implement "host" target substitution

Author: zdivelbiss

src/cargo/core/compiler/compile_kind.rs

@@ -84,15 +84,27 @@ impl CompileKind {
         fallback: CompileKindFallback,
     ) -> CargoResult<Vec<CompileKind>> {
         let dedup = |targets: &[String]| {
-            Ok(targets
+            let deduplicated_targets = targets
                 .iter()
-                .map(|value| Ok(CompileKind::Target(CompileTarget::new(value)?)))
+                .map(|value| {
+                    // This neatly substitutes the manually-specified `host` target directive
+                    // with the compiling machine's target triple.
+
+                    if value.as_str() == "host" {
+                        let host_triple = env!("RUST_HOST_TARGET");
+                        Ok(CompileKind::Target(CompileTarget::new(host_triple)?))
+                    } else {
+                        Ok(CompileKind::Target(CompileTarget::new(value.as_str())?))
+                    }
+                })
                 // First collect into a set to deduplicate any `--target` passed
                 // more than once...
                 .collect::<CargoResult<BTreeSet<_>>>()?
                 // ... then generate a flat list for everything else to use.
                 .into_iter()
-                .collect())
+                .collect();
+
+            Ok(deduplicated_targets)
         };
 
         if !targets.is_empty() {

src/cargo/util/command_prelude.rs

@@ -1257,6 +1257,9 @@ fn get_target_triples() -> Vec<clap_complete::CompletionCandidate> {
         }
     }
 
+    // Allow tab-completion for `host` as the desired target.
+    candidates.push(clap_complete::CompletionCandidate::new("host"));
+
     candidates
 }