Auto merge of #33905 - eddyb:mir-overflow, r=nikomatsakis

[MIR] Implement overflow checking

The initial set of changes is from @Aatch's #33255 PR, rebased on master, plus:

Added an `Assert` terminator to MIR, to simplify working with overflow and bounds checks.
With this terminator, error cases can be accounted for directly, instead of looking for lang item calls.
It also keeps the MIR slimmer, with no extra explicit blocks for the actual panic calls.

Warnings can be produced when the `Assert` is known to always panic at runtime, e.g.:
```rust
warning: index out of bounds: the len is 1 but the index is 3
 --> <anon>:1:14
1 |> fn main() { &[std::io::stdout()][3]; }
  |>              ^^^^^^^^^^^^^^^^^^^^^^
```

Generalized the `OperandValue::FatPtr` optimization to any aggregate pair of immediates.
This allows us to generate the same IR for overflow checks as old trans, not something worse.
For example, addition on `i16` calls `llvm.sadd.with.overflow.i16`, which returns `{i16, i1}`.
However, the Rust type `(i16, bool)`, has to be `{i16, i8}`, only an immediate `bool` is `i1`.
But if we split the pair into an `i16` and an `i1`, we can pass them around as such for free.

The latest addition is a rebase of #34054, updated to work for pairs too. Closes #34054, fixes #33873.

Last but not least, the `#[rustc_inherit_overflow_checks]` attribute was introduced to control the
overflow checking behavior of generic or `#[inline]` functions, when translated in another crate.

It is **not** intended to be used by crates other than `libcore`, which is in the unusual position of
being distributed as only an optimized build with no checks, even when used from debug mode.
Before MIR-based translation, this worked out fine, as the decision for overflow was made at
translation time, in the crate being compiled, but MIR stored in `rlib` has to contain the checks.

To avoid always generating the checks and slowing everything down, a decision was made to
use an attribute in the few spots of `libcore` that need it (see #33255 for previous discussion):
* `core::ops::{Add, Sub, Mul, Neg, Shl, Shr}` implementations for integers, which have `#[inline]` methods and can be used in generic abstractions from other crates
* `core::ops::{Add, Sub, Mul, Neg, Shl, Shr}Assign` same as above, for augmented assignment
* `pow` and `abs` methods on integers, which intentionally piggy-back on built-in multiplication and negation, respectively, to get overflow checks
* `core::iter::{Iterator, Chain, Peek}::count` and `core::iter::Enumerate::{next, nth}`, also documented as panicking on overflow, from addition, counting elements of an iterator in an `usize`

Author: bors

src/libcore/iter/iterator.rs

@@ -172,6 +172,7 @@ pub trait Iterator {
     /// assert_eq!(a.iter().count(), 5);
     /// ```
     #[inline]
+    #[rustc_inherit_overflow_checks]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn count(self) -> usize where Self: Sized {
         // Might overflow.

src/libcore/iter/mod.rs

@@ -510,6 +510,7 @@ impl<A, B> Iterator for Chain<A, B> where
     }
 
     #[inline]
+    #[rustc_inherit_overflow_checks]
     fn count(self) -> usize {
         match self.state {
             ChainState::Both => self.a.count() + self.b.count(),
@@ -932,6 +933,7 @@ impl<I> Iterator for Enumerate<I> where I: Iterator {
     ///
     /// Might panic if the index of the element overflows a `usize`.
     #[inline]
+    #[rustc_inherit_overflow_checks]
     fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
         self.iter.next().map(|a| {
             let ret = (self.count, a);
@@ -947,6 +949,7 @@ impl<I> Iterator for Enumerate<I> where I: Iterator {
     }
 
     #[inline]
+    #[rustc_inherit_overflow_checks]
     fn nth(&mut self, n: usize) -> Option<(usize, I::Item)> {
         self.iter.nth(n).map(|a| {
             let i = self.count + n;
@@ -1008,6 +1011,7 @@ impl<I: Iterator> Iterator for Peekable<I> {
     }
 
     #[inline]
+    #[rustc_inherit_overflow_checks]
     fn count(self) -> usize {
         (if self.peeked.is_some() { 1 } else { 0 }) + self.iter.count()
     }