Auto merge of #51564 - SimonSapin:try-int, r=alexcrichton

Implement always-fallible TryFrom for usize/isize conversions that are infallible on some platforms

This reverts commit 837d6c70233715a0ae8e15c703d40e3046a2f36a "Remove TryFrom impls that might become conditionally-infallible with a portability lint".

This fixes #49415 by adding (restoring) missing `TryFrom` impls for integer conversions to or from `usize` or `isize`, by making them always fallible at the type system level (that is, with `Error=TryFromIntError`) even though they happen to be infallible on some platforms (for some values of `size_of::<usize>()`).

They had been removed to allow the possibility to conditionally having some of them be infallible `From` impls instead, depending on the platforms, and have the [portability lint](https://github.com/rust-lang/rfcs/pull/1868) warn when they are used in code that is not already opting into non-portability. For example `#[allow(some_lint)] usize::from(x: u64)` would be valid on code that only targets 64-bit platforms.

This PR gives up on this possiblity for two reasons:

* Based on discussion with @aturon, it seems that the portability lint is not happening any time soon. It’s better to have the conversions be available *at all* than keep blocking them for so long. Portability-lint-gated platform-specific APIs can always be added separately later.

* For code that is fine with fallibility, the alternative would force it to opt into "non-portability" even though there would be no real portability issue.

1 files changed, 2 insertions, 18 deletions

diff --git a/src/libstd/io/cursor.rs b/src/libstd/io/cursor.rs
index 8ac52572810..aadd33b3954 100644
--- a/src/libstd/io/cursor.rs
+++ b/src/libstd/io/cursor.rs
@@ -10,6 +10,7 @@
 
 use io::prelude::*;
 
+use core::convert::TryInto;
 use cmp;
 use io::{self, Initializer, SeekFrom, Error, ErrorKind};
 
@@ -259,26 +260,9 @@ fn slice_write(pos_mut: &mut u64, slice: &mut [u8], buf: &[u8]) -> io::Result<us
     Ok(amt)
 }
 
-/// Compensate removal of some impls per
-/// https://github.com/rust-lang/rust/pull/49305#issuecomment-376293243
-#[cfg(any(target_pointer_width = "16",
-          target_pointer_width = "32"))]
-fn try_into(n: u64) -> Result<usize, ()> {
-    if n <= (<usize>::max_value() as u64) {
-        Ok(n as usize)
-    } else {
-        Err(())
-    }
-}
-
-#[cfg(any(target_pointer_width = "64"))]
-fn try_into(n: u64) -> Result<usize, ()> {
-    Ok(n as usize)
-}
-
 // Resizing write implementation
 fn vec_write(pos_mut: &mut u64, vec: &mut Vec<u8>, buf: &[u8]) -> io::Result<usize> {
-    let pos: usize = try_into(*pos_mut).map_err(|_| {
+    let pos: usize = (*pos_mut).try_into().map_err(|_| {
         Error::new(ErrorKind::InvalidInput,
                     "cursor position exceeds maximum possible vector length")
     })?;