don't make empty ident when printing ' ident from extern "'"

compiler/rustc_span/src/symbol.rs

@@ -2463,9 +2463,10 @@ pub const STDLIB_STABLE_CRATES: &[Symbol] = &[sym::std, sym::core, sym::alloc, s
 
 #[derive(Copy, Clone, Eq, HashStable_Generic, Encodable, Decodable)]
 pub struct Ident {
-    // `name` should never be the empty symbol. If you are considering that,
-    // you are probably conflating "empty identifier with "no identifier" and
-    // you should use `Option<Ident>` instead.
+    /// `name` should never be the empty symbol. If you are considering that,
+    /// you are probably conflating "empty identifier with "no identifier" and
+    /// you should use `Option<Ident>` instead.
+    /// Trying to construct an `Ident` with an empty name will trigger debug assertions.
     pub name: Symbol,
     pub span: Span,
 }
@@ -2508,6 +2509,8 @@ impl Ident {
         Ident::new(self.name, span.with_ctxt(self.span.ctxt()))
     }
 
+    /// Creates a new ident with the same span and name with leading quote removed, if any.
+    /// If called on an empty ident, or with name just a single quote, returns an empty ident which is invalid.
     pub fn without_first_quote(self) -> Ident {
         Ident::new(Symbol::intern(self.as_str().trim_start_matches('\'')), self.span)
     }
@@ -3095,10 +3098,15 @@ impl Ident {
     }
 
     pub fn is_raw_lifetime_guess(self) -> bool {
-        let name_without_apostrophe = self.without_first_quote();
-        name_without_apostrophe.name != self.name
-            && name_without_apostrophe.name.can_be_raw()
-            && name_without_apostrophe.is_reserved_lifetime()
+        // Check that the name isn't just a single quote.
+        // `self.without_first_quote()` would return empty ident, which triggers debug assert.
+        if self.name.as_str() == "'" {
+            return false;
+        }
+        let ident_without_apostrophe = self.without_first_quote();
+        ident_without_apostrophe.name != self.name
+            && ident_without_apostrophe.name.can_be_raw()
+            && ident_without_apostrophe.is_reserved_lifetime()
     }
 
     pub fn guess_print_mode(self) -> IdentPrintMode {

tests/ui/extern/extern-single-quote-issue-147365.rs

@@ -0,0 +1,9 @@
+//@ needs-rustc-debug-assertions
+
+// https://github.com/rust-lang/rust/issues/147365
+// Ensures we don't trigger debug assert by creating an empty Ident when determining whether
+// the single quote is a raw lifetime.
+
+extern "'" {} //~ ERROR invalid ABI: found `'`
+
+fn main() {}

tests/ui/extern/extern-single-quote-issue-147365.stderr

@@ -0,0 +1,16 @@
+error[E0703]: invalid ABI: found `'`
+  --> $DIR/extern-single-quote-issue-147365.rs:7:8
+   |
+LL | extern "'" {}
+   |        ^^^ invalid ABI
+   |
+   = note: invoke `rustc --print=calling-conventions` for a full list of supported calling conventions
+help: there's a similarly named valid ABI `C`
+   |
+LL - extern "'" {}
+LL + extern "C" {}
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0703`.