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.}`

lib/std/pragmas.nim

@@ -0,0 +1,19 @@
+# see `semLowerLetVarCustomPragma` for compiler support that enables these
+# lowerings
+
+template byaddr*(lhs, typ, expr) =
+  ## Allows a syntax for lvalue reference, exact analog to
+  ## `auto& a = expr;` in C++
+  runnableExamples:
+    var s = @[10,11,12]
+    var a {.byaddr.} = s[0]
+    a+=100
+    doAssert s == @[110,11,12]
+    doAssert a is int
+    var b {.byaddr.}: int = s[0]
+    doAssert a.addr == b.addr
+  when typ is type(nil):
+    let tmp = addr(expr)
+  else:
+    let tmp: ptr typ = addr(expr)
+  template lhs: untyped = tmp[]