Rollup merge of #82849 - camsteffen:option-get-or-default, r=joshtriplett

Add Option::get_or_default

Tracking issue: #82901

The original issue is #55042, which was closed, but for an invalid reason (see discussion there). Opening this to reconsider (I hope that's okay). It seems like the only gap for `Option` being "entry-like".

I ran into a need for this method where I had a `Vec<Option<MyData>>` and wanted to do `vec[n].get_or_default().my_data_method()`. Using an `Option` as an inner component of a data structure is probably where the need for this will normally arise.

compiler/rustc_mir/src/lib.rs

@@ -25,6 +25,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(stmt_expr_attributes)]
 #![feature(trait_alias)]
 #![feature(option_expect_none)]
+#![feature(option_get_or_default)]
 #![feature(or_patterns)]
 #![feature(once_cell)]
 #![feature(control_flow_enum)]

compiler/rustc_mir/src/transform/coverage/graph.rs

@@ -392,10 +392,7 @@ impl BasicCoverageBlockData {
             }
         }
         let operand = counter_kind.as_operand_id();
-        if let Some(replaced) = self
-            .edge_from_bcbs
-            .get_or_insert_with(FxHashMap::default)
-            .insert(from_bcb, counter_kind)
+        if let Some(replaced) = self.edge_from_bcbs.get_or_default().insert(from_bcb, counter_kind)
         {
             Error::from_string(format!(
                 "attempt to set an edge counter more than once; from_bcb: \