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.

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.
     ///

src/librustc_mir/build/matches/util.rs

@@ -4,7 +4,6 @@ use crate::hair::*;
 use rustc::mir::*;
 use smallvec::SmallVec;
 use std::u32;
-use std::convert::TryInto;
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub fn field_match_pairs<'pat>(

src/librustc_mir/const_eval.rs

@@ -5,7 +5,6 @@ use std::error::Error;
 use std::borrow::{Borrow, Cow};
 use std::hash::Hash;
 use std::collections::hash_map::Entry;
-use std::convert::TryInto;
 
 use rustc::hir::def::DefKind;
 use rustc::hir::def_id::DefId;

src/librustc_mir/hair/pattern/_match.rs

@@ -253,7 +253,6 @@ use arena::TypedArena;
 
 use smallvec::{smallvec, SmallVec};
 use std::cmp::{self, max, min, Ordering};
-use std::convert::TryInto;
 use std::fmt;
 use std::iter::{FromIterator, IntoIterator};
 use std::ops::RangeInclusive;

src/librustc_mir/interpret/operand.rs

@@ -1,8 +1,6 @@
 //! Functions concerning immediate values and operands, and reading from operands.
 //! All high-level functions to read from memory work on operands as sources.
 
-use std::convert::{TryInto, TryFrom};
-
 use rustc::{mir, ty};
 use rustc::ty::layout::{
     self, Size, LayoutOf, TyLayout, HasDataLayout, IntegerExt, PrimitiveExt, VariantIdx,

src/librustc_mir/interpret/place.rs

@@ -2,7 +2,6 @@
 //! into a place.
 //! All high-level functions to write to memory work on places as destinations.
 
-use std::convert::TryFrom;
 use std::hash::Hash;
 
 use rustc::mir;

src/librustc_mir/util/elaborate_drops.rs

@@ -10,8 +10,6 @@ use rustc::ty::util::IntTypeExt;
 use rustc_index::vec::Idx;
 use crate::util::patch::MirPatch;
 
-use std::convert::TryInto;
-
 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 pub enum DropFlagState {
     Present, // i.e., initialized

src/librustc_parse/lexer/mod.rs

@@ -9,7 +9,6 @@ use rustc_lexer::Base;
 use rustc_lexer::unescape;
 
 use std::char;
-use std::convert::TryInto;
 use rustc_data_structures::sync::Lrc;
 use log::debug;
 

src/libstd/io/cursor.rs

@@ -3,8 +3,6 @@ use crate::io::prelude::*;
 use crate::cmp;
 use crate::io::{self, Initializer, SeekFrom, Error, ErrorKind, IoSlice, IoSliceMut};
 
-use core::convert::TryInto;
-
 /// A `Cursor` wraps an in-memory buffer and provides it with a
 /// [`Seek`] implementation.
 ///

src/libstd/sys/unix/fs.rs

@@ -752,7 +752,6 @@ impl File {
 
         #[cfg(not(target_os = "android"))]
         {
-            use crate::convert::TryInto;
             let size: off64_t = size
                 .try_into()
                 .map_err(|e| io::Error::new(io::ErrorKind::InvalidInput, e))?;

src/libstd/sys/unix/time.rs

@@ -4,7 +4,6 @@ use crate::time::Duration;
 use core::hash::{Hash, Hasher};
 
 pub use self::inner::{Instant, SystemTime, UNIX_EPOCH};
-use crate::convert::TryInto;
 
 const NSEC_PER_SEC: u64 = 1_000_000_000;
 

src/libstd/sys/windows/time.rs

@@ -3,7 +3,6 @@ use crate::fmt;
 use crate::mem;
 use crate::sys::c;
 use crate::time::Duration;
-use crate::convert::TryInto;
 
 use core::hash::{Hash, Hasher};
 

src/test/ui/const-generics/invalid-const-arg-for-type-param.rs

@@ -1,9 +1,8 @@
-use std::convert::TryInto;
-
 struct S;
 
 fn main() {
     let _: u32 = 5i32.try_into::<32>().unwrap(); //~ ERROR wrong number of const arguments
+    //~^ ERROR wrong number of type arguments
     S.f::<0>(); //~ ERROR no method named `f`
     S::<0>; //~ ERROR  wrong number of const arguments
 }

src/test/ui/const-generics/invalid-const-arg-for-type-param.stderr

@@ -1,11 +1,17 @@
 error[E0107]: wrong number of const arguments: expected 0, found 1
-  --> $DIR/invalid-const-arg-for-type-param.rs:6:34
+  --> $DIR/invalid-const-arg-for-type-param.rs:4:34
    |
 LL |     let _: u32 = 5i32.try_into::<32>().unwrap();
    |                                  ^^ unexpected const argument
 
+error[E0107]: wrong number of type arguments: expected 1, found 0
+  --> $DIR/invalid-const-arg-for-type-param.rs:4:23
+   |
+LL |     let _: u32 = 5i32.try_into::<32>().unwrap();
+   |                       ^^^^^^^^ expected 1 type argument
+
 error[E0599]: no method named `f` found for type `S` in the current scope
-  --> $DIR/invalid-const-arg-for-type-param.rs:7:7
+  --> $DIR/invalid-const-arg-for-type-param.rs:6:7
    |
 LL | struct S;
    | --------- method `f` not found for this
@@ -14,12 +20,12 @@ LL |     S.f::<0>();
    |       ^ method not found in `S`
 
 error[E0107]: wrong number of const arguments: expected 0, found 1
-  --> $DIR/invalid-const-arg-for-type-param.rs:8:9
+  --> $DIR/invalid-const-arg-for-type-param.rs:7:9
    |
 LL |     S::<0>;
    |         ^ unexpected const argument
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0107, E0599.
 For more information about an error, try `rustc --explain E0107`.

src/test/ui/numeric/numeric-cast.fixed

@@ -1,8 +1,5 @@
 // run-rustfix
 
-// The `try_into` suggestion doesn't include this, but we do suggest it after applying it
-use std::convert::TryInto;
-
 fn foo<N>(_x: N) {}
 
 fn main() {

src/test/ui/numeric/numeric-cast.rs

@@ -1,8 +1,5 @@
 // run-rustfix
 
-// The `try_into` suggestion doesn't include this, but we do suggest it after applying it
-use std::convert::TryInto;
-
 fn foo<N>(_x: N) {}
 
 fn main() {