Special-case .llvm in mangler to fix segfaults

This commit special cases `.llvm` in the mangler to print `.llvm$6d$`
instead. This will avoid segfaults when names in a user's Rust code are
`llvm`.

src/librustc_codegen_utils/symbol_names.rs

@@ -613,6 +613,9 @@ impl fmt::Write for SymbolPrinter<'_, '_> {
                 // for ':' and '-'
                 '-' | ':' => self.path.temp_buf.push('.'),
 
+                // Avoid segmentation fault on some platforms, see #60925.
+                'm' if self.path.temp_buf.ends_with(".llv") => self.path.temp_buf.push_str("$6d$"),
+
                 // These are legal symbols
                 'a'..='z' | 'A'..='Z' | '0'..='9' | '_' | '.' | '$' => self.path.temp_buf.push(c),
 

src/test/ui/issue-53912.rs

@@ -0,0 +1,37 @@
+// compile-pass
+
+// This test is the same code as in ui/symbol-names/issue-60925.rs but this checks that the
+// reproduction compiles successfully and doesn't segfault, whereas that test just checks that the
+// symbol mangling fix produces the correct result.
+
+fn dummy() {}
+
+mod llvm {
+    pub(crate) struct Foo;
+}
+mod foo {
+    pub(crate) struct Foo<T>(T);
+
+    impl Foo<::llvm::Foo> {
+        pub(crate) fn foo() {
+            for _ in 0..0 {
+                for _ in &[::dummy()] {
+                    ::dummy();
+                    ::dummy();
+                    ::dummy();
+                }
+            }
+        }
+    }
+
+    pub(crate) fn foo() {
+        Foo::foo();
+        Foo::foo();
+    }
+}
+
+pub fn foo() {
+    foo::foo();
+}
+
+fn main() {}

src/test/ui/symbol-names/issue-60925.rs

@@ -0,0 +1,39 @@
+#![feature(rustc_attrs)]
+
+// This test is the same code as in ui/issue-53912.rs but this test checks that the symbol mangling
+// fix produces the correct result, whereas that test just checks that the reproduction compiles
+// successfully and doesn't segfault
+
+fn dummy() {}
+
+mod llvm {
+    pub(crate) struct Foo;
+}
+mod foo {
+    pub(crate) struct Foo<T>(T);
+
+    impl Foo<::llvm::Foo> {
+        #[rustc_symbol_name]
+//~^ ERROR _ZN11issue_609253foo36Foo$LT$issue_60925..llv$6d$..Foo$GT$3foo17h059a991a004536adE
+        pub(crate) fn foo() {
+            for _ in 0..0 {
+                for _ in &[::dummy()] {
+                    ::dummy();
+                    ::dummy();
+                    ::dummy();
+                }
+            }
+        }
+    }
+
+    pub(crate) fn foo() {
+        Foo::foo();
+        Foo::foo();
+    }
+}
+
+pub fn foo() {
+    foo::foo();
+}
+
+fn main() {}

src/test/ui/symbol-names/issue-60925.stderr

@@ -0,0 +1,8 @@
+error: symbol-name(_ZN11issue_609253foo36Foo$LT$issue_60925..llv$6d$..Foo$GT$3foo17h059a991a004536adE)
+  --> $DIR/issue-60925.rs:16:9
+   |
+LL |         #[rustc_symbol_name]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+