Add Config::without_compiler

[fitzgen]

This provides us an allocation-free path for constructing a Config.

Why not rely on builds when cfg(not(any(feature = "cranelift", feature = "winch")))? Because we need to test our various OOM-handling and allocation-free code paths in this workspace, and without some way to create a config without a compiler otherwise, cargo's feature resolver will enable those features in the workspace, enabling the compiler in the config, but we don't intend to make compiler configurations handle OOM.

[alexcrichton]

Bikeshed on this: WDYT about:

• Leave Config::new as the only constructor
• Add a Config::enable_compiler(&mut self, bool) method
• Defer the default Cranelift configuration to happening in build_compiler

That way we keep just a single constructor and this, in theory, would still compose well with other options. (and try_compiler would still remain)

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[fitzgen]

Defer the default Cranelift configuration to happening in build_compiler

This seems kind of difficult since we don't really know if our defaults were overridden with other Config::cranelift_flag_{enable,set} calls or whatever unless we start doing more introspection on the given flags and duplicating state between Config and the compiler, which feels pretty nasty.

[alexcrichton]

Could the defaults be pushed into the Cranelift/Winch compiler builder then? Adding a new constructor is a really heavy hammer I'd prefer to avoid, so I'd ideally like to push pretty hard on solving the problems of using the existing constructor

[fitzgen]

Could the defaults be pushed into the Cranelift/Winch compiler builder then? Adding a new constructor is a really heavy hammer I'd prefer to avoid, so I'd ideally like to push pretty hard on solving the problems of using the existing constructor

Done, want to take another look?

[alexcrichton]

Thanks!