Auto merge of #66129 - Nadrieril:refactor-slice-pat-usefulness, r=<try>

Refactor slice pattern usefulness checking

This PR changes how variable-length slice patterns are handled in usefulness checking.
The objectives are: cleaning up that code to make it easier to understand, and paving the way to handling fixed-length slices more cleverly too, for #53820.

Before this, variable-length slice patterns were eagerly expanded into a union of fixed-length slices. Now they have their own special constructor, which allows expanding them a bit more lazily.
As a nice side-effect, this improves diagnostics.

This PR shows a slight performance improvement, mostly due to 149792b. This will probably have to be reverted in some way when we implement or-patterns.

src/test/ui/consts/const_let_refutable.stderr

@@ -1,8 +1,8 @@
-error[E0005]: refutable pattern in function argument: `&[]`, `&[_]` and `&[_, _, _]` not covered
+error[E0005]: refutable pattern in function argument: `&[]`, `&[_]` and `&[_, _, _, ..]` not covered
   --> $DIR/const_let_refutable.rs:3:16
    |
 LL | const fn slice([a, b]: &[i32]) -> i32 {
-   |                ^^^^^^ patterns `&[]`, `&[_]` and `&[_, _, _]` not covered
+   |                ^^^^^^ patterns `&[]`, `&[_]` and `&[_, _, _, ..]` not covered
 
 error[E0723]: can only call other `const fn` within a `const fn`, but `const <&i32 as std::ops::Add>::add` is not stable as `const fn`
   --> $DIR/const_let_refutable.rs:4:5

src/test/ui/pattern/usefulness/match-byte-array-patterns-2.stderr

@@ -6,11 +6,11 @@ LL |     match buf {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `&[]`, `&[_]`, `&[_, _]` and 3 more not covered
+error[E0004]: non-exhaustive patterns: `&[..]` not covered
   --> $DIR/match-byte-array-patterns-2.rs:10:11
    |
 LL |     match buf {
-   |           ^^^ patterns `&[]`, `&[_]`, `&[_, _]` and 3 more not covered
+   |           ^^^ pattern `&[..]` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 

src/test/ui/pattern/usefulness/match-slice-patterns.rs

@@ -2,7 +2,7 @@
 
 fn check(list: &[Option<()>]) {
     match list {
-    //~^ ERROR `&[_, Some(_), None, _]` not covered
+    //~^ ERROR `&[_, Some(_), .., None, _]` not covered
         &[] => {},
         &[_] => {},
         &[_, _] => {},

src/test/ui/pattern/usefulness/match-slice-patterns.stderr

@@ -1,8 +1,8 @@
-error[E0004]: non-exhaustive patterns: `&[_, Some(_), None, _]` not covered
+error[E0004]: non-exhaustive patterns: `&[_, Some(_), .., None, _]` not covered
   --> $DIR/match-slice-patterns.rs:4:11
    |
 LL |     match list {
-   |           ^^^^ pattern `&[_, Some(_), None, _]` not covered
+   |           ^^^^ pattern `&[_, Some(_), .., None, _]` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 

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

@@ -44,7 +44,7 @@ fn main() {
     }
     let vec = vec![0.5f32];
     let vec: &[f32] = &vec;
-    match *vec { //~ ERROR non-exhaustive patterns: `[_, _, _, _]` not covered
+    match *vec { //~ ERROR non-exhaustive patterns: `[_, _, _, _, ..]` not covered
         [0.1, 0.2, 0.3] => (),
         [0.1, 0.2] => (),
         [0.1] => (),

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

@@ -66,11 +66,11 @@ LL |     match *vec {
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `[_, _, _, _]` not covered
+error[E0004]: non-exhaustive patterns: `[_, _, _, _, ..]` not covered
   --> $DIR/non-exhaustive-match.rs:47:11
    |
 LL |     match *vec {
-   |           ^^^^ pattern `[_, _, _, _]` not covered
+   |           ^^^^ pattern `[_, _, _, _, ..]` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 

src/test/ui/pattern/usefulness/slice-patterns.rs

@@ -0,0 +1,110 @@
+#![feature(slice_patterns)]
+#![deny(unreachable_patterns)]
+
+fn main() {
+    let s: &[bool] = &[true; 0];
+    let s0: &[bool; 0] = &[];
+    let s1: &[bool; 1] = &[false; 1];
+    let s2: &[bool; 2] = &[false; 2];
+    let s3: &[bool; 3] = &[false; 3];
+
+    let [] = s0;
+    let [_] = s1;
+    let [_, _] = s2;
+
+    let [..] = s;
+    let [..] = s0;
+    let [..] = s1;
+    let [..] = s2;
+    let [..] = s3;
+
+    let [_, _, ..] = s2;
+    let [_, .., _] = s2;
+    let [.., _, _] = s2;
+
+    match s1 {
+        [true, ..] => {}
+        [.., false] => {}
+    }
+    match s2 {
+    //~^ ERROR `&[false, true]` not covered
+        [true, ..] => {}
+        [.., false] => {}
+    }
+    match s3 {
+    //~^ ERROR `&[false, _, true]` not covered
+        [true, ..] => {}
+        [.., false] => {}
+    }
+    match s {
+    //~^ ERROR `&[false, .., true]` not covered
+        [] => {}
+        [true, ..] => {}
+        [.., false] => {}
+    }
+
+    match s3 {
+    //~^ ERROR `&[false, _, _]` not covered
+        [true, .., true] => {}
+    }
+    match s {
+    //~^ ERROR `&[_, ..]` not covered
+        [] => {}
+    }
+    match s {
+    //~^ ERROR `&[_, _, ..]` not covered
+        [] => {}
+        [_] => {}
+    }
+    match s {
+    //~^ ERROR `&[false, ..]` not covered
+        [] => {}
+        [true, ..] => {}
+    }
+    match s {
+    //~^ ERROR `&[false, _, ..]` not covered
+        [] => {}
+        [_] => {}
+        [true, ..] => {}
+    }
+    match s {
+    //~^ ERROR `&[_, .., false]` not covered
+        [] => {}
+        [_] => {}
+        [.., true] => {}
+    }
+
+    match s {
+        [true, ..] => {}
+        [true, ..] => {} //~ ERROR unreachable pattern
+        [true] => {} //~ ERROR unreachable pattern
+        [..] => {}
+    }
+    match s {
+        [.., true] => {}
+        [.., true] => {} //~ ERROR unreachable pattern
+        [true] => {} //~ ERROR unreachable pattern
+        [..] => {}
+    }
+    match s {
+        [false, .., true] => {}
+        [false, .., true] => {} //~ ERROR unreachable pattern
+        [false, true] => {} //~ ERROR unreachable pattern
+        [false] => {}
+        [..] => {}
+    }
+    match s {
+    //~^ ERROR `&[_, _, .., true]` not covered
+        [] => {}
+        [_] => {}
+        [_, _] => {}
+        [.., false] => {}
+    }
+    match s {
+    //~^ ERROR `&[true, _, .., _]` not covered
+        [] => {}
+        [_] => {}
+        [_, _] => {}
+        [false, .., false] => {}
+    }
+}

src/test/ui/pattern/usefulness/slice-patterns.stderr

@@ -0,0 +1,133 @@
+error[E0004]: non-exhaustive patterns: `&[false, true]` not covered
+  --> $DIR/slice-patterns.rs:29:11
+   |
+LL |     match s2 {
+   |           ^^ pattern `&[false, true]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[false, _, true]` not covered
+  --> $DIR/slice-patterns.rs:34:11
+   |
+LL |     match s3 {
+   |           ^^ pattern `&[false, _, true]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[false, .., true]` not covered
+  --> $DIR/slice-patterns.rs:39:11
+   |
+LL |     match s {
+   |           ^ pattern `&[false, .., true]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[false, _, _]` not covered
+  --> $DIR/slice-patterns.rs:46:11
+   |
+LL |     match s3 {
+   |           ^^ pattern `&[false, _, _]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[_, ..]` not covered
+  --> $DIR/slice-patterns.rs:50:11
+   |
+LL |     match s {
+   |           ^ pattern `&[_, ..]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[_, _, ..]` not covered
+  --> $DIR/slice-patterns.rs:54:11
+   |
+LL |     match s {
+   |           ^ pattern `&[_, _, ..]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[false, ..]` not covered
+  --> $DIR/slice-patterns.rs:59:11
+   |
+LL |     match s {
+   |           ^ pattern `&[false, ..]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[false, _, ..]` not covered
+  --> $DIR/slice-patterns.rs:64:11
+   |
+LL |     match s {
+   |           ^ pattern `&[false, _, ..]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[_, .., false]` not covered
+  --> $DIR/slice-patterns.rs:70:11
+   |
+LL |     match s {
+   |           ^ pattern `&[_, .., false]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error: unreachable pattern
+  --> $DIR/slice-patterns.rs:79:9
+   |
+LL |         [true, ..] => {}
+   |         ^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/slice-patterns.rs:2:9
+   |
+LL | #![deny(unreachable_patterns)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/slice-patterns.rs:80:9
+   |
+LL |         [true] => {}
+   |         ^^^^^^
+
+error: unreachable pattern
+  --> $DIR/slice-patterns.rs:85:9
+   |
+LL |         [.., true] => {}
+   |         ^^^^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/slice-patterns.rs:86:9
+   |
+LL |         [true] => {}
+   |         ^^^^^^
+
+error: unreachable pattern
+  --> $DIR/slice-patterns.rs:91:9
+   |
+LL |         [false, .., true] => {}
+   |         ^^^^^^^^^^^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/slice-patterns.rs:92:9
+   |
+LL |         [false, true] => {}
+   |         ^^^^^^^^^^^^^
+
+error[E0004]: non-exhaustive patterns: `&[_, _, .., true]` not covered
+  --> $DIR/slice-patterns.rs:96:11
+   |
+LL |     match s {
+   |           ^ pattern `&[_, _, .., true]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[true, _, .., _]` not covered
+  --> $DIR/slice-patterns.rs:103:11
+   |
+LL |     match s {
+   |           ^ pattern `&[true, _, .., _]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error: aborting due to 17 previous errors
+
+For more information about this error, try `rustc --explain E0004`.

src/test/ui/uninhabited/uninhabited-matches-feature-gated.stderr

@@ -30,11 +30,11 @@ LL |     let _ = match x {};
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
-error[E0004]: non-exhaustive patterns: `&[_]` not covered
+error[E0004]: non-exhaustive patterns: `&[_, ..]` not covered
   --> $DIR/uninhabited-matches-feature-gated.rs:21:19
    |
 LL |     let _ = match x {
-   |                   ^ pattern `&[_]` not covered
+   |                   ^ pattern `&[_, ..]` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms