pretty-pretty extremal constants!

While many programmers may intuitively appreciate the significance of
"magic numbers" like −2147483648, Rust is about empowering everyone to
build reliable and efficient software! It's a bit more legible to
print the constant names (even noisy fully-qualified-paths thereof).

The bit-manipulation methods mirror those in
`librustc_mir::hair::pattern::_match::all_constructors`; thanks to the
immortal Varkor for guidance.

Resolves #56393.

src/librustc/ty/print/pretty.rs

@@ -6,12 +6,13 @@ use crate::middle::cstore::{ExternCrate, ExternCrateSource};
 use crate::middle::region;
 use crate::ty::{self, DefIdTree, ParamConst, Ty, TyCtxt, TypeFoldable};
 use crate::ty::subst::{Kind, Subst, UnpackedKind};
-use crate::ty::layout::Size;
-use crate::mir::interpret::{ConstValue, sign_extend, Scalar};
+use crate::ty::layout::{Integer, IntegerExt, Size};
+use crate::mir::interpret::{ConstValue, sign_extend, Scalar, truncate};
 use syntax::ast;
 use rustc_apfloat::ieee::{Double, Single};
 use rustc_apfloat::Float;
 use rustc_target::spec::abi::Abi;
+use syntax::attr::{SignedInt, UnsignedInt};
 use syntax::symbol::{kw, InternedString};
 
 use std::cell::Cell;
@@ -899,15 +900,31 @@ pub trait PrettyPrinter<'tcx>:
                     return Ok(self);
                 },
                 ty::Uint(ui) => {
-                    p!(write("{}{}", data, ui));
+                    let bit_size = Integer::from_attr(&self.tcx(), UnsignedInt(ui)).size();
+                    let max = truncate(u128::max_value(), bit_size);
+
+                    if data == max {
+                        p!(write("std::{}::MAX", ui))
+                    } else {
+                        p!(write("{}{}", data, ui))
+                    };
                     return Ok(self);
                 },
                 ty::Int(i) =>{
+                    let bit_size = Integer::from_attr(&self.tcx(), SignedInt(i))
+                        .size().bits() as u128;
+                    let min = 1u128 << (bit_size - 1);
+                    let max = min - 1;
+
                     let ty = self.tcx().lift_to_global(&ct.ty).unwrap();
                     let size = self.tcx().layout_of(ty::ParamEnv::empty().and(ty))
                         .unwrap()
                         .size;
-                    p!(write("{}{}", sign_extend(data, size) as i128, i));
+                    match data {
+                        d if d == min => p!(write("std::{}::MIN", i)),
+                        d if d == max => p!(write("std::{}::MAX", i)),
+                        _ => p!(write("{}{}", sign_extend(data, size) as i128, i))
+                    }
                     return Ok(self);
                 },
                 ty::Char => {

src/test/ui/consts/const-match-check.eval1.stderr

@@ -1,8 +1,8 @@
-error[E0005]: refutable pattern in local binding: `-2147483648i32..=-1i32` not covered
+error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` not covered
   --> $DIR/const-match-check.rs:25:15
    |
 LL |     A = { let 0 = 0; 0 },
-   |               ^ pattern `-2147483648i32..=-1i32` not covered
+   |               ^ pattern `std::i32::MIN..=-1i32` not covered
 
 error: aborting due to previous error
 

src/test/ui/consts/const-match-check.eval2.stderr

@@ -1,8 +1,8 @@
-error[E0005]: refutable pattern in local binding: `-2147483648i32..=-1i32` not covered
+error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` not covered
   --> $DIR/const-match-check.rs:31:24
    |
 LL |     let x: [i32; { let 0 = 0; 0 }] = [];
-   |                        ^ pattern `-2147483648i32..=-1i32` not covered
+   |                        ^ pattern `std::i32::MIN..=-1i32` not covered
 
 error: aborting due to previous error
 

src/test/ui/consts/const-match-check.matchck.stderr

@@ -1,26 +1,26 @@
-error[E0005]: refutable pattern in local binding: `-2147483648i32..=-1i32` not covered
+error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` not covered
   --> $DIR/const-match-check.rs:4:22
    |
 LL | const X: i32 = { let 0 = 0; 0 };
-   |                      ^ pattern `-2147483648i32..=-1i32` not covered
+   |                      ^ pattern `std::i32::MIN..=-1i32` not covered
 
-error[E0005]: refutable pattern in local binding: `-2147483648i32..=-1i32` not covered
+error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` not covered
   --> $DIR/const-match-check.rs:8:23
    |
 LL | static Y: i32 = { let 0 = 0; 0 };
-   |                       ^ pattern `-2147483648i32..=-1i32` not covered
+   |                       ^ pattern `std::i32::MIN..=-1i32` not covered
 
-error[E0005]: refutable pattern in local binding: `-2147483648i32..=-1i32` not covered
+error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` not covered
   --> $DIR/const-match-check.rs:13:26
    |
 LL |     const X: i32 = { let 0 = 0; 0 };
-   |                          ^ pattern `-2147483648i32..=-1i32` not covered
+   |                          ^ pattern `std::i32::MIN..=-1i32` not covered
 
-error[E0005]: refutable pattern in local binding: `-2147483648i32..=-1i32` not covered
+error[E0005]: refutable pattern in local binding: `std::i32::MIN..=-1i32` not covered
   --> $DIR/const-match-check.rs:19:26
    |
 LL |     const X: i32 = { let 0 = 0; 0 };
-   |                          ^ pattern `-2147483648i32..=-1i32` not covered
+   |                          ^ pattern `std::i32::MIN..=-1i32` not covered
 
 error: aborting due to 4 previous errors
 

src/test/ui/exhaustive_integer_patterns.stderr

@@ -10,11 +10,11 @@ note: lint level defined here
 LL | #![deny(unreachable_patterns)]
    |         ^^^^^^^^^^^^^^^^^^^^
 
-error[E0004]: non-exhaustive patterns: `128u8..=255u8` not covered
+error[E0004]: non-exhaustive patterns: `128u8..=std::u8::MAX` not covered
   --> $DIR/exhaustive_integer_patterns.rs:28:11
    |
 LL |     match x {
-   |           ^ pattern `128u8..=255u8` not covered
+   |           ^ pattern `128u8..=std::u8::MAX` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
@@ -32,19 +32,19 @@ error: unreachable pattern
 LL |         -2..=20 => {}
    |         ^^^^^^^
 
-error[E0004]: non-exhaustive patterns: `-128i8..=-8i8`, `-6i8`, `121i8..=124i8` and 1 more not covered
+error[E0004]: non-exhaustive patterns: `std::i8::MIN..=-8i8`, `-6i8`, `121i8..=124i8` and 1 more not covered
   --> $DIR/exhaustive_integer_patterns.rs:41:11
    |
 LL |     match x {
-   |           ^ patterns `-128i8..=-8i8`, `-6i8`, `121i8..=124i8` and 1 more not covered
+   |           ^ patterns `std::i8::MIN..=-8i8`, `-6i8`, `121i8..=124i8` and 1 more not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `-128i8` not covered
+error[E0004]: non-exhaustive patterns: `std::i8::MIN` not covered
   --> $DIR/exhaustive_integer_patterns.rs:82:11
    |
 LL |     match 0i8 {
-   |           ^^^ pattern `-128i8` not covered
+   |           ^^^ pattern `std::i8::MIN` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
@@ -56,19 +56,19 @@ LL |     match 0i16 {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `128u8..=255u8` not covered
+error[E0004]: non-exhaustive patterns: `128u8..=std::u8::MAX` not covered
   --> $DIR/exhaustive_integer_patterns.rs:108:11
    |
 LL |     match 0u8 {
-   |           ^^^ pattern `128u8..=255u8` not covered
+   |           ^^^ pattern `128u8..=std::u8::MAX` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `(0u8, Some(_))` and `(2u8..=255u8, Some(_))` not covered
+error[E0004]: non-exhaustive patterns: `(0u8, Some(_))` and `(2u8..=std::u8::MAX, Some(_))` not covered
   --> $DIR/exhaustive_integer_patterns.rs:120:11
    |
 LL |     match (0u8, Some(())) {
-   |           ^^^^^^^^^^^^^^^ patterns `(0u8, Some(_))` and `(2u8..=255u8, Some(_))` not covered
+   |           ^^^^^^^^^^^^^^^ patterns `(0u8, Some(_))` and `(2u8..=std::u8::MAX, Some(_))` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
@@ -80,19 +80,19 @@ LL |     match (0u8, true) {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `340282366920938463463374607431768211455u128` not covered
+error[E0004]: non-exhaustive patterns: `std::u128::MAX` not covered
   --> $DIR/exhaustive_integer_patterns.rs:145:11
    |
 LL |     match 0u128 {
-   |           ^^^^^ pattern `340282366920938463463374607431768211455u128` not covered
+   |           ^^^^^ pattern `std::u128::MAX` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `5u128..=340282366920938463463374607431768211455u128` not covered
+error[E0004]: non-exhaustive patterns: `5u128..=std::u128::MAX` not covered
   --> $DIR/exhaustive_integer_patterns.rs:149:11
    |
 LL |     match 0u128 {
-   |           ^^^^^ pattern `5u128..=340282366920938463463374607431768211455u128` not covered
+   |           ^^^^^ pattern `5u128..=std::u128::MAX` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 

src/test/ui/for/for-loop-refutable-pattern-error-message.stderr

@@ -1,8 +1,8 @@
-error[E0005]: refutable pattern in `for` loop binding: `&-2147483648i32..=0i32` not covered
+error[E0005]: refutable pattern in `for` loop binding: `&std::i32::MIN..=0i32` not covered
   --> $DIR/for-loop-refutable-pattern-error-message.rs:2:9
    |
 LL |     for &1 in [1].iter() {}
-   |         ^^ pattern `&-2147483648i32..=0i32` not covered
+   |         ^^ pattern `&std::i32::MIN..=0i32` not covered
 
 error: aborting due to previous error
 

src/test/ui/match/match-non-exhaustive.stderr

@@ -1,8 +1,8 @@
-error[E0004]: non-exhaustive patterns: `-2147483648i32..=0i32` and `2i32..=2147483647i32` not covered
+error[E0004]: non-exhaustive patterns: `std::i32::MIN..=0i32` and `2i32..=std::i32::MAX` not covered
   --> $DIR/match-non-exhaustive.rs:2:11
    |
 LL |     match 0 { 1 => () }
-   |           ^ patterns `-2147483648i32..=0i32` and `2i32..=2147483647i32` not covered
+   |           ^ patterns `std::i32::MIN..=0i32` and `2i32..=std::i32::MAX` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 

src/test/ui/non-exhaustive/non-exhaustive-match.rs

@@ -12,8 +12,8 @@ fn main() {
     match Some(10) { //~ ERROR non-exhaustive patterns: `Some(_)` not covered
       None => {}
     }
-    match (2, 3, 4) { //~ ERROR non-exhaustive patterns: `(_, _, -2147483648i32..=3i32)`
-                      //  and `(_, _, 5i32..=2147483647i32)` not covered
+    match (2, 3, 4) { //~ ERROR non-exhaustive patterns: `(_, _, std::i32::MIN..=3i32)`
+                      //  and `(_, _, 5i32..=std::i32::MAX)` not covered
       (_, _, 4) => {}
     }
     match (T::A, T::A) { //~ ERROR non-exhaustive patterns: `(A, A)` not covered

src/test/ui/non-exhaustive/non-exhaustive-match.stderr

@@ -28,11 +28,11 @@ LL |     match Some(10) {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `(_, _, -2147483648i32..=3i32)` and `(_, _, 5i32..=2147483647i32)` not covered
+error[E0004]: non-exhaustive patterns: `(_, _, std::i32::MIN..=3i32)` and `(_, _, 5i32..=std::i32::MAX)` not covered
   --> $DIR/non-exhaustive-match.rs:15:11
    |
 LL |     match (2, 3, 4) {
-   |           ^^^^^^^^^ patterns `(_, _, -2147483648i32..=3i32)` and `(_, _, 5i32..=2147483647i32)` not covered
+   |           ^^^^^^^^^ patterns `(_, _, std::i32::MIN..=3i32)` and `(_, _, 5i32..=std::i32::MAX)` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 

src/test/ui/precise_pointer_size_matching.stderr

@@ -1,16 +1,16 @@
-error[E0004]: non-exhaustive patterns: `$ISIZE_MIN..=-6isize` and `21isize..=$ISIZE_MAX` not covered
+error[E0004]: non-exhaustive patterns: `std::isize::MIN..=-6isize` and `21isize..=std::isize::MAX` not covered
   --> $DIR/precise_pointer_size_matching.rs:24:11
    |
 LL |     match 0isize {
-   |           ^^^^^^ patterns `$ISIZE_MIN..=-6isize` and `21isize..=$ISIZE_MAX` not covered
+   |           ^^^^^^ patterns `std::isize::MIN..=-6isize` and `21isize..=std::isize::MAX` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `0usize` and `21usize..=$USIZE_MAX` not covered
+error[E0004]: non-exhaustive patterns: `0usize` and `21usize..=std::usize::MAX` not covered
   --> $DIR/precise_pointer_size_matching.rs:29:11
    |
 LL |     match 0usize {
-   |           ^^^^^^ patterns `0usize` and `21usize..=$USIZE_MAX` not covered
+   |           ^^^^^^ patterns `0usize` and `21usize..=std::usize::MAX` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 

src/test/ui/refutable-pattern-errors.rs

@@ -3,5 +3,5 @@ fn func((1, (Some(1), 2..=3)): (isize, (Option<isize>, isize))) { }
 
 fn main() {
     let (1, (Some(1), 2..=3)) = (1, (None, 2));
-    //~^ ERROR refutable pattern in local binding: `(-2147483648i32..=0i32, _)` not covered
+    //~^ ERROR refutable pattern in local binding: `(std::i32::MIN..=0i32, _)` not covered
 }

src/test/ui/refutable-pattern-errors.stderr

@@ -4,11 +4,11 @@ error[E0005]: refutable pattern in function argument: `(_, _)` not covered
 LL | fn func((1, (Some(1), 2..=3)): (isize, (Option<isize>, isize))) { }
    |         ^^^^^^^^^^^^^^^^^^^^^ pattern `(_, _)` not covered
 
-error[E0005]: refutable pattern in local binding: `(-2147483648i32..=0i32, _)` not covered
+error[E0005]: refutable pattern in local binding: `(std::i32::MIN..=0i32, _)` not covered
   --> $DIR/refutable-pattern-errors.rs:5:9
    |
 LL |     let (1, (Some(1), 2..=3)) = (1, (None, 2));
-   |         ^^^^^^^^^^^^^^^^^^^^^ pattern `(-2147483648i32..=0i32, _)` not covered
+   |         ^^^^^^^^^^^^^^^^^^^^^ pattern `(std::i32::MIN..=0i32, _)` not covered
 
 error: aborting due to 2 previous errors