[NFC] Add notes on self argument of borrow accessor being non-lexical

Author: meg-gupta

docs/SIL/Ownership.md

@@ -814,6 +814,11 @@ SILValue. Instead, it constrains the lifetime of an addressable
 variable. Since the constraint is applied to the in-memory
 representation, no additional lexical SILValue is required.
 
+The `self` function argument in a borrow accessor is not lexical.
+`self` is already borrowed for the duration of the borrow accessor and the
+dependency between borrowed return value and `self` will be represented in SIL,
+making lexical lifetime unnecessary.
+
 ### Deinit Barriers
 
 Deinit barriers (see Instruction.isDeinitBarrier(_:)) are instructions

lib/SILOptimizer/Utils/SILInliner.cpp

@@ -769,6 +769,8 @@ Scope scopeForArgument(Scope nonlexicalScope, SILValue callArg, unsigned index,
   }
 
   // Use non-lexical scope for functions returning @guaranteed results.
+  // TODO: Represent the SILFunctionArgument of borrow accessors as non-lexical
+  // during SILGen.
   if (callee->getConventions().hasGuaranteedResult()) {
     return nonlexicalScope;
   }