Auto merge of #50912 - varkor:exhaustive-integer-matching, r=arielb1

Exhaustive integer matching

This adds a new feature flag `exhaustive_integer_patterns` that enables exhaustive matching of integer types by their values. For example, the following is now accepted:
```rust
#![feature(exhaustive_integer_patterns)]
#![feature(exclusive_range_pattern)]

fn matcher(x: u8) {
  match x { // ok
    0 .. 32 => { /* foo */ }
    32 => { /* bar */ }
    33 ..= 255 => { /* baz */ }
  }
}
```
This matching is permitted on all integer (signed/unsigned and char) types. Sensible error messages are also provided. For example:
```rust
fn matcher(x: u8) {
  match x { //~ ERROR
    0 .. 32 => { /* foo */ }
  }
}
```
results in:
```
error[E0004]: non-exhaustive patterns: `32u8...255u8` not covered
 --> matches.rs:3:9
  |
6 |   match x {
  |         ^ pattern `32u8...255u8` not covered
```

This implements https://github.com/rust-lang/rfcs/issues/1550 for https://github.com/rust-lang/rust/issues/50907. While there hasn't been a full RFC for this feature, it was suggested that this might be a feature that obviously complements the existing exhaustiveness checks (e.g. for `bool`) and so a feature gate would be sufficient for now.

src/test/ui/exhaustive_integer_patterns.rs

@@ -0,0 +1,173 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(exhaustive_integer_patterns)]
+#![feature(exclusive_range_pattern)]
+#![deny(unreachable_patterns)]
+
+use std::{char, usize, u8, u16, u32, u64, u128, isize, i8, i16, i32, i64, i128};
+
+fn main() {
+    let x: u8 = 0;
+
+    // A single range covering the entire domain.
+    match x {
+        0 ..= 255 => {} // ok
+    }
+
+    // A combination of ranges and values.
+    // These are currently allowed to be overlapping.
+    match x {
+        0 ..= 32 => {}
+        33 => {}
+        34 .. 128 => {}
+        100 ..= 200 => {}
+        200 => {} //~ ERROR unreachable pattern
+        201 ..= 255 => {}
+    }
+
+    // An incomplete set of values.
+    match x { //~ ERROR non-exhaustive patterns
+        0 .. 128 => {}
+    }
+
+    // A more incomplete set of values.
+    match x { //~ ERROR non-exhaustive patterns
+        0 ..= 10 => {}
+        20 ..= 30 => {}
+        35 => {}
+        70 .. 255 => {}
+    }
+
+    let x: i8 = 0;
+    match x { //~ ERROR non-exhaustive patterns
+        -7 => {}
+        -5..=120 => {}
+        -2..=20 => {} //~ ERROR unreachable pattern
+        125 => {}
+    }
+
+    // Let's test other types too!
+    let c: char = '\u{0}';
+    match c {
+        '\u{0}' ..= char::MAX => {} // ok
+    }
+
+    // We can actually get away with just covering the
+    // following two ranges, which correspond to all
+    // valid Unicode Scalar Values.
+    match c {
+        '\u{0000}' ..= '\u{D7FF}' => {}
+        '\u{E000}' ..= '\u{10_FFFF}' => {}
+    }
+
+    match 0usize {
+        0 ..= usize::MAX => {} // ok
+    }
+
+    match 0u16 {
+        0 ..= u16::MAX => {} // ok
+    }
+
+    match 0u32 {
+        0 ..= u32::MAX => {} // ok
+    }
+
+    match 0u64 {
+        0 ..= u64::MAX => {} // ok
+    }
+
+    match 0u128 {
+        0 ..= u128::MAX => {} // ok
+    }
+
+    match 0isize {
+        isize::MIN ..= isize::MAX => {} // ok
+    }
+
+    match 0i8 {
+        -128 ..= 127 => {} // ok
+    }
+
+    match 0i8 { //~ ERROR non-exhaustive patterns
+        -127 ..= 127 => {}
+    }
+
+    match 0i16 {
+        i16::MIN ..= i16::MAX => {} // ok
+    }
+
+    match 0i16 { //~ ERROR non-exhaustive patterns
+        i16::MIN ..= -1 => {}
+        1 ..= i16::MAX => {}
+    }
+
+    match 0i32 {
+        i32::MIN ..= i32::MAX => {} // ok
+    }
+
+    match 0i64 {
+        i64::MIN ..= i64::MAX => {} // ok
+    }
+
+    match 0i128 {
+        i128::MIN ..= i128::MAX => {} // ok
+    }
+
+    // Make sure that guards don't factor into the exhaustiveness checks.
+    match 0u8 { //~ ERROR non-exhaustive patterns
+        0 .. 128 => {}
+        128 ..= 255 if true => {}
+    }
+
+    match 0u8 {
+        0 .. 128 => {}
+        128 ..= 255 if false => {}
+        128 ..= 255 => {} // ok, because previous arm was guarded
+    }
+
+    // Now things start getting a bit more interesting. Testing products!
+    match (0u8, Some(())) { //~ ERROR non-exhaustive patterns
+        (1, _) => {}
+        (_, None) => {}
+    }
+
+    match (0u8, true) { //~ ERROR non-exhaustive patterns
+        (0 ..= 125, false) => {}
+        (128 ..= 255, false) => {}
+        (0 ..= 255, true) => {}
+    }
+
+    match (0u8, true) { // ok
+        (0 ..= 125, false) => {}
+        (128 ..= 255, false) => {}
+        (0 ..= 255, true) => {}
+        (125 .. 128, false) => {}
+    }
+
+    match 0u8 { // ok
+        0 .. 2 => {}
+        1 ..= 2 => {}
+        _ => {}
+    }
+
+    const LIM: u128 = u128::MAX - 1;
+    match 0u128 { //~ ERROR non-exhaustive patterns
+        0 ..= LIM => {}
+    }
+
+    match 0u128 { //~ ERROR non-exhaustive patterns
+        0 ..= 4 => {}
+    }
+
+    match 0u128 { //~ ERROR non-exhaustive patterns
+        4 ..= u128::MAX => {}
+    }
+}

src/test/ui/exhaustive_integer_patterns.stderr

@@ -0,0 +1,87 @@
+error: unreachable pattern
+  --> $DIR/exhaustive_integer_patterns.rs:32:9
+   |
+LL |         200 => {} //~ ERROR unreachable pattern
+   |         ^^^
+   |
+note: lint level defined here
+  --> $DIR/exhaustive_integer_patterns.rs:13:9
+   |
+LL | #![deny(unreachable_patterns)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error[E0004]: non-exhaustive patterns: `128u8..=255u8` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:37:11
+   |
+LL |     match x { //~ ERROR non-exhaustive patterns
+   |           ^ pattern `128u8..=255u8` not covered
+
+error[E0004]: non-exhaustive patterns: `11u8..=19u8`, `31u8..=34u8`, `36u8..=69u8` and 1 more not covered
+  --> $DIR/exhaustive_integer_patterns.rs:42:11
+   |
+LL |     match x { //~ ERROR non-exhaustive patterns
+   |           ^ patterns `11u8..=19u8`, `31u8..=34u8`, `36u8..=69u8` and 1 more not covered
+
+error: unreachable pattern
+  --> $DIR/exhaustive_integer_patterns.rs:53:9
+   |
+LL |         -2..=20 => {} //~ ERROR unreachable pattern
+   |         ^^^^^^^
+
+error[E0004]: non-exhaustive patterns: `-128i8..=-8i8`, `-6i8`, `121i8..=124i8` and 1 more not covered
+  --> $DIR/exhaustive_integer_patterns.rs:50:11
+   |
+LL |     match x { //~ ERROR non-exhaustive patterns
+   |           ^ patterns `-128i8..=-8i8`, `-6i8`, `121i8..=124i8` and 1 more not covered
+
+error[E0004]: non-exhaustive patterns: `-128i8` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:99:11
+   |
+LL |     match 0i8 { //~ ERROR non-exhaustive patterns
+   |           ^^^ pattern `-128i8` not covered
+
+error[E0004]: non-exhaustive patterns: `0i16` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:107:11
+   |
+LL |     match 0i16 { //~ ERROR non-exhaustive patterns
+   |           ^^^^ pattern `0i16` not covered
+
+error[E0004]: non-exhaustive patterns: `128u8..=255u8` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:125:11
+   |
+LL |     match 0u8 { //~ ERROR non-exhaustive patterns
+   |           ^^^ pattern `128u8..=255u8` not covered
+
+error[E0004]: non-exhaustive patterns: `(0u8, Some(_))` and `(2u8..=255u8, Some(_))` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:137:11
+   |
+LL |     match (0u8, Some(())) { //~ ERROR non-exhaustive patterns
+   |           ^^^^^^^^^^^^^^^ patterns `(0u8, Some(_))` and `(2u8..=255u8, Some(_))` not covered
+
+error[E0004]: non-exhaustive patterns: `(126u8..=127u8, false)` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:142:11
+   |
+LL |     match (0u8, true) { //~ ERROR non-exhaustive patterns
+   |           ^^^^^^^^^^^ pattern `(126u8..=127u8, false)` not covered
+
+error[E0004]: non-exhaustive patterns: `340282366920938463463374607431768211455u128` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:162:11
+   |
+LL |     match 0u128 { //~ ERROR non-exhaustive patterns
+   |           ^^^^^ pattern `340282366920938463463374607431768211455u128` not covered
+
+error[E0004]: non-exhaustive patterns: `5u128..=340282366920938463463374607431768211455u128` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:166:11
+   |
+LL |     match 0u128 { //~ ERROR non-exhaustive patterns
+   |           ^^^^^ pattern `5u128..=340282366920938463463374607431768211455u128` not covered
+
+error[E0004]: non-exhaustive patterns: `0u128..=3u128` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:170:11
+   |
+LL |     match 0u128 { //~ ERROR non-exhaustive patterns
+   |           ^^^^^ pattern `0u128..=3u128` not covered
+
+error: aborting due to 13 previous errors
+
+For more information about this error, try `rustc --explain E0004`.

src/test/ui/feature-gate-exhaustive_integer_patterns.rs

@@ -0,0 +1,16 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let x: u8 = 0;
+    match x { //~ ERROR non-exhaustive patterns: `_` not covered
+        0 ..= 255 => {}
+    }
+}

src/test/ui/feature-gate-exhaustive_integer_patterns.stderr

@@ -0,0 +1,9 @@
+error[E0004]: non-exhaustive patterns: `_` not covered
+  --> $DIR/feature-gate-exhaustive_integer_patterns.rs:13:11
+   |
+LL |     match x { //~ ERROR non-exhaustive patterns: `_` not covered
+   |           ^ pattern `_` not covered
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0004`.