Emit unwind tables by default with -Cpanic=abort on linux

Otherwise backtraces (in e.g. the default panic hook) are incomplete.

More details

The linux backtrace unwinder relies on unwind tables to work properly.
The default panic hook prints backtraces. Backtraces with
`-C panic=abort` used to work in Rust 1.22 but broke in Rust 1.23, because
in 1.23 we stopped emitting unwind tables with `-C panic=abort` (see
24cc38e).

In 1.45 (see cda9946) a workaround in the form of
`-C force-unwind-tables=yes` was added.

More history

`-C panic=abort` was added in [Rust
1.10](https://blog.rust-lang.org/2016/07/07/Rust-1.10/#what-s-in-1-10-stable)
and the motivation was binary size and compile time. But given how
confusing that behavior has turned out to be, it is better to make
binary size optimzation opt-in with `-C force-unwind-tables=no` rather
than default since the current default breaks backtraces.

Besides, if binary size is a primary concern, there are many other
tricks that can be used that has a higher impact.

TODO:
- [ ] better commit message
- [ ] more tests?
- [ ] figure out how to present this in release notes
- [ ] check if all tests pass

Author: Enselic

compiler/rustc_session/src/session.rs

@@ -802,10 +802,10 @@ 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.
+        //
+        // FIXME: Update the above comment
         self.target.requires_uwtable
-            || self.opts.cg.force_unwind_tables.unwrap_or(
-                self.panic_strategy() == PanicStrategy::Unwind || self.target.default_uwtable,
-            )
+            || self.opts.cg.force_unwind_tables.unwrap_or(self.target.default_uwtable)
     }
 
     /// Returns the number of query threads that should be used for this

compiler/rustc_target/src/spec/base/android.rs

@@ -8,10 +8,6 @@ pub(crate) fn opts() -> TargetOptions {
     base.tls_model = TlsModel::Emulated;
     base.has_thread_local = false;
     base.supported_sanitizers = SanitizerSet::ADDRESS;
-    // This is for backward compatibility, see https://github.com/rust-lang/rust/issues/49867
-    // for context. (At that time, there was no `-C force-unwind-tables`, so the only solution
-    // was to always emit `uwtable`).
-    base.default_uwtable = true;
     base.crt_static_respected = true;
     base
 }

compiler/rustc_target/src/spec/base/linux.rs

@@ -12,6 +12,9 @@ pub(crate) fn opts() -> TargetOptions {
         relro_level: RelroLevel::Full,
         has_thread_local: true,
         crt_static_respected: true,
+        // We want backtraces to work by default and they rely on unwind tables
+        // (regardless of `-C panic` strategy).
+        default_uwtable: true,
         supported_split_debuginfo: Cow::Borrowed(&[
             SplitDebuginfo::Packed,
             SplitDebuginfo::Unpacked,

tests/run-make/panic-abort-eh_frame/rmake.rs

@@ -20,5 +20,5 @@ fn main() {
         .edition("2021")
         .arg("-Zvalidate-mir")
         .run();
-    llvm_objdump().arg("--dwarf=frames").input("foo.o").run().assert_stdout_not_contains("DW_CFA");
+    llvm_objdump().arg("--dwarf=frames").input("foo.o").run().assert_stdout_contains("DW_CFA");
 }