astgen: clean up source line calculation and management

Clarify that `astgen.advanceSourceCursor` already increments absolute
values of the line and columns numbers; i.e., `GenZir.calcLine` is thus
not only obsolete but wrong by design.

Incidentally, this clean up allows for specifying the `FnDecl` line
numbers for DWARF use correctly as relative values with respect to
the start of the parent `Decl`. This `Decl` in turn has its line number
information specified relatively to its parent `Decl`, and so on, until
we reach the global scope.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/Module.zig b/src/Module.zig
index 35b7efb016..e2e2505927 100644
--- a/src/Module.zig
+++ b/src/Module.zig
@@ -375,6 +375,7 @@ pub const Decl = struct {
     src_node: Ast.Node.Index,
     /// Line number corresponding to `src_node`. Stored separately so that source files
     /// do not need to be loaded into memory in order to compute debug line numbers.
+    /// This value is absolute.
     src_line: u32,
     /// Index to ZIR `extra` array to the entry in the parent's decl structure
     /// (the part that says "for every decls_len"). The first item at this index is
@@ -4122,7 +4123,8 @@ fn scanDecl(iter: *ScanDeclIter, decl_sub_index: usize, flags: u4) SemaError!voi
     const has_linksection_or_addrspace = (flags & 0b1000) != 0;
     // zig fmt: on
 
-    const line = iter.parent_decl.relativeToLine(zir.extra[decl_sub_index + 4]);
+    const line_off = zir.extra[decl_sub_index + 4];
+    const line = iter.parent_decl.relativeToLine(line_off);
     const decl_name_index = zir.extra[decl_sub_index + 5];
     const decl_index = zir.extra[decl_sub_index + 6];
     const decl_block_inst_data = zir.instructions.items(.data)[decl_index].pl_node;