They're long dead since the switch to flexible targets, but was not
removed like their consumers were. Interesting they even got maintained
by various porters out there!

Technically [syntax-breaking] as they're public API, but since they're
unused in the compiler, the potential breakage IMO should be minimal.

The diagnostic emitter now accounts for labels with no text message,
presenting the underline on its own, without drawing the line for the
non existing message below it. Go from

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ----^^^^^^^----
  |   |   |
  |   |   `b` is a good letter
  |
```

to

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ----^^^^^^^----
  |       |
  |       `b` is a good letter
```

and from

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ^^^^-------^^^^
  |   |   |
  |   |
  |   `a` is a good letter
```

to

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ^^^^-------^^^^ `a` is a good letter
```

This calling convention is used to define interrup handlers on MSP430
microcontrollers. Usage looks like this:

``` rust
#[no_mangle]
#[link_section = "__interrupt_vector_10"]
pub static TIM0_VECTOR: unsafe extern "msp430-interrupt" fn() = tim0;

unsafe extern "msp430-interrupt" fn tim0() {
  P1OUT.write(0x00);
}
```

which generates the following assembly:

``` asm
Disassembly of section __interrupt_vector_10:

0000fff2 <TIM0_VECTOR>:
    fff2:       10 c0           interrupt service routine at 0xc010

Disassembly of section .text:

0000c010 <_ZN3msp4tim017h3193b957fd6a4fd4E>:
    c010:       c2 43 21 00     mov.b   #0,     &0x0021 ;r3 As==00
    c014:       00 13           reti
        ...
```

* Add support for `#[proc_macro]`

* Reactivate `proc_macro` feature and gate `#[proc_macro_attribute]` under it

* Have `#![feature(proc_macro)]` imply `#![feature(use_extern_macros)]`,
error on legacy import of proc macros via `#[macro_use]`

This is a clearer name since they represent [a, b, c] array literals.

This marks the pushpop_unsafe feature as removed inside the feature_gate.
It was added in commit 1829fa51 and then
removed again in commit d399098f .
Seems that the second commit forgot to mark it as removed in feature_gate.rs.

This enables us to remove another element from the whitelist of non gate
tested unstable lang features (issue #39059).

This removes the safe_suggestion feature from feature_gate.rs.
It was added in commit 164f0105
and then removed again in commit c11fe553 .

As the removal was in the same PR #38099 as the addition, we don't move it to
the "removed" section.

Removes an element from the whitelist of non gate tested unstable lang features (issue #39059).

CC #34761

This commit stabilizes the `proc_macro` and `proc_macro_lib` features in the
compiler to stabilize the "Macros 1.1" feature of the language. Many more
details can be found on the tracking issue, #35900.

Closes #35900

Stage 1 can’t really handle negative 128-bit literals, but an equivalent bit-not is fine

Dangling a carrot in front of a donkey.

This commit includes manual merge conflict resolution changes from a rebase by @est31.

This commit introduces 128-bit integers. Stage 2 builds and produces a working compiler which
understands and supports 128-bit integers throughout.

The general strategy used is to have rustc_i128 module which provides aliases for iu128, equal to
iu64 in stage9 and iu128 later. Since nowhere in rustc we rely on large numbers being supported,
this strategy is good enough to get past the first bootstrap stages to end up with a fully working
128-bit capable compiler.

In order for this strategy to work, number of locations had to be changed to use associated
max_value/min_value instead of MAX/MIN constants as well as the min_value (or was it max_value?)
had to be changed to use xor instead of shift so both 64-bit and 128-bit based consteval works
(former not necessarily producing the right results in stage1).

This commit includes manual merge conflict resolution changes from a rebase by @est31.

Historically this was done to accommodate bugs in lints, but there hasn't been a
bug in a lint since this feature was added which the warnings affected. Let's
completely purge warnings from all our stages by denying warnings in all stages.
This will also assist in tracking down `stage0` code to be removed whenever
we're updating the bootstrap compiler.