tests: add sanity-check assembly test for every target

Adds a basic assembly test checking that each target can produce assembly
and update the target tier policy to require this.

Signed-off-by: David Wood <david@davidtw.co>

Author: davidtwco

Diff for: src/doc/rustc/src/target-tier-policy.md

@@ -246,6 +246,8 @@ approved by the appropriate team for that shared code before acceptance.
     introducing unconditional uses of features that another variation of the
     target may not have; use conditional compilation or runtime detection, as
     appropriate, to let each target run code supported by that target.
+- Tier 3 targets must be able to produce assembly using at least one of
+  rustc's supported backends from any host target.
 
 If a tier 3 target stops meeting these requirements, or the target maintainers
 no longer have interest or time, or the target shows no signs of activity and

Diff for: src/tools/tidy/src/lib.rs

@@ -70,6 +70,7 @@ pub mod pal;
 pub mod rustdoc_css_themes;
 pub mod rustdoc_gui_tests;
 pub mod style;
+pub mod target_policy;
 pub mod target_specific_tests;
 pub mod tests_placement;
 pub mod ui_tests;

Diff for: src/tools/tidy/src/main.rs

@@ -109,6 +109,7 @@ fn main() {
         // Checks that only make sense for the compiler.
         check!(error_codes, &root_path, &[&compiler_path, &librustdoc_path], verbose);
         check!(fluent_alphabetical, &compiler_path, bless);
+        check!(target_policy, &root_path);
 
         // Checks that only make sense for the std libs.
         check!(pal, &library_path);

Diff for: src/tools/tidy/src/target_policy.rs

@@ -0,0 +1,45 @@
+//! Tests for target tier policy compliance.
+//!
+//! As of writing, only checks that sanity-check assembly test for targets doesn't miss any targets.
+
+use crate::walk::{filter_not_rust, walk};
+use std::{collections::HashSet, path::Path};
+
+const TARGET_DEFINITIONS_PATH: &str = "compiler/rustc_target/src/spec/targets/";
+const ASSEMBLY_TEST_PATH: &str = "tests/assembly/targets.rs";
+const REVISION_LINE_START: &str = "// revisions: ";
+
+pub fn check(root_path: &Path, bad: &mut bool) {
+    let mut targets_to_find = HashSet::new();
+
+    let definitions_path = root_path.join(TARGET_DEFINITIONS_PATH);
+    for defn in ignore::WalkBuilder::new(&definitions_path)
+        .max_depth(Some(1))
+        .filter_entry(|e| !filter_not_rust(e.path()))
+        .build()
+    {
+        let defn = defn.unwrap();
+        // Skip directory itself.
+        if defn.path() == definitions_path {
+            continue;
+        }
+
+        let path = defn.path();
+        let target_name = path.file_stem().unwrap().to_string_lossy().into_owned();
+        let _ = targets_to_find.insert(target_name);
+    }
+
+    walk(&root_path.join(ASSEMBLY_TEST_PATH), |_, _| false, &mut |_, contents| {
+        for line in contents.lines() {
+            let Some(_) = line.find(REVISION_LINE_START) else {
+                continue;
+            };
+            let (_, target_name) = line.split_at(REVISION_LINE_START.len());
+            targets_to_find.remove(target_name);
+        }
+    });
+
+    for target in targets_to_find {
+        tidy_error!(bad, "{ASSEMBLY_TEST_PATH}: missing assembly test for {target}")
+    }
+}