Auto merge of #88759 - Amanieu:panic_in_drop, r=nagisa,eddyb

Add -Z panic-in-drop={unwind,abort} command-line option

This PR changes `Drop` to abort if an unwinding panic attempts to escape it, making the process abort instead. This has several benefits:
- The current behavior when unwinding out of `Drop` is very unintuitive and easy to miss: unwinding continues, but the remaining drops in scope are simply leaked.
- A lot of unsafe code doesn't expect drops to unwind, which can lead to unsoundness:
  - https://github.com/servo/rust-smallvec/issues/14
  - https://github.com/bluss/arrayvec/issues/3
- There is a code size and compilation time cost to this: LLVM needs to generate extra landing pads out of all calls in a drop implementation. This can compound when functions are inlined since unwinding will then continue on to process drops in the callee, which can itself unwind, etc.
  - Initial measurements show a 3% size reduction and up to 10% compilation time reduction on some crates (`syn`).

One thing to note about `-Z panic-in-drop=abort` is that *all* crates must be built with this option for it to be sound since it makes the compiler assume that dropping `Box<dyn Any>` will never unwind.

cc https://github.com/rust-lang/lang-team/issues/97

1 files changed, 14 insertions, 2 deletions

diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 447be84b5a7..bb29a87035e 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -349,6 +349,7 @@ mod desc {
     pub const parse_threads: &str = parse_number;
     pub const parse_passes: &str = "a space-separated list of passes, or `all`";
     pub const parse_panic_strategy: &str = "either `unwind` or `abort`";
+    pub const parse_opt_panic_strategy: &str = parse_panic_strategy;
     pub const parse_relro_level: &str = "one of: `full`, `partial`, or `off`";
     pub const parse_sanitizers: &str =
         "comma separated list of sanitizers: `address`, `hwaddress`, `leak`, `memory` or `thread`";
@@ -549,7 +550,7 @@ mod parse {
         }
     }
 
-    crate fn parse_panic_strategy(slot: &mut Option<PanicStrategy>, v: Option<&str>) -> bool {
+    crate fn parse_opt_panic_strategy(slot: &mut Option<PanicStrategy>, v: Option<&str>) -> bool {
         match v {
             Some("unwind") => *slot = Some(PanicStrategy::Unwind),
             Some("abort") => *slot = Some(PanicStrategy::Abort),
@@ -558,6 +559,15 @@ mod parse {
         true
     }
 
+    crate fn parse_panic_strategy(slot: &mut PanicStrategy, v: Option<&str>) -> bool {
+        match v {
+            Some("unwind") => *slot = PanicStrategy::Unwind,
+            Some("abort") => *slot = PanicStrategy::Abort,
+            _ => return false,
+        }
+        true
+    }
+
     crate fn parse_relro_level(slot: &mut Option<RelroLevel>, v: Option<&str>) -> bool {
         match v {
             Some(s) => match s.parse::<RelroLevel>() {
@@ -958,7 +968,7 @@ options! {
         "optimization level (0-3, s, or z; default: 0)"),
     overflow_checks: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "use overflow checks for integer arithmetic"),
-    panic: Option<PanicStrategy> = (None, parse_panic_strategy, [TRACKED],
+    panic: Option<PanicStrategy> = (None, parse_opt_panic_strategy, [TRACKED],
         "panic strategy to compile crate with"),
     passes: Vec<String> = (Vec::new(), parse_list, [TRACKED],
         "a list of extra LLVM passes to run (space separated)"),
@@ -1186,6 +1196,8 @@ options! {
         "pass `-install_name @rpath/...` to the macOS linker (default: no)"),
     panic_abort_tests: bool = (false, parse_bool, [TRACKED],
         "support compiling tests with panic=abort (default: no)"),
+    panic_in_drop: PanicStrategy = (PanicStrategy::Unwind, parse_panic_strategy, [TRACKED],
+        "panic strategy for panics in drops"),
     parse_only: bool = (false, parse_bool, [UNTRACKED],
         "parse only; do not compile, assemble, or link (default: no)"),
     partially_uninit_const_threshold: Option<usize> = (None, parse_opt_number, [TRACKED],