From 12f2bcde63285bf2b2f9f9ac615d4edf139a3f0e Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 18 Sep 2024 16:45:27 -0400
Subject: [PATCH] Check params for unsafety in THIR

---
 .../rustc_mir_build/src/check_unsafety.rs     | 14 ++++++++++++++
 tests/ui/unsafe/union-pat-in-param.rs         | 19 +++++++++++++++++++
 tests/ui/unsafe/union-pat-in-param.stderr     | 19 +++++++++++++++++++
 3 files changed, 52 insertions(+)
 create mode 100644 tests/ui/unsafe/union-pat-in-param.rs
 create mode 100644 tests/ui/unsafe/union-pat-in-param.stderr

diff --git a/compiler/rustc_mir_build/src/check_unsafety.rs b/compiler/rustc_mir_build/src/check_unsafety.rs
index c7fcfe3ce2a..57390dd58f4 100644
--- a/compiler/rustc_mir_build/src/check_unsafety.rs
+++ b/compiler/rustc_mir_build/src/check_unsafety.rs
@@ -218,6 +218,13 @@ fn visit_inner_body(&mut self, def: LocalDefId) {
                 warnings: self.warnings,
                 suggest_unsafe_block: self.suggest_unsafe_block,
             };
+            // params in THIR may be unsafe, e.g. a union pattern.
+            for param in &inner_thir.params {
+                if let Some(param_pat) = param.pat.as_deref() {
+                    inner_visitor.visit_pat(param_pat);
+                }
+            }
+            // Visit the body.
             inner_visitor.visit_expr(&inner_thir[expr]);
             // Unsafe blocks can be used in the inner body, make sure to take it into account
             self.safety_context = inner_visitor.safety_context;
@@ -1032,6 +1039,13 @@ pub(crate) fn check_unsafety(tcx: TyCtxt<'_>, def: LocalDefId) {
         warnings: &mut warnings,
         suggest_unsafe_block: true,
     };
+    // params in THIR may be unsafe, e.g. a union pattern.
+    for param in &thir.params {
+        if let Some(param_pat) = param.pat.as_deref() {
+            visitor.visit_pat(param_pat);
+        }
+    }
+    // Visit the body.
     visitor.visit_expr(&thir[expr]);
 
     warnings.sort_by_key(|w| w.block_span);
diff --git a/tests/ui/unsafe/union-pat-in-param.rs b/tests/ui/unsafe/union-pat-in-param.rs
new file mode 100644
index 00000000000..8454bfb20dc
--- /dev/null
+++ b/tests/ui/unsafe/union-pat-in-param.rs
@@ -0,0 +1,19 @@
+union U {
+    a: &'static i32,
+    b: usize,
+}
+
+fn fun(U { a }: U) {
+    //~^ ERROR access to union field is unsafe
+    dbg!(*a);
+}
+
+fn main() {
+    fun(U { b: 0 });
+
+    let closure = |U { a }| {
+        //~^ ERROR access to union field is unsafe
+        dbg!(*a);
+    };
+    closure(U { b: 0 });
+}
diff --git a/tests/ui/unsafe/union-pat-in-param.stderr b/tests/ui/unsafe/union-pat-in-param.stderr
new file mode 100644
index 00000000000..b9709b7cc9b
--- /dev/null
+++ b/tests/ui/unsafe/union-pat-in-param.stderr
@@ -0,0 +1,19 @@
+error[E0133]: access to union field is unsafe and requires unsafe function or block
+  --> $DIR/union-pat-in-param.rs:6:12
+   |
+LL | fn fun(U { a }: U) {
+   |            ^ access to union field
+   |
+   = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
+
+error[E0133]: access to union field is unsafe and requires unsafe function or block
+  --> $DIR/union-pat-in-param.rs:14:24
+   |
+LL |     let closure = |U { a }| {
+   |                        ^ access to union field
+   |
+   = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0133`.