new syntax for lvalue references: var b {.byaddr.} = expr (#13508)

* new syntax for lvalue references: `var b {.byaddr.} = expr`
* on type mismatch, `???(0, 0)` not shown anymore
* * compiler now lowers `var a: {.foo.}: MyType = expr` to foo(a, MyType, expr)
* new pragmas.byaddr defined in pure library code exploiting this lowering
* skip `template foo() {.pragma.}`

changelog.md

@@ -149,7 +149,7 @@ echo f
 - `std/oswalkdir` was buggy, it's now deprecated and reuses `std/os` procs
 - `net.newContext` now performs SSL Certificate checking on Linux and OSX.
   Define `nimDisableCertificateValidation` to disable it globally.
-
+- new syntax for lvalue references: `var b {.byaddr.} = expr` enabled by `import pragmas`
 
 ## Language additions
 
@@ -159,6 +159,9 @@ echo f
 - `=sink` type bound operator is now optional. Compiler can now use combination
   of `=destroy` and `copyMem` to move objects efficiently.
 
+- `var a {.foo.}: MyType = expr` now lowers to `foo(a, MyType, expr)` for non builtin pragmas,
+  enabling things like lvalue references, see `pragmas.byaddr`
+
 ## Language changes
 
 - Unsigned integer operators have been fixed to allow promotion of the first operand.