Add inherent try_from and try_into methods to integer types

This allows these methods to be used without importing the corresponding traits.

This causes new `unused-imports` warnings for existing imports used only by method calls. This is a non-trivial amount of churn, maybe worth it?

The warnings can be fixed by removing imports (like this PR does in rustc and tests), or by adding `#[allow(unused_import)]` to them until the minimum supported Rust version is incremented to one that has this PR.

The new methods are insta-stable in order to avoid causing `unstable-name-collisions` warnings. These would be harder to deal with since they happen at calls sites rather than trait imports.

Author: SimonSapin

src/libcore/convert.rs

@@ -451,8 +451,6 @@ pub trait TryInto<T>: Sized {
 /// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
 ///
 /// ```
-/// use std::convert::TryFrom;
-///
 /// let big_number = 1_000_000_000_000i64;
 /// // Silently truncates `big_number`, requires detecting
 /// // and handling the truncation after the fact.

src/libcore/iter/range.rs

@@ -1,4 +1,3 @@
-use crate::convert::TryFrom;
 use crate::mem;
 use crate::ops::{self, Add, Sub, Try};
 use crate::usize;

src/libcore/num/dec2flt/rawfp.rs

@@ -18,7 +18,7 @@
 //! take the universally-correct slow path (Algorithm M) for very small and very large numbers.
 //! That algorithm needs only next_float() which does handle subnormals and zeros.
 use crate::cmp::Ordering::{Equal, Greater, Less};
-use crate::convert::{TryFrom, TryInto};
+use crate::convert::TryFrom;
 use crate::fmt::{Debug, LowerExp};
 use crate::num::dec2flt::num::{self, Big};
 use crate::num::dec2flt::table;

src/libcore/num/mod.rs

@@ -4,7 +4,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use crate::convert::TryFrom;
+use crate::convert::{TryFrom, TryInto};
 use crate::fmt;
 use crate::intrinsics;
 use crate::mem;
@@ -278,6 +278,28 @@ $EndFeature, "
             }
         }
 
+        doc_comment! {
+            "FIXME docs",
+            #[stable(feature = "int_inherent_try_from_try_into", since = "1.41.0")]
+            #[inline]
+            pub fn try_from<T>(value: T) -> Result<Self, <Self as TryFrom<T>>::Error>
+                where Self: TryFrom<T>
+            {
+                TryFrom::try_from(value)
+            }
+        }
+
+        doc_comment! {
+            "FIXME docs",
+            #[stable(feature = "int_inherent_try_from_try_into", since = "1.41.0")]
+            #[inline]
+            pub fn try_into<T>(self) -> Result<T, <Self as TryInto<T>>::Error>
+                where Self: TryInto<T>
+            {
+                TryInto::try_into(self)
+            }
+        }
+
         doc_comment! {
             concat!("Converts a string slice in a given base to an integer.
 
@@ -2340,6 +2362,28 @@ stringify!($MaxV), ");", $EndFeature, "
             pub const fn max_value() -> Self { !0 }
         }
 
+        doc_comment! {
+            "FIXME docs",
+            #[stable(feature = "int_inherent_try_from_try_into", since = "1.41.0")]
+            #[inline]
+            pub fn try_from<T>(value: T) -> Result<Self, <Self as TryFrom<T>>::Error>
+                where Self: TryFrom<T>
+            {
+                TryFrom::try_from(value)
+            }
+        }
+
+        doc_comment! {
+            "FIXME docs",
+            #[stable(feature = "int_inherent_try_from_try_into", since = "1.41.0")]
+            #[inline]
+            pub fn try_into<T>(self) -> Result<T, <Self as TryInto<T>>::Error>
+                where Self: TryInto<T>
+            {
+                TryInto::try_into(self)
+            }
+        }
+
         doc_comment! {
             concat!("Converts a string slice in a given base to an integer.
 

src/libcore/tests/iter.rs

@@ -1,5 +1,4 @@
 use core::cell::Cell;
-use core::convert::TryFrom;
 use core::iter::*;
 use core::{i8, i16, isize};
 use core::usize;

src/libcore/tests/num/mod.rs

@@ -1,4 +1,4 @@
-use core::convert::{TryFrom, TryInto};
+use core::convert::TryFrom;
 use core::cmp::PartialEq;
 use core::fmt::Debug;
 use core::marker::Copy;

src/librustc/mir/interpret/pointer.rs

@@ -5,7 +5,6 @@ use crate::ty::layout::{self, HasDataLayout, Size};
 
 use rustc_macros::HashStable;
 
-use std::convert::TryFrom;
 use std::fmt::{self, Display};
 
 /// Used by `check_in_alloc` to indicate context of check

src/librustc_apfloat/ieee.rs

@@ -2,7 +2,6 @@ use crate::{Category, ExpInt, IEK_INF, IEK_NAN, IEK_ZERO};
 use crate::{Float, FloatConvert, ParseError, Round, Status, StatusAnd};
 
 use core::cmp::{self, Ordering};
-use core::convert::TryFrom;
 use core::fmt::{self, Write};
 use core::marker::PhantomData;
 use core::mem;

src/librustc_metadata/rmeta/table.rs

@@ -2,7 +2,6 @@ use crate::rmeta::*;
 
 use rustc_index::vec::Idx;
 use rustc_serialize::{Encodable, opaque::Encoder};
-use std::convert::TryInto;
 use std::marker::PhantomData;
 use std::num::NonZeroUsize;
 use log::debug;

src/librustc_mir/build/matches/mod.rs

@@ -26,8 +26,6 @@ mod simplify;
 mod test;
 mod util;
 
-use std::convert::TryFrom;
-
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// Generates MIR for a `match` expression.
     ///