Add feature to set path-based visibility of macro_rules to pub by default

[yaahc]

Bare minimum impl to probe for issues that need to be resolved before I go and add the feature flag machinery.

This has already exposed a shortcoming in our unreachable_pub lint where it is erroneously firing for some internal macros as pub even tho they don't have path-based scoping on their own.

experiment for #148610

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

error: unreachable `pub` item
  --> library/core/src/num/mod.rs:85:1
   |
85 | macro_rules! u8_xe_bytes_doc {
   | -^^^^^^^^^^^^^^^^^^^^^^^^^^^
   | |
   | help: consider restricting its visibility: `pub(crate)`
   |
   = help: or consider exporting it for use by other crates

error: unreachable `pub` item
  --> library/core/src/num/mod.rs:97:1
   |
97 | macro_rules! i8_xe_bytes_doc {
   | -^^^^^^^^^^^^^^^^^^^^^^^^^^^
   | |
   | help: consider restricting its visibility: `pub(crate)`
   |
   = help: or consider exporting it for use by other crates

error: unreachable `pub` item
   --> library/core/src/num/mod.rs:110:1
    |
110 | macro_rules! usize_isize_to_xe_bytes_doc {
    | -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

error: unreachable `pub` item
   --> library/core/src/num/mod.rs:121:1
    |
121 | macro_rules! usize_isize_from_xe_bytes_doc {
    | -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

---

error: unreachable `pub` item
    --> library/core/src/num/mod.rs:1599:17
     |
1402 | / macro_rules! from_str_int_impl {
1403 | |     ($signedness:ident $($int_ty:ty)+) => {$(
1404 | |         #[stable(feature = "rust1", since = "1.0.0")]
1405 | |         #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
...    |
1599 | |                 macro_rules! unwrap_or_PIE {
     | |                 -^^^^^^^^^^^^^^^^^^^^^^^^^
     | |                 |
     | |                 help: consider restricting its visibility: `pub(crate)`
...    |
1663 | |     )*}
1664 | | }
     | |_- in this expansion of `from_str_int_impl!`
1665 |
1666 |   from_str_int_impl! { signed isize i8 i16 i32 i64 i128 }
     |   ------------------------------------------------------- in this macro invocation
     |
     = help: or consider exporting it for use by other crates

error: unreachable `pub` item
    --> library/core/src/num/mod.rs:1617:21
     |
1402 | / macro_rules! from_str_int_impl {
1403 | |     ($signedness:ident $($int_ty:ty)+) => {$(
1404 | |         #[stable(feature = "rust1", since = "1.0.0")]
1405 | |         #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
...    |
1617 | |                     macro_rules! run_unchecked_loop {
     | |                     -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     | |                     |
     | |                     help: consider restricting its visibility: `pub(crate)`
...    |
1663 | |     )*}
1664 | | }
     | |_- in this expansion of `from_str_int_impl!`
1665 |
1666 |   from_str_int_impl! { signed isize i8 i16 i32 i64 i128 }
     |   ------------------------------------------------------- in this macro invocation
     |
     = help: or consider exporting it for use by other crates

error: unreachable `pub` item
    --> library/core/src/num/mod.rs:1633:21
     |
1402 | / macro_rules! from_str_int_impl {
1403 | |     ($signedness:ident $($int_ty:ty)+) => {$(
1404 | |         #[stable(feature = "rust1", since = "1.0.0")]
1405 | |         #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
...    |
1633 | |                     macro_rules! run_checked_loop {
     | |                     -^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     | |                     |
     | |                     help: consider restricting its visibility: `pub(crate)`
...    |
1663 | |     )*}
1664 | | }
     | |_- in this expansion of `from_str_int_impl!`
1665 |
1666 |   from_str_int_impl! { signed isize i8 i16 i32 i64 i128 }
     |   ------------------------------------------------------- in this macro invocation
     |
     = help: or consider exporting it for use by other crates

error: unreachable `pub` item
    --> library/core/src/num/mod.rs:1599:17
     |
1402 | / macro_rules! from_str_int_impl {
1403 | |     ($signedness:ident $($int_ty:ty)+) => {$(
1404 | |         #[stable(feature = "rust1", since = "1.0.0")]
1405 | |         #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
...    |
1599 | |                 macro_rules! unwrap_or_PIE {
     | |                 -^^^^^^^^^^^^^^^^^^^^^^^^^
     | |                 |
     | |                 help: consider restricting its visibility: `pub(crate)`
...    |
1663 | |     )*}
1664 | | }
     | |_- in this expansion of `from_str_int_impl!`
...
1667 |   from_str_int_impl! { unsigned usize u8 u16 u32 u64 u128 }
     |   --------------------------------------------------------- in this macro invocation
     |
     = help: or consider exporting it for use by other crates

error: unreachable `pub` item
    --> library/core/src/num/mod.rs:1617:21
     |
1402 | / macro_rules! from_str_int_impl {
1403 | |     ($signedness:ident $($int_ty:ty)+) => {$(
1404 | |         #[stable(feature = "rust1", since = "1.0.0")]
1405 | |         #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
...    |
1617 | |                     macro_rules! run_unchecked_loop {
     | |                     -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     | |                     |
     | |                     help: consider restricting its visibility: `pub(crate)`
...    |
1663 | |     )*}
1664 | | }
     | |_- in this expansion of `from_str_int_impl!`
...
1667 |   from_str_int_impl! { unsigned usize u8 u16 u32 u64 u128 }
     |   --------------------------------------------------------- in this macro invocation
     |
     = help: or consider exporting it for use by other crates

error: unreachable `pub` item
    --> library/core/src/num/mod.rs:1633:21
     |
1402 | / macro_rules! from_str_int_impl {
1403 | |     ($signedness:ident $($int_ty:ty)+) => {$(
1404 | |         #[stable(feature = "rust1", since = "1.0.0")]
1405 | |         #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
...    |
1633 | |                     macro_rules! run_checked_loop {
     | |                     -^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     | |                     |
     | |                     help: consider restricting its visibility: `pub(crate)`
...    |
1663 | |     )*}
1664 | | }
     | |_- in this expansion of `from_str_int_impl!`
...
1667 |   from_str_int_impl! { unsigned usize u8 u16 u32 u64 u128 }
     |   --------------------------------------------------------- in this macro invocation
     |
     = help: or consider exporting it for use by other crates

error: unreachable `pub` item
---

error: unreachable `pub` item
  --> library/core/src/cmp/bytewise.rs:74:1
   |
74 | macro_rules! is_bytewise_comparable_array_length {
   | -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   | |
   | help: consider restricting its visibility: `pub(crate)`
   |
   = help: or consider exporting it for use by other crates

---

error: unreachable `pub` item
   --> library/core/src/ops/arith.rs:480:1
    |
480 | macro_rules! div_impl_integer {
    | -^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

error: unreachable `pub` item
   --> library/core/src/ops/arith.rs:509:1
    |
509 | macro_rules! div_impl_float {
    | -^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

---

error: unreachable `pub` item
   --> library/core/src/ops/bit.rs:479:1
    |
479 | macro_rules! shl_impl {
    | -^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

---

error: unreachable `pub` item
   --> library/core/src/ops/bit.rs:601:1
    |
601 | macro_rules! shr_impl {
    | -^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

error: unreachable `pub` item
   --> library/core/src/ops/bit.rs:621:1
    |
621 | macro_rules! shr_impl_all {
    | -^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

---

error: unreachable `pub` item
   --> library/core/src/ops/bit.rs:815:1
    |
815 | macro_rules! bitor_assign_impl {
    | -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

error: unreachable `pub` item
   --> library/core/src/ops/bit.rs:890:1
    |
890 | macro_rules! bitxor_assign_impl {
    | -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

---

error: unreachable `pub` item
    --> library/core/src/ops/bit.rs:1041:1
     |
1041 | macro_rules! shr_assign_impl {
     | -^^^^^^^^^^^^^^^^^^^^^^^^^^^
     | |
     | help: consider restricting its visibility: `pub(crate)`
     |
     = help: or consider exporting it for use by other crates

---

error: unreachable `pub` item
   --> library/core/src/iter/adapters/zip.rs:152:1
    |
152 | macro_rules! zip_impl_general_defaults {
    | -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | help: consider restricting its visibility: `pub(crate)`
    |
    = help: or consider exporting it for use by other crates

---

error: unreachable `pub` item
    --> library/core/src/net/ip_addr.rs:2403:1
     |
2403 | macro_rules! bitop_impls {
     | -^^^^^^^^^^^^^^^^^^^^^^^
     | |
     | help: consider restricting its visibility: `pub(crate)`
     |
     = help: or consider exporting it for use by other crates

---

error: unreachable `pub` item
    --> library/core/src/sync/atomic.rs:2581:1
     |
2581 | macro_rules! if_8_bit {
     | -^^^^^^^^^^^^^^^^^^^^
     | |
     | help: consider restricting its visibility: `pub(crate)`
     |
     = help: or consider exporting it for use by other crates