Use unsafex.StringAlias in linker.pathKey

[jhump]

This fixes the old reflection-based approach to convert a slice to an array (so it is comparable and can be used as a map key) to instead use unsafex.StringAlias. This elides a copy that was inadvertently happening in the old implementation and is also hopefully simpler to read and understand than the old way (though it does mean that keys in the map will be less scrutable in a debugger since the underlying slice data will make for completely unreadable/junk strings).

[mcy]

I am assuming you're cool with the risk here that Go Protobuf mutates a particular SourcePath, yeah?

[jhump]

I am assuming you're cool with the risk here that Go Protobuf mutates a particular SourcePath, yeah?

Yeah you're right, this needs to clone the slice. For some reason I was thinking this was used differently. I was mistakenly thinking of how I used this in the old compiler, where the slice that is used to create the key is internal, and we can ensure it's never exposed or mutated. But in this case, they come directly from a mutable *descriptorpb.SourceCodeInfo, so we can't rely on them not changing.

Reworking...

[jhump]

Reworking...

@mcy, I made changes that should make this safe in the face of the input path slices being mutated.

[jhump]

Previously, it looks like we indexed the last location for a given path (loc.Next == 0).

The new code indexes the first one, which I believe is what protobuf-go does and is much more useful since the caller can then use Next in the returned location to traverse other locations for the same path.