Auto merge of #143613 - Enselic:panic-abort-uwtables, r=petrochenkov

Fix backtraces with `-C panic=abort` on linux; emit unwind tables by default

The linux backtrace unwinder relies on unwind tables to work properly, and generating and printing a backtrace is done by for example the default panic hook.

Begin emitting unwind tables by default again with `-C panic=abort` (see history below) so that backtraces work.

Closes https://github.com/rust-lang/rust/issues/81902 which is **regression-from-stable-to-stable**
Closes https://github.com/rust-lang/rust/issues/94815

### History

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 https://github.com/rust-lang/rust/pull/45031 and https://github.com/rust-lang/rust/issues/81902#issuecomment-3046487084).

In 1.45 a workaround in the form of `-C force-unwind-tables=yes` was added (see https://github.com/rust-lang/rust/pull/69984).

`-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 optimization 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.

# Release Note Entry Draft:

## Compatibility Notes

* [Fix backtraces with `-C panic=abort` on Linux by generating unwind tables by default](https://github.com/rust-lang/rust/pull/143613). Build with `-C force-unwind-tables=no` to keep omitting unwind tables.

try-job: aarch64-apple
try-job: armhf-gnu
try-job: aarch64-msvc-1

4 files changed, 10 insertions, 5 deletions

diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 25b46241c52..172672a80fb 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -758,7 +758,8 @@ impl Session {
         //     ELF x86-64 abi, but it can be disabled for some compilation units.
         //
         // Typically when we're compiling with `-C panic=abort` we don't need
-        // `uwtable` because we can't generate any exceptions!
+        // `uwtable` because we can't generate any exceptions! But note that
+        // some targets require unwind tables to generate backtraces.
         // 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.
diff --git a/compiler/rustc_target/src/spec/base/android.rs b/compiler/rustc_target/src/spec/base/android.rs
index 0426ea44c6a..df2757aaabf 100644
--- a/compiler/rustc_target/src/spec/base/android.rs
+++ b/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
 }
diff --git a/compiler/rustc_target/src/spec/base/linux.rs b/compiler/rustc_target/src/spec/base/linux.rs
index 9982c254eca..26e4590cf5e 100644
--- a/compiler/rustc_target/src/spec/base/linux.rs
+++ b/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,
diff --git a/compiler/rustc_target/src/spec/targets/arm_unknown_linux_gnueabihf.rs b/compiler/rustc_target/src/spec/targets/arm_unknown_linux_gnueabihf.rs
index a3f5389f0aa..0c711d5e71a 100644
--- a/compiler/rustc_target/src/spec/targets/arm_unknown_linux_gnueabihf.rs
+++ b/compiler/rustc_target/src/spec/targets/arm_unknown_linux_gnueabihf.rs
@@ -19,6 +19,11 @@ pub(crate) fn target() -> Target {
             max_atomic_width: Some(64),
             mcount: "\u{1}__gnu_mcount_nc".into(),
             llvm_mcount_intrinsic: Some("llvm.arm.gnu.eabi.mcount".into()),
+            // The default on linux is to have `default_uwtable=true`, but on
+            // this target we get an "`__aeabi_unwind_cpp_pr0` not defined"
+            // linker error, so set it to `true` here.
+            // FIXME(#146996): Remove this override once #146996 has been fixed.
+            default_uwtable: false,
             ..base::linux_gnu::opts()
         },
     }