rustc_session: accumulate multiple -Zsanitizer target modifiers

When passing multiple `-Zsanitizer` flags to the compiler (e.g.,
`-Zsanitizer=address -Zsanitizer=shadow-call-stack`), the options parser
was overwriting the previous values in `target_modifiers` instead of
accumulating them. This resulted in only the last sanitizer being
recorded in the crate metadata's target modifiers, even though the
frontend correctly enabled all of them.

The only way to provide multiple sanitizers was to combine them into a
comma-separated list passed to a single `-Zsanitizer=` flag, but this
does not fit well into the GN build system where different targets may
pass different combinations of sanitizer flags.

Consequently, this caused spurious "incompatible target modifiers"
ABI mismatch errors when linking against dependencies compiled with
accumulated flags (e.g., `-Zsanitizer=address,shadow-call-stack`).

Fix this by entry-modifying the `target_modifiers` map to accumulate
the sanitizers as a comma-separated list when the option is
`sanitizer`.

Also add a codegen test verifying that both CFI and SafeStack
attributes/metadata are present when enabled together via multiple
flags.

Test: ./x.py test tests/codegen-llvm/sanitizer/multiple-sanitizers.rs

Author: PiJoules

compiler/rustc_session/src/options.rs

@@ -726,7 +726,25 @@ fn build_options<O: Default>(
                 }
                 if let Some(tmod) = *tmod {
                     let v = value.map_or(String::new(), ToOwned::to_owned);
-                    collected_options.target_modifiers.insert(tmod, v);
+
+                    // Accumulate all the -Zsanitizer flags into a single target modifier.
+                    match tmod {
+                        OptionsTargetModifiers::UnstableOptions(
+                            UnstableOptionsTargetModifiers::Sanitizer,
+                        ) => {
+                            collected_options
+                                .target_modifiers
+                                .entry(tmod)
+                                .and_modify(|existing| {
+                                    existing.push(',');
+                                    existing.push_str(&v);
+                                })
+                                .or_insert(v);
+                        }
+                        _ => {
+                            collected_options.target_modifiers.insert(tmod, v);
+                        }
+                    }
                 }
                 if let Some(mitigation) = mitigation {
                     collected_options.mitigations.reset_mitigation(*mitigation, index);

tests/codegen-llvm/sanitizer/multiple-sanitizers.rs

@@ -0,0 +1,16 @@
+// Verifies that multiple compatible sanitizers (CFI + SafeStack) can be enabled together
+// and their target modifiers accumulate instead of overwriting.
+//
+//@ only-x86_64
+//@ only-linux
+//@ compile-flags: -Zsanitizer=cfi -Zsanitizer=safestack -Clto -Ccodegen-units=1 -C unsafe-allow-abi-mismatch=sanitizer
+
+#![crate_type = "lib"]
+
+// CHECK: ; Function Attrs:{{.*}}safestack
+// CHECK: define{{.*}}foo{{.*}}!type
+pub fn foo(f: fn(i32) -> i32, arg: i32) -> i32 {
+    f(arg)
+}
+
+// CHECK: attributes #0 = {{.*}}safestack{{.*}}

tests/ui/target_modifiers/sanitizers-safestack-and-kcfi.missed_both.stderr

@@ -1,5 +1,5 @@
 error: mixing `-Zsanitizer` will cause an ABI mismatch in crate `sanitizers_safestack_and_kcfi`
-  --> $DIR/sanitizers-safestack-and-kcfi.rs:16:1
+  --> $DIR/sanitizers-safestack-and-kcfi.rs:18:1
    |
 LL | #![feature(no_core)]
    | ^

tests/ui/target_modifiers/sanitizers-safestack-and-kcfi.missed_kcfi.stderr

@@ -1,5 +1,5 @@
 error: mixing `-Zsanitizer` will cause an ABI mismatch in crate `sanitizers_safestack_and_kcfi`
-  --> $DIR/sanitizers-safestack-and-kcfi.rs:16:1
+  --> $DIR/sanitizers-safestack-and-kcfi.rs:18:1
    |
 LL | #![feature(no_core)]
    | ^

tests/ui/target_modifiers/sanitizers-safestack-and-kcfi.missed_safestack.stderr

@@ -1,5 +1,5 @@
 error: mixing `-Zsanitizer` will cause an ABI mismatch in crate `sanitizers_safestack_and_kcfi`
-  --> $DIR/sanitizers-safestack-and-kcfi.rs:16:1
+  --> $DIR/sanitizers-safestack-and-kcfi.rs:18:1
    |
 LL | #![feature(no_core)]
    | ^

tests/ui/target_modifiers/sanitizers-safestack-and-kcfi.rs

@@ -4,14 +4,16 @@
 //@ aux-build:safestack-and-kcfi.rs
 //@ compile-flags: -Cpanic=abort
 
-//@ revisions: good good_reverted missed_safestack missed_kcfi missed_both
+//@ revisions: good good_reverted good_multiple missed_safestack missed_kcfi missed_both
 //@[good] compile-flags: -Zsanitizer=safestack,kcfi
 //@[good_reverted] compile-flags: -Zsanitizer=kcfi,safestack
+//@[good_multiple] compile-flags: -Zsanitizer=safestack -Zsanitizer=kcfi
 //@[missed_safestack] compile-flags: -Zsanitizer=kcfi
 //@[missed_kcfi] compile-flags: -Zsanitizer=safestack
 // [missed_both] no additional compile-flags:
 //@[good] check-pass
 //@[good_reverted] check-pass
+//@[good_multiple] check-pass
 
 #![feature(no_core)]
 //[missed_safestack]~^ ERROR mixing `-Zsanitizer` will cause an ABI mismatch in crate `sanitizers_safestack_and_kcfi`