Rollup merge of rust-lang#128627 - khuey:DUMMY_SP-line-no, r=nnethercote

Special case DUMMY_SP to emit line 0/column 0 locations on DWARF platforms.

Line 0 has a special meaning in DWARF. From the version 5 spec:

    The compiler may emit the value 0 in cases
    where an instruction cannot be attributed to any
    source line.

DUMMY_SP spans cannot be attributed to any line. However, because rustc internally stores line numbers starting at zero, lookup_debug_loc() adjusts every line number by one. Special casing DUMMY_SP to actually emit line 0 ensures rustc communicates to the debugger that there's no meaningful source code for this instruction, rather than telling the debugger to jump to line 1 randomly.

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -570,7 +570,17 @@ impl<'ll, 'tcx> DebugInfoMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         inlined_at: Option<&'ll DILocation>,
         span: Span,
     ) -> &'ll DILocation {
-        let DebugLoc { line, col, .. } = self.lookup_debug_loc(span.lo());
+        // When emitting debugging information, DWARF (i.e. everything but MSVC)
+        // treats line 0 as a magic value meaning that the code could not be
+        // attributed to any line in the source. That's also exactly what dummy
+        // spans are. Make that equivalence here, rather than passing dummy spans
+        // to lookup_debug_loc, which will return line 1 for them.
+        let (line, col) = if span.is_dummy() && !self.sess().target.is_like_msvc {
+            (0, 0)
+        } else {
+            let DebugLoc { line, col, .. } = self.lookup_debug_loc(span.lo());
+            (line, col)
+        };
 
         unsafe { llvm::LLVMRustDIBuilderCreateDebugLocation(line, col, scope, inlined_at) }
     }

tests/debuginfo/dummy_span.rs

@@ -0,0 +1,45 @@
+//@ min-lldb-version: 310
+
+//@ compile-flags:-g
+
+// === GDB TESTS ===================================================================================
+
+// gdb-command:run 7
+
+// gdb-command:next
+// gdb-command:next
+// gdb-check:[...]#loc1[...]
+// gdb-command:next
+// gdb-check:[...]#loc2[...]
+
+// === LLDB TESTS ==================================================================================
+
+// lldb-command:run 7
+
+// lldb-command:next
+// lldb-command:next
+// lldb-command:frame select
+// lldb-check:[...]#loc1[...]
+// lldb-command:next
+// lldb-command:frame select
+// lldb-check:[...]#loc2[...]
+
+use std::env;
+use std::num::ParseIntError;
+
+fn main() -> Result<(), ParseIntError> {
+    let args = env::args();
+    let number_str = args.skip(1).next().unwrap();
+    let number = number_str.parse::<i32>()?;
+    zzz(); // #break
+    if number % 7 == 0 {
+        // This generates code with a dummy span for
+        // some reason. If that ever changes this
+        // test will not test what it wants to test.
+        return Ok(()); // #loc1
+    }
+    println!("{}", number);
+    Ok(())
+} // #loc2
+
+fn zzz() { () }