> Generally this is a bit nicer than having explicit lambdas, but I thought the 'best-case' scenario would be if GCC saw into the stack layout of the calling function and could manipulate the calling functions stored stack variables (and saved registers).

A modern compiler IR is probably going to be an SSA-based infinite virtual register set model. In such a model, any variable without its address taken ends up being a register (which may happen to be spilled to the stack). Referencing the variable via a nested function means its a local variable whose address escapes, which kills a lot of optimization potential. It's probably possible to adjust SSA to handle this, but it's a lot of work for little benefit, especially since closure models (closures being regular objects with an unnameable type and an overloaded call operator) have taken over nested functions in language design and thus it isn't really applicable for modern languages.

> After all, a debugger can track what variable goes where at every line of code, so this can be done.

Variable value tracking breaks down pretty much the moment any optimization happens.