Auto merge of #84373 - cjgillot:resolve-span, r=michaelwoerister,petrochenkov

Encode spans relative to the enclosing item

The aim of this PR is to avoid recomputing queries when code is moved without modification.

MCP at https://github.com/rust-lang/compiler-team/issues/443

This is achieved by :
1. storing the HIR owner LocalDefId information inside the span;
2. encoding and decoding spans relative to the enclosing item in the incremental on-disk cache;
3. marking a dependency to the `source_span(LocalDefId)` query when we translate a span from the short (`Span`) representation to its explicit (`SpanData`) representation.

Since all client code uses `Span`, step 3 ensures that all manipulations
of span byte positions actually create the dependency edge between
the caller and the `source_span(LocalDefId)`.
This query return the actual absolute span of the parent item.
As a consequence, any source code motion that changes the absolute byte position of a node will either:
- modify the distance to the parent's beginning, so change the relative span's hash;
- dirty `source_span`, and trigger the incremental recomputation of all code that
  depends on the span's absolute byte position.

With this scheme, I believe the dependency tracking to be accurate.

For the moment, the spans are marked during lowering.
I'd rather do this during def-collection,
but the AST MutVisitor is not practical enough just yet.
The only difference is that we attach macro-expanded spans
to their expansion point instead of the macro itself.

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -196,7 +196,11 @@ impl CoverageSpan {
     /// body_span), returns the macro name symbol.
     pub fn visible_macro(&self, body_span: Span) -> Option<Symbol> {
         if let Some(current_macro) = self.current_macro() {
-            if self.expn_span.parent().unwrap_or_else(|| bug!("macro must have a parent")).ctxt()
+            if self
+                .expn_span
+                .parent_callsite()
+                .unwrap_or_else(|| bug!("macro must have a parent"))
+                .ctxt()
                 == body_span.ctxt()
             {
                 return Some(current_macro);