Track span of function in method calls, and use this in #[track_caller]

Fixes #69977

When we parse a chain of method calls like `foo.a().b().c()`, each
`MethodCallExpr` gets assigned a span that starts at the beginning of
the call chain (`foo`). While this is useful for diagnostics, it means
that `Location::caller` will return the same location for every call
in a call chain.

This PR makes us separately record the span of the function name and
arguments for a method call (e.g. `b()` in `foo.a().b().c()`). This
`Span` is passed through HIR lowering and MIR building to
`TerminatorKind::Call`, where it is used in preference to
`Terminator.source_info.span` when determining `Location::caller`.

This new span is also useful for diagnostics where we want to emphasize
a particular method call - for an example, see
https://github.com/rust-lang/rust/pull/72389#discussion_r436035990

1 files changed, 3 insertions, 1 deletions

diff --git a/src/librustc_save_analysis/dump_visitor.rs b/src/librustc_save_analysis/dump_visitor.rs
index 489d38b00df..39cb39ba619 100644
--- a/src/librustc_save_analysis/dump_visitor.rs
+++ b/src/librustc_save_analysis/dump_visitor.rs
@@ -1375,7 +1375,9 @@ impl<'l, 'tcx> Visitor<'tcx> for DumpVisitor<'l, 'tcx> {
                 let res = self.save_ctxt.get_path_res(hir_expr.hir_id);
                 self.process_struct_lit(ex, path, fields, adt.variant_of_res(res), *base)
             }
-            hir::ExprKind::MethodCall(ref seg, _, args) => self.process_method_call(ex, seg, args),
+            hir::ExprKind::MethodCall(ref seg, _, args, _) => {
+                self.process_method_call(ex, seg, args)
+            }
             hir::ExprKind::Field(ref sub_ex, _) => {
                 self.visit_expr(&sub_ex);