kind) -> &[Cfg] {
         let info = match kind {
             Kind::Host => &self.host_info,
+            Kind::HostSandbox => &self.host_sandbox_info,
             Kind::Target => &self.target_info,
         };
         info.cfg()
@@ -145,6 +157,10 @@ impl<'a, 'cfg> BuildContext<'a, 'cfg> {
         &self.rustc.host
     }
 
+    pub fn host_sandbox_triple(&self) -> &str {
+        HOST_SANDBOX_TARGET
+    }
+
     pub fn target_triple(&self) -> &str {
         self.build_config
             .requested_target
@@ -157,6 +173,7 @@ impl<'a, 'cfg> BuildContext<'a, 'cfg> {
     fn target_config(&self, kind: Kind) -> &TargetConfig {
         match kind {
             Kind::Host => &self.host_config,
+            Kind::HostSandbox => &self.host_sandbox_config,
             Kind::Target => &self.target_config,
         }
     }
@@ -181,6 +198,7 @@ impl<'a, 'cfg> BuildContext<'a, 'cfg> {
     fn info(&self, kind: Kind) -> &TargetInfo {
         match kind {
             Kind::Host => &self.host_info,
+            Kind::HostSandbox => &self.host_sandbox_info,
             Kind::Target => &self.target_info,
         }
     }
@@ -196,6 +214,7 @@ impl<'a, 'cfg> BuildContext<'a, 'cfg> {
     pub fn script_override(&self, lib_name: &str, kind: Kind) -> Option<&BuildOutput> {
         match kind {
             Kind::Host => self.host_config.overrides.get(lib_name),
+            Kind::HostSandbox => self.host_sandbox_config.overrides.get(lib_name),
             Kind::Target => self.target_config.overrides.get(lib_name),
         }
     }
diff --git a/src/cargo/core/compiler/build_context/target_info.rs b/src/cargo/core/compiler/build_context/target_info.rs
index b5b74fd9024..9daea43a4a0 100644
--- a/src/cargo/core/compiler/build_context/target_info.rs
+++ b/src/cargo/core/compiler/build_context/target_info.rs
@@ -110,7 +110,7 @@ impl TargetInfo {
         let mut pipelining_test = process.clone();
         pipelining_test.args(&["--error-format=json", "--json=artifacts"]);
         let supports_pipelining = match kind {
-            Kind::Host => Some(rustc.cached_output(&pipelining_test).is_ok()),
+            Kind::Host | Kind::HostSandbox => Some(rustc.cached_output(&pipelining_test).is_ok()),
             Kind::Target => None,
         };
 
@@ -118,8 +118,14 @@ impl TargetInfo {
             .as_ref()
             .map(|s| s.as_str())
             .unwrap_or(&rustc.host);
-        if kind == Kind::Target {
-            process.arg("--target").arg(target_triple);
+        match kind {
+            Kind::Host => {}
+            Kind::HostSandbox => {
+                process.arg("--target").arg(super::HOST_SANDBOX_TARGET);
+            }
+            Kind::Target => {
+                process.arg("--target").arg(target_triple);
+            }
         }
 
         let crate_type_process = process.clone();
@@ -161,6 +167,13 @@ impl TargetInfo {
                 }
                 rustlib
             }
+            Kind::HostSandbox => {
+                rustlib.push("lib");
+                rustlib.push("rustlib");
+                rustlib.push(super::HOST_SANDBOX_TARGET);
+                rustlib.push("lib");
+                rustlib
+            }
             Kind::Target => {
                 rustlib.push("lib");
                 rustlib.push("rustlib");
diff --git a/src/cargo/core/compiler/context/compilation_files.rs b/src/cargo/core/compiler/context/compilation_files.rs
index f7e0ee7cfb8..07cc0a738e2 100644
--- a/src/cargo/core/compiler/context/compilation_files.rs
+++ b/src/cargo/core/compiler/context/compilation_files.rs
@@ -10,7 +10,7 @@ use log::info;
 
 use super::{BuildContext, Context, FileFlavor, Kind, Layout};
 use crate::core::compiler::{CompileMode, Unit};
-use crate::core::{TargetKind, Workspace};
+use crate::core::{LibKind, TargetKind, Workspace};
 use crate::util::{self, CargoResult};
 
 /// The `Metadata` is a hash used to make unique file names for each unit in a build.
@@ -53,6 +53,8 @@ impl fmt::Display for Metadata {
 pub struct CompilationFiles<'a, 'cfg> {
     /// The target directory layout for the host (and target if it is the same as host).
     pub(super) host: Layout,
+    /// The target directory layout for the host sandbox architecture
+    pub(super) host_sandbox: Layout,
     /// The target directory layout for the target (if different from then host).
     pub(super) target: Option<Layout>,
     /// Additional directory to include a copy of the outputs.
@@ -93,6 +95,7 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
     pub(super) fn new(
         roots: &[Unit<'a>],
         host: Layout,
+        host_sandbox: Layout,
         target: Option<Layout>,
         export_dir: Option<PathBuf>,
         ws: &'a Workspace<'cfg>,
@@ -110,6 +113,7 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
         CompilationFiles {
             ws,
             host,
+            host_sandbox,
             target,
             export_dir,
             roots: roots.to_vec(),
@@ -122,6 +126,7 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
     pub fn layout(&self, kind: Kind) -> &Layout {
         match kind {
             Kind::Host => &self.host,
+            Kind::HostSandbox => &self.host_sandbox,
             Kind::Target => self.target.as_ref().unwrap_or(&self.host),
         }
     }
@@ -345,10 +350,10 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
 
         let out_dir = self.out_dir(unit);
         let link_stem = self.link_stem(unit);
-        let info = if unit.kind == Kind::Host {
-            &bcx.host_info
-        } else {
-            &bcx.target_info
+        let info = match unit.kind {
+            Kind::Host => &bcx.host_info,
+            Kind::HostSandbox => &bcx.host_sandbox_info,
+            Kind::Target => &bcx.target_info,
         };
         let file_stem = self.file_stem(unit);
 
@@ -405,8 +410,15 @@ impl<'a, 'cfg: 'a> CompilationFiles<'a, 'cfg> {
             }
             TargetKind::ExampleLib(ref kinds) | TargetKind::Lib(ref kinds) => {
                 for kind in kinds {
+                    let crate_type =
+                        if kind == &LibKind::ProcMacro && unit.kind == Kind::HostSandbox {
+                            // wasm32-unknown-unknown doesn't support proc-macro, so just use cdylib
+                            "cdylib"
+                        } else {
+                            kind.crate_type()
+                        };
                     add(
-                        kind.crate_type(),
+                        crate_type,
                         if kind.linkable() {
                             FileFlavor::Linkable { rmeta: false }
                         } else {
diff --git a/src/cargo/core/compiler/context/mod.rs b/src/cargo/core/compiler/context/mod.rs
index f82026e5281..813f95a7428 100644
--- a/src/cargo/core/compiler/context/mod.rs
+++ b/src/cargo/core/compiler/context/mod.rs
@@ -298,6 +298,7 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
             "debug"
         };
         let host_layout = Layout::new(self.bcx.ws, None, dest)?;
+        let host_sandbox_layout = Layout::new(self.bcx.ws, Some(super::build_context::HOST_SANDBOX_TARGET), dest)?;
         let target_layout = match self.bcx.build_config.requested_target.as_ref() {
             Some(target) => Some(Layout::new(self.bcx.ws, Some(target), dest)?),
             None => None,
@@ -309,6 +310,7 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
         let files = CompilationFiles::new(
             units,
             host_layout,
+            host_sandbox_layout,
             target_layout,
             export_dir,
             self.bcx.ws,
@@ -327,6 +329,10 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
             .host
             .prepare()
             .chain_err(|| internal("couldn't prepare build directories"))?;
+        self.files_mut()
+            .host_sandbox
+            .prepare()
+            .chain_err(|| internal("couldn't prepare build directories"))?;
         if let Some(ref mut target) = self.files.as_mut().unwrap().target {
             target
                 .prepare()
diff --git a/src/cargo/core/compiler/context/unit_dependencies.rs b/src/cargo/core/compiler/context/unit_dependencies.rs
index 5cb41e6385d..407f121576b 100644
--- a/src/cargo/core/compiler/context/unit_dependencies.rs
+++ b/src/cargo/core/compiler/context/unit_dependencies.rs
@@ -187,7 +187,7 @@ fn compute_deps<'a, 'cfg, 'tmp>(
         {
             let unit = new_unit(bcx, pkg, lib, dep_unit_for, Kind::Target, mode);
             ret.push((unit, dep_unit_for));
-            let unit = new_unit(bcx, pkg, lib, dep_unit_for, Kind::Host, mode);
+            let unit = new_unit(bcx, pkg, lib, dep_unit_for, unit.kind.for_target(lib), mode);
             ret.push((unit, dep_unit_for));
         } else {
             let unit = new_unit(bcx, pkg, lib, dep_unit_for, unit.kind.for_target(lib), mode);
@@ -199,6 +199,7 @@ fn compute_deps<'a, 'cfg, 'tmp>(
     // all we need. If this isn't a build script, then it depends on the
     // build script if there is one.
     if unit.target.is_custom_build() {
+        assert_eq!(unit.kind, Kind::Host, "unit.target {:?}", unit.target);
         return Ok(ret);
     }
     ret.extend(dep_build_script(unit, bcx));
diff --git a/src/cargo/core/compiler/custom_build.rs b/src/cargo/core/compiler/custom_build.rs
index 321d8cdee76..d6fb43a5a75 100644
--- a/src/cargo/core/compiler/custom_build.rs
+++ b/src/cargo/core/compiler/custom_build.rs
@@ -163,6 +163,7 @@ fn build_work<'a, 'cfg>(cx: &mut Context<'a, 'cfg>, unit: &Unit<'a>) -> CargoRes
             "TARGET",
             &match unit.kind {
                 Kind::Host => bcx.host_triple(),
+                Kind::HostSandbox => bcx.host_sandbox_triple(),
                 Kind::Target => bcx.target_triple(),
             },
         )
diff --git a/src/cargo/core/compiler/mod.rs b/src/cargo/core/compiler/mod.rs
index 5567537f095..04c986c7157 100644
--- a/src/cargo/core/compiler/mod.rs
+++ b/src/cargo/core/compiler/mod.rs
@@ -51,7 +51,11 @@ use crate::util::{internal, join_paths, profile};
 /// These will be the same unless cross-compiling.
 #[derive(PartialEq, Eq, Hash, Debug, Clone, Copy, PartialOrd, Ord, Serialize)]
 pub enum Kind {
+    /// Actual native host architecture
     Host,
+    /// For host, but in a wasm sandbox
+    HostSandbox,
+    /// Target architecture
     Target,
 }
 
@@ -947,16 +951,27 @@ fn build_base_args<'a, 'cfg>(
         }
     }
 
-    if unit.kind == Kind::Target {
-        opt(
-            cmd,
-            "--target",
-            "",
-            bcx.build_config
-                .requested_target
-                .as_ref()
-                .map(|s| s.as_ref()),
-        );
+    match unit.kind {
+        Kind::Host => {}
+        Kind::HostSandbox => {
+            opt(
+                cmd,
+                "--target",
+                "",
+                Some(cx.bcx.host_sandbox_triple().as_ref()),
+            );
+        }
+        Kind::Target => {
+            opt(
+                cmd,
+                "--target",
+                "",
+                bcx.build_config
+                    .requested_target
+                    .as_ref()
+                    .map(|s| s.as_ref()),
+            );
+        }
     }
 
     opt(cmd, "-C", "ar=", bcx.ar(unit.kind).map(|s| s.as_ref()));
@@ -1109,7 +1124,16 @@ impl Kind {
         // that needs to be on the host we lift ourselves up to `Host`.
         match self {
             Kind::Host => Kind::Host,
-            Kind::Target if target.for_host() => Kind::Host,
+            Kind::HostSandbox if target.is_custom_build() => Kind::Host,
+            Kind::HostSandbox => Kind::HostSandbox,
+            Kind::Target if target.for_host() => {
+                if target.wasm_sandbox() {
+                    assert!(!target.is_custom_build(), "target {:?}", target);
+                    Kind::HostSandbox
+                } else {
+                    Kind::Host
+                }
+            }
             Kind::Target => Kind::Target,
         }
     }
diff --git a/src/cargo/core/compiler/unit.rs b/src/cargo/core/compiler/unit.rs
index 00c9841cc14..e9efad967f9 100644
--- a/src/cargo/core/compiler/unit.rs
+++ b/src/cargo/core/compiler/unit.rs
@@ -140,6 +140,9 @@ impl<'a> UnitInterner<'a> {
         kind: Kind,
         mode: CompileMode,
     ) -> Unit<'a> {
+        // building custom_build implies Host
+        assert!(!(target.is_custom_build() && mode == CompileMode::Build) || kind == Kind::Host,
+            "target {:?} kind {:?} mode {:?}", target, kind, mode);
         let inner = self.intern_inner(&UnitInner {
             pkg,
             target,
diff --git a/src/cargo/core/manifest.rs b/src/cargo/core/manifest.rs
index acda0e5700b..1e047a56ef0 100644
--- a/src/cargo/core/manifest.rs
+++ b/src/cargo/core/manifest.rs
@@ -241,6 +241,7 @@ pub struct Target {
     harness: bool, // whether to use the test harness (--test)
     for_host: bool,
     proc_macro: bool,
+    wasm_sandbox: bool,
     edition: Edition,
 }
 
@@ -296,6 +297,7 @@ struct SerializedTarget<'a> {
     /// Corresponds to `--crate-type` compiler attribute.
     /// See https://doc.rust-lang.org/reference/linkage.html
     crate_types: Vec<&'a str>,
+    wasm_sandbox: bool,
     name: &'a str,
     src_path: Option<&'a PathBuf>,
     edition: &'a str,
@@ -315,6 +317,7 @@ impl ser::Serialize for Target {
         SerializedTarget {
             kind: &self.kind,
             crate_types: self.rustc_crate_types(),
+            wasm_sandbox: self.wasm_sandbox,
             name: &self.name,
             src_path,
             edition: &self.edition.to_string(),
@@ -384,6 +387,7 @@ compact_debug! {
                 harness
                 for_host
                 proc_macro
+                wasm_sandbox
                 edition
             )]
         }
@@ -614,6 +618,7 @@ impl Target {
             harness: true,
             for_host: false,
             proc_macro: false,
+            wasm_sandbox: false,
             edition,
             tested: true,
             benched: true,
@@ -768,6 +773,9 @@ impl Target {
     pub fn proc_macro(&self) -> bool {
         self.proc_macro
     }
+    pub fn wasm_sandbox(&self) -> bool {
+        self.wasm_sandbox
+    }
     pub fn edition(&self) -> Edition {
         self.edition
     }
@@ -867,7 +875,17 @@ impl Target {
     pub fn rustc_crate_types(&self) -> Vec<&str> {
         match self.kind {
             TargetKind::Lib(ref kinds) | TargetKind::ExampleLib(ref kinds) => {
-                kinds.iter().map(LibKind::crate_type).collect()
+                kinds
+                    .iter()
+                    .map(|kind| {
+                        // wasm32-unknown-unknown doesn't support proc-macro, so just use cdylib
+                        if self.wasm_sandbox() && kind == &LibKind::ProcMacro {
+                            "cdylib"
+                        } else {
+                            kind.crate_type()
+                        }
+                    })
+                    .collect()
             }
             TargetKind::CustomBuild
             | TargetKind::Bench
@@ -908,6 +926,10 @@ impl Target {
         self.proc_macro = proc_macro;
         self
     }
+    pub fn set_wasm_sandbox(&mut self, wasm_sandbox: bool) -> &mut Target {
+        self.wasm_sandbox = wasm_sandbox;
+        self
+    }
     pub fn set_edition(&mut self, edition: Edition) -> &mut Target {
         self.edition = edition;
         self
diff --git a/src/cargo/ops/cargo_compile.rs b/src/cargo/ops/cargo_compile.rs
index dde087e0406..07e63064087 100644
--- a/src/cargo/ops/cargo_compile.rs
+++ b/src/cargo/ops/cargo_compile.rs
@@ -640,10 +640,10 @@ fn generate_targets<'a>(
             _ => target_mode,
         };
         // Plugins or proc macros should be built for the host.
-        let kind = if target.for_host() {
-            Kind::Host
-        } else {
-            default_arch_kind
+        let kind = match (target.for_host(), target.wasm_sandbox()) {
+            (true, false) => Kind::Host,
+            (true, true) => Kind::HostSandbox,
+            (false, _) => default_arch_kind,
         };
         let profile = profiles.get_profile(
             pkg.package_id(),
diff --git a/src/cargo/util/toml/mod.rs b/src/cargo/util/toml/mod.rs
index 5d9e000b807..0de14eb15d4 100644
--- a/src/cargo/util/toml/mod.rs
+++ b/src/cargo/util/toml/mod.rs
@@ -1517,6 +1517,8 @@ struct TomlTarget {
     proc_macro: Option<bool>,
     #[serde(rename = "proc_macro")]
     proc_macro2: Option<bool>,
+    #[serde(rename = "wasm-sandbox")]
+    wasm_sandbox: Option<bool>,
     harness: Option<bool>,
     #[serde(rename = "required-features")]
     required_features: Option<Vec<String>>,
diff --git a/src/cargo/util/toml/targets.rs b/src/cargo/util/toml/targets.rs
index a834fad5e1f..6241eff739b 100644
--- a/src/cargo/util/toml/targets.rs
+++ b/src/cargo/util/toml/targets.rs
@@ -769,7 +769,8 @@ fn configure(features: &Features, toml: &TomlTarget, target: &mut Target) -> Car
             (None, None) => t2.for_host(),
             (Some(true), _) | (_, Some(true)) => true,
             (Some(false), _) | (_, Some(false)) => false,
-        });
+        })
+        .set_wasm_sandbox(toml.wasm_sandbox.unwrap_or_else(|| t2.wasm_sandbox()));
     if let Some(edition) = toml.edition.clone() {
         features
             .require(Feature::edition())