v.pref: support v -trace-calls run file.v (#15450)

vlib/v/transformer/transformer.v

@@ -226,6 +226,7 @@ pub fn (mut t Transformer) stmt(mut node ast.Stmt) ast.Stmt {
 			}
 		}
 		ast.FnDecl {
+			t.fn_decl(mut node)
 			t.index.indent(true)
 			for mut stmt in node.stmts {
 				stmt = t.stmt(mut stmt)
@@ -1034,3 +1035,51 @@ pub fn (mut t Transformer) sql_expr(mut node ast.SqlExpr) ast.Expr {
 	}
 	return node
 }
+
+// fn_decl mutates `node`.
+// if `pref.trace_calls` is true ast Nodes for `eprintln(...)` is prepended to the `FnDecl`'s
+// stmts list to let the gen backend generate the target specific code for the print.
+pub fn (mut t Transformer) fn_decl(mut node ast.FnDecl) {
+	if t.pref.trace_calls {
+		// Skip `C.fn()` and all of builtin
+		// builtin could probably be traced also but would need
+		// special cases for, at least, println/eprintln
+		if node.no_body || node.is_builtin {
+			return
+		}
+		call_expr := t.gen_trace_print_call_expr(node)
+		expr_stmt := ast.ExprStmt{
+			expr: call_expr
+		}
+		node.stmts.prepend(expr_stmt)
+	}
+}
+
+// gen_trace_print_expr_stmt generates an ast.CallExpr representation of a
+// `eprint(...)` V code statement.
+fn (t Transformer) gen_trace_print_call_expr(node ast.FnDecl) ast.CallExpr {
+	print_str := '> trace ' + node.stringify(t.table, node.mod, map[string]string{})
+
+	call_arg := ast.CallArg{
+		expr: ast.StringLiteral{
+			val: print_str
+		}
+		typ: ast.string_type_idx
+	}
+	args := [call_arg]
+
+	fn_name := 'eprintln'
+	call_expr := ast.CallExpr{
+		name: fn_name
+		args: args
+		mod: node.mod
+		pos: node.pos
+		language: node.language
+		scope: node.scope
+		comments: [ast.Comment{
+			text: 'fn $node.short_name trace call'
+		}]
+	}
+
+	return call_expr
+}