WIP let a {.rom.} = constExpr

[timotheecour]

WIP proposal as alternative to the buggy let a = constExpr.unsafeAddr refs nim-lang#16794

note: const with ref types should be merged as-is (refs nim-lang#15528) and doesn't interfere with this proposal

nim-lang/RFCs#257 can be fixed in a way that doesn't break the type system as follows:

proposal 1a (mutually exclusive with 1b)

let b {.rom.} = [1,2] # b ends in ROM memory
func fn(): int = b[0] # no side effect thanks to {.rom.}
let b2 = b[0].addr
const b3 = b

IMO semantics for 1a is better than 1b, because semantically, b{.rom.} is closer to a let (gets generated in backend, has backend layout etc);

proposal 1b

const a = [1,2]
const b {.addressable.} = [1,2]
func fn(): int = b[0] # no side effect thanks to {.rom.}
let b2 = b[0].addr
const b3 = b

proposal 2 (can be done independently of 1a or 1b)

const a = [1,2]
let b {.immutable.} = [1,2]
func fn(): int = b[0] # no side effect thanks to {.rom.}
let b2 = b[0].addr

note: that variant would also allow RT values inside definition of b eg:

var x = 2
let b {.immutable.} = [1, x]

but this would preclude using b at CT in subsequent expressions

notes

can be implemented using sections, refs https://www.keil.com/support/man/docs/armclang_ref/armclang_ref_cmf1493390581125.htm

int x1 = 5;                     // Goes in .data section (default)
int y1;                         // Goes in .bss section (default)
const int z1 = 42;              // Goes in .rodata section (default)
char *s1 = "abc1";              // s1 goes in .data section (default). String "abc1" goes in .conststring section.

#pragma clang section bss="myBSS" data="myData" rodata="myRodata"
int x2 = 5;                     // Goes in myData section.
int y2;                         // Goes in myBss section.
const int z2 = 42;              // Goes in myRodata section.
char *s2 = "abc2";              // s2 goes in myData section. String "abc2" goes in .conststring section.

note

use this instead of the more hacky prepareStrMutation (nim-lang#17213) (refs nim-lang#17173)

[juancarlospaco]

rom