toggle-source-debugging sets certain compiler parameters, and also turns leaf case optimizations on (when called with nil) or off (when called with t). For all these parameters, the value nil reduces compilation speed.
toggle-source-debugging is called in the configuration file a-dot-lispworks.lisp, and the initial state of LispWorks such that source level debugging is on.
The parameters relate to information required for source level debugging, cross-referencing and finding all changed definitions.
The parameters (all in the compiler package) are:
When true, the compiler produces information for the Cross Referencer.
When true, the cross-referencing information produced by the compiler is loaded when the corresponding file is loaded.
When true, the Cross Referencer notices when a function is redefined, including an interpreted redefinition..
When true, the compiler generates information used by the debugger.
toggle-source-debugging modifies the status of the variables, and then returns the new value. To check whether all the variables are set to true, without modifying them, use source-debugging-on-p.
Cross-referencing information is used by the functions who-calls, who-binds, who-references, who-sets, and friends.
In LispWorks 4.2 and earlier, toggle-source-debugging controlled source file recording information. In LispWorks 4.3 and later, this is controlled independently by *record-source-files*.
LispWorks User Guide and Reference Manual - 20 Sep 2017