Set the TARGET env variable when running rustc

The goal is to be able to do things differently in a proc macro
depending on the target platform. It is already defined for build script
and it would help to have it for macros as well.

Closes: #10714

src/cargo/core/compiler/compilation.rs

@@ -183,7 +183,7 @@ impl<'cfg> Compilation<'cfg> {
             self.rustc_process.clone()
         };
 
-        let cmd = fill_rustc_tool_env(rustc, unit);
+        let cmd = fill_rustc_tool_env(rustc, unit, &self.host);
         self.fill_env(cmd, &unit.pkg, None, unit.kind, true)
     }
 
@@ -194,7 +194,7 @@ impl<'cfg> Compilation<'cfg> {
         script_meta: Option<Metadata>,
     ) -> CargoResult<ProcessBuilder> {
         let rustdoc = ProcessBuilder::new(&*self.config.rustdoc()?);
-        let cmd = fill_rustc_tool_env(rustdoc, unit);
+        let cmd = fill_rustc_tool_env(rustdoc, unit, &self.host);
         let mut cmd = self.fill_env(cmd, &unit.pkg, script_meta, unit.kind, true)?;
         cmd.retry_with_argfile(true);
         unit.target.edition().cmd_edition_arg(&mut cmd);
@@ -373,7 +373,7 @@ impl<'cfg> Compilation<'cfg> {
 
 /// Prepares a rustc_tool process with additional environment variables
 /// that are only relevant in a context that has a unit
-fn fill_rustc_tool_env(mut cmd: ProcessBuilder, unit: &Unit) -> ProcessBuilder {
+fn fill_rustc_tool_env(mut cmd: ProcessBuilder, unit: &Unit, host_triple: &str) -> ProcessBuilder {
     if unit.target.is_executable() {
         let name = unit
             .target
@@ -383,6 +383,11 @@ fn fill_rustc_tool_env(mut cmd: ProcessBuilder, unit: &Unit) -> ProcessBuilder {
         cmd.env("CARGO_BIN_NAME", name);
     }
     cmd.env("CARGO_CRATE_NAME", unit.target.crate_name());
+    let target_triple = match &unit.kind {
+        CompileKind::Host => host_triple,
+        CompileKind::Target(t) => t.short_name(),
+    };
+    cmd.env("TARGET", target_triple);
     cmd
 }
 

src/doc/src/reference/environment-variables.md

@@ -247,6 +247,9 @@ corresponding environment variable is set to the empty string, `""`.
 * `OUT_DIR` --- If the package has a build script, this is set to the folder where the build
               script should place its output. See below for more information.
               (Only set during compilation.)
+* `TARGET` --- the target triple that is being compiled for. Native code should be
+             compiled for this triple. See the [Target Triple] description
+             for more information. (Only set during compilation.)
 * `CARGO_BIN_EXE_<name>` --- The absolute path to a binary target's executable.
   This is only set when building an [integration test] or benchmark. This may
   be used with the [`env` macro] to find the executable to run for testing

tests/testsuite/build_script.rs

@@ -5101,6 +5101,7 @@ fn duplicate_script_with_extra_env() {
                     compile_error!{"expected mycfg set"}
                     // Compile-time assertion.
                     assert_eq!(env!("CRATE_TARGET"), "{target}");
+                    assert_eq!(env!("TARGET"), "{target}");
                     // Run-time assertion.
                     assert_eq!(std::env::var("CRATE_TARGET").unwrap(), "{target}");
                 }

tests/testsuite/cross_compile.rs

@@ -30,7 +30,7 @@ fn simple_cross() {
                         assert_eq!(std::env::var("TARGET").unwrap(), "{}");
                     }}
                 "#,
-                cross_compile::alternate()
+                cross_compile::alternate(),
             ),
         )
         .file(
@@ -40,9 +40,11 @@ fn simple_cross() {
                     use std::env;
                     fn main() {{
                         assert_eq!(env::consts::ARCH, "{}");
+                        assert_eq!(env!("TARGET"), "{}");
                     }}
                 "#,
-                cross_compile::alternate_arch()
+                cross_compile::alternate_arch(),
+                cross_compile::alternate()
             ),
         )
         .build();
@@ -101,9 +103,11 @@ fn simple_cross_config() {
                     use std::env;
                     fn main() {{
                         assert_eq!(env::consts::ARCH, "{}");
+                        assert_eq!(env!("TARGET"), "{}");
                     }}
                 "#,
-                cross_compile::alternate_arch()
+                cross_compile::alternate_arch(),
+                cross_compile::alternate()
             ),
         )
         .build();