Allow using -C force-unwind-tables=no when panic=unwind

compiler/rustc_session/src/session.rs

@@ -807,8 +807,11 @@ impl Session {
         // This is used to control the emission of the `uwtable` attribute on
         // LLVM functions.
         //
-        // At the very least, unwind tables are needed when compiling with
-        // `-C panic=unwind`.
+        // Unwind tables are needed when compiling with `-C panic=unwind`, but
+        // LLVM won't omit unwind tables unless the function is also marked as
+        // `nounwind`, so users are allowed to disable `uwtable` emission.
+        // Historically rustc always emits `uwtable` attributes by default, so
+        // even they can be disabled, they're still emitted by default.
         //
         // On some targets (including windows), however, exceptions include
         // other events such as illegal instructions, segfaults, etc. This means
@@ -821,13 +824,10 @@ impl Session {
         // If a target requires unwind tables, then they must be emitted.
         // Otherwise, we can defer to the `-C force-unwind-tables=<yes/no>`
         // value, if it is provided, or disable them, if not.
-        if self.panic_strategy() == PanicStrategy::Unwind {
-            true
-        } else if self.target.requires_uwtable {
-            true
-        } else {
-            self.opts.cg.force_unwind_tables.unwrap_or(self.target.default_uwtable)
-        }
+        self.target.requires_uwtable
+            || self.opts.cg.force_unwind_tables.unwrap_or(
+                self.panic_strategy() == PanicStrategy::Unwind || self.target.default_uwtable,
+            )
     }
 
     /// Returns the symbol name for the registrar function,
@@ -1483,13 +1483,6 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
 
     // Unwind tables cannot be disabled if the target requires them.
     if let Some(include_uwtables) = sess.opts.cg.force_unwind_tables {
-        if sess.panic_strategy() == PanicStrategy::Unwind && !include_uwtables {
-            sess.err(
-                "panic=unwind requires unwind tables, they cannot be disabled \
-                     with `-C force-unwind-tables=no`.",
-            );
-        }
-
         if sess.target.requires_uwtable && !include_uwtables {
             sess.err(
                 "target requires unwind tables, they cannot be disabled with \

src/test/assembly/panic-no-unwind-no-uwtable.rs

@@ -0,0 +1,8 @@
+// assembly-output: emit-asm
+// only-x86_64-unknown-linux-gnu
+// compile-flags: -C panic=unwind -C force-unwind-tables=n -O
+
+#![crate_type = "lib"]
+
+// CHECK-NOT: .cfi_startproc
+pub fn foo() {}

src/test/assembly/panic-unwind-no-uwtable.rs

@@ -0,0 +1,12 @@
+// assembly-output: emit-asm
+// only-x86_64-unknown-linux-gnu
+// compile-flags: -C panic=unwind -C force-unwind-tables=n
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: foo:
+// CHECK: .cfi_startproc
+#[no_mangle]
+fn foo() {
+    panic!();
+}

src/test/codegen/force-no-unwind-tables.rs

@@ -3,5 +3,9 @@
 
 #![crate_type="lib"]
 
+// CHECK-LABEL: define{{.*}}void @foo
 // CHECK-NOT: attributes #{{.*}} uwtable
-pub fn foo() {}
+#[no_mangle]
+fn foo() {
+    panic!();
+}

src/test/codegen/panic-unwind-default-uwtable.rs

@@ -0,0 +1,6 @@
+// compile-flags: -C panic=unwind -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+
+// CHECK: attributes #{{.*}} uwtable
+pub fn foo() {}

src/test/ui/unwind-no-uwtable.rs

@@ -0,0 +1,34 @@
+// run-pass
+// ignore-windows target requires uwtable
+// ignore-wasm32-bare no proper panic=unwind support
+// compile-flags: -C panic=unwind -C force-unwind-tables=n
+
+use std::panic::{self, AssertUnwindSafe};
+
+struct Increase<'a>(&'a mut u8);
+
+impl Drop for Increase<'_> {
+    fn drop(&mut self) {
+        *self.0 += 1;
+    }
+}
+
+#[inline(never)]
+fn unwind() {
+    panic!();
+}
+
+#[inline(never)]
+fn increase(count: &mut u8) {
+    let _increase = Increase(count);
+    unwind();
+}
+
+fn main() {
+    let mut count = 0;
+    assert!(panic::catch_unwind(AssertUnwindSafe(
+        #[inline(never)]
+        || increase(&mut count)
+    )).is_err());
+    assert_eq!(count, 1);
+}