From 9802c0ccd0cf408260961eb61581b61363c4b47b Mon Sep 17 00:00:00 2001
From: Jon Gjengset <jongje@amazon.com>
Date: Wed, 16 Dec 2020 17:38:21 -0800
Subject: Default to host platform for cargo metadata

This modifies the logic for calling cargo metadata so that it will use
the host platform if no explicit target platform is given. This is
needed since cargo metadata defaults to outputting information for _all_
targets.

Fixes #6908.
---
 crates/project_model/src/cargo_workspace.rs | 32 +++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

(limited to 'crates/project_model')

diff --git a/crates/project_model/src/cargo_workspace.rs b/crates/project_model/src/cargo_workspace.rs
index 894c5c952..91fb645da 100644
--- a/crates/project_model/src/cargo_workspace.rs
+++ b/crates/project_model/src/cargo_workspace.rs
@@ -16,6 +16,7 @@ use paths::{AbsPath, AbsPathBuf};
 use rustc_hash::FxHashMap;
 
 use crate::cfg_flag::CfgFlag;
+use crate::utf8_stdout;
 
 /// `CargoWorkspace` represents the logical structure of, well, a Cargo
 /// workspace. It pretty closely mirrors `cargo metadata` output.
@@ -166,8 +167,35 @@ impl CargoWorkspace {
         if let Some(parent) = cargo_toml.parent() {
             meta.current_dir(parent.to_path_buf());
         }
-        if let Some(target) = config.target.as_ref() {
-            meta.other_options(vec![String::from("--filter-platform"), target.clone()]);
+        let target = if let Some(target) = config.target.as_ref() {
+            Some(target.clone())
+        } else {
+            // cargo metadata defaults to giving information for _all_ targets.
+            // In the absence of a preference from the user, we use the host platform.
+            let mut rustc = Command::new(toolchain::rustc());
+            rustc.current_dir(cargo_toml.parent().unwrap()).arg("-vV");
+            log::debug!("Discovering host platform by {:?}", rustc);
+            match utf8_stdout(rustc) {
+                Ok(stdout) => {
+                    let field = "host: ";
+                    let target =
+                        stdout.lines().find(|l| l.starts_with(field)).map(|l| &l[field.len()..]);
+                    if let Some(target) = target {
+                        Some(target.to_string())
+                    } else {
+                        // If we fail to resolve the host platform, it's not the end of the world.
+                        log::info!("rustc -vV did not report host platform, got:\n{}", stdout);
+                        None
+                    }
+                }
+                Err(e) => {
+                    log::warn!("Failed to discover host platform: {}", e);
+                    None
+                }
+            }
+        };
+        if let Some(target) = target {
+            meta.other_options(vec![String::from("--filter-platform"), target]);
         }
         let mut meta = meta.exec().with_context(|| {
             format!("Failed to run `cargo metadata --manifest-path {}`", cargo_toml.display())
-- 
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