Simplify Span by explicitly not supporting ctxt

[nzig]

Reading through the code, I was confused for a while because the code in libsyntax_pos/span_encoding.rs appears to support storing the SyntaxContext inside a compressed Span. However, no bits are actually allocated for SyntaxContext.

Since this is unlikely to ever be used, I think the code would be clearer for the next newcomer if this was simply removed.

See #44646
r? @michaelwoerister

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[kennytm]

cc @petrochenkov

[petrochenkov]

Let's keep it as is.
Today it's 0, tomorrow it can be some other value. There are some plans about changing span's size or the meaning of base.

[nzig]

If we don't change it then I'll add a comment explaining this. It's particularly confusing to me because INLINE_OFFSETS has an entry for CTXT_INDEX that is essentially an arbitrary value, because it has 0 size.

[petrochenkov]

Sure, feel free to add a comment.
Or maybe it would be better to add the ctxt field to the "picture"

// -------------------------------------------------------------
// | base 31:8  | len 7:1  | ctxt (currently 0 bits) | tag 0:0 |
// -------------------------------------------------------------

[nzig]

Should that be a separate pull request or should I modify this one?

[petrochenkov]

@nzig
There's not much difference, whatever is more convenient for you.

[nzig]

OK. I'll make a separate one.