discuss (guaranteed) tail calls

Author: folkertdev

text/0000-cmse-calling-conventions.md

@@ -109,6 +109,13 @@ Currently both ABIs disallow the use of c-variadics. For `cmse-nonsecure-entry`,
 
 For `cmse-nonsecure-call`, we may support and stabilize c-variadics at some point in the future.
 
+## No tail calls
+
+Neither cmse ABI can support (guaranteed) tail calls, per the LLVM source:
+
+> For both the non-secure calls and the returns from a CMSE entry function, the function needs to do some extra work after the call, or before the return, respectively, thus it cannot end with a tail call
+
+ For `cmse-nonsecure-entry` we emit an error when `become` is used to call a function with this ABI. For `cmse-nonsecure-call` it is not (currently) possible to construct an example that triggers the error, because rust requires the caller and callee of a guaranteed tail call to have the same ABI, and `cmse-nonsecure-call` cannot be used in function definitions.
 ### Support for `const fn`
 
 No special support for calling cmse functions is needed.