Rollup merge of rust-lang#70562 - lcnr:const-arr_len, r=Centril

infer array len from pattern

closes rust-lang#70529

This still errors in the following case

```rust
#![feature(const_generics)]
fn arr<const N: usize>() -> [u8; N] {
    todo!()
}

fn main() {
    match arr() {
        [5, ..] => (),
        //~^ ERROR cannot pattern-match on an array without a fixed length
        [_, _] => (),
    }
}
```
Considering that this should be rare and is harder to implement I would merge this PR without *fixing* the above.

Author: Centril

src/librustc_error_codes/error_codes/E0730.md

@@ -7,8 +7,8 @@ Example of erroneous code:
 
 fn is_123<const N: usize>(x: [u32; N]) -> bool {
     match x {
-        [1, 2, 3] => true, // error: cannot pattern-match on an
-                           //        array without a fixed length
+        [1, 2, ..] => true, // error: cannot pattern-match on an
+                            //        array without a fixed length
         _ => false
     }
 }

src/librustc_typeck/check/pat.rs

@@ -1355,16 +1355,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> Ty<'tcx> {
         let err = self.tcx.types.err;
         let expected = self.structurally_resolved_type(span, expected);
-        let (inner_ty, slice_ty, expected) = match expected.kind {
+        let (element_ty, slice_ty, inferred) = match expected.kind {
             // An array, so we might have something like `let [a, b, c] = [0, 1, 2];`.
-            ty::Array(inner_ty, len) => {
+            ty::Array(element_ty, len) => {
                 let min = before.len() as u64 + after.len() as u64;
-                let slice_ty = self
-                    .check_array_pat_len(span, slice, len, min)
-                    .map_or(err, |len| self.tcx.mk_array(inner_ty, len));
-                (inner_ty, slice_ty, expected)
+                let (slice_ty, expected) =
+                    self.check_array_pat_len(span, element_ty, expected, slice, len, min);
+                (element_ty, slice_ty, expected)
             }
-            ty::Slice(inner_ty) => (inner_ty, expected, expected),
+            ty::Slice(element_ty) => (element_ty, expected, expected),
             // The expected type must be an array or slice, but was neither, so error.
             _ => {
                 if !expected.references_error() {
@@ -1376,30 +1375,33 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Type check all the patterns before `slice`.
         for elt in before {
-            self.check_pat(&elt, inner_ty, def_bm, ti);
+            self.check_pat(&elt, element_ty, def_bm, ti);
         }
         // Type check the `slice`, if present, against its expected type.
         if let Some(slice) = slice {
             self.check_pat(&slice, slice_ty, def_bm, ti);
         }
         // Type check the elements after `slice`, if present.
         for elt in after {
-            self.check_pat(&elt, inner_ty, def_bm, ti);
+            self.check_pat(&elt, element_ty, def_bm, ti);
         }
-        expected
+        inferred
     }
 
     /// Type check the length of an array pattern.
     ///
-    /// Return the length of the variable length pattern,
-    /// if it exists and there are no errors.
+    /// Returns both the type of the variable length pattern
+    /// (or `tcx.err` in case there is none),
+    /// and the potentially inferred array type.
     fn check_array_pat_len(
         &self,
         span: Span,
+        element_ty: Ty<'tcx>,
+        arr_ty: Ty<'tcx>,
         slice: Option<&'tcx Pat<'tcx>>,
         len: &ty::Const<'tcx>,
         min_len: u64,
-    ) -> Option<u64> {
+    ) -> (Ty<'tcx>, Ty<'tcx>) {
         if let Some(len) = len.try_eval_usize(self.tcx, self.param_env) {
             // Now we know the length...
             if slice.is_none() {
@@ -1409,21 +1411,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 if min_len != len {
                     self.error_scrutinee_inconsistent_length(span, min_len, len);
                 }
-            } else if let r @ Some(_) = len.checked_sub(min_len) {
+            } else if let Some(pat_len) = len.checked_sub(min_len) {
                 // The variable-length pattern was there,
                 // so it has an array type with the remaining elements left as its size...
-                return r;
+                return (self.tcx.mk_array(element_ty, pat_len), arr_ty);
             } else {
                 // ...however, in this case, there were no remaining elements.
                 // That is, the slice pattern requires more than the array type offers.
                 self.error_scrutinee_with_rest_inconsistent_length(span, min_len, len);
             }
+        } else if slice.is_none() {
+            // We have a pattern with a fixed length,
+            // which we can use to infer the length of the array.
+            let updated_arr_ty = self.tcx.mk_array(element_ty, min_len);
+            self.demand_eqtype(span, updated_arr_ty, arr_ty);
+            return (self.tcx.types.err, updated_arr_ty);
         } else {
-            // No idea what the length is, which happens if we have e.g.,
-            // `let [a, b] = arr` where `arr: [T; N]` where `const N: usize`.
+            // We have a variable-length pattern and don't know the array length.
+            // This happens if we have e.g.,
+            // `let [a, b, ..] = arr` where `arr: [T; N]` where `const N: usize`.
             self.error_scrutinee_unfixed_length(span);
         }
-        None
+        (self.tcx.types.err, arr_ty)
     }
 
     fn error_scrutinee_inconsistent_length(&self, span: Span, min_len: u64, size: u64) {

src/test/ui/array-slice-vec/infer_array_len.rs

@@ -0,0 +1,21 @@
+// see issue #70529
+struct A;
+
+impl From<A> for [u8; 2] {
+    fn from(a: A) -> Self {
+        [0; 2]
+    }
+}
+
+impl From<A> for [u8; 3] {
+    fn from(a: A) -> Self {
+        [0; 3]
+    }
+}
+
+
+fn main() {
+    let a = A;
+    let [_, _] = a.into();
+    //~^ ERROR type annotations needed
+}

src/test/ui/array-slice-vec/infer_array_len.stderr

@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed
+  --> $DIR/infer_array_len.rs:19:9
+   |
+LL |     let [_, _] = a.into();
+   |         ^^^^^^ consider giving this pattern a type
+   |
+   = note: type must be known at this point
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/array-slice-vec/match_arr_unknown_len.rs

@@ -0,0 +1,11 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+fn is_123<const N: usize>(x: [u32; N]) -> bool {
+    match x {
+        [1, 2] => true, //~ ERROR mismatched types
+        _ => false
+    }
+}
+
+fn main() {}

src/test/ui/array-slice-vec/match_arr_unknown_len.stderr

@@ -0,0 +1,20 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/match_arr_unknown_len.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0308]: mismatched types
+  --> $DIR/match_arr_unknown_len.rs:6:9
+   |
+LL |         [1, 2] => true,
+   |         ^^^^^^ expected `2usize`, found `N`
+   |
+   = note: expected array `[u32; 2]`
+              found array `[u32; _]`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/const-generics/infer_arg_from_pat.rs

@@ -0,0 +1,27 @@
+// run-pass
+//
+// see issue #70529
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+struct A<const N: usize> {
+    arr: [u8; N],
+}
+
+impl<const N: usize> A<N> {
+    fn new() -> Self {
+        A {
+            arr: [0; N],
+        }
+    }
+
+    fn value(&self) -> usize {
+        N
+    }
+}
+
+fn main() {
+    let a = A::new();
+    let [_, _] = a.arr;
+    assert_eq!(a.value(), 2);
+}

src/test/ui/const-generics/infer_arg_from_pat.stderr

@@ -0,0 +1,8 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/infer_arg_from_pat.rs:4:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+

src/test/ui/const-generics/infer_arr_len_from_pat.rs

@@ -0,0 +1,13 @@
+// check-pass
+//
+// see issue #70529
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+fn as_chunks<const N: usize>() -> [u8; N] {
+    loop {}
+}
+
+fn main() {
+    let [_, _] = as_chunks();
+}

src/test/ui/const-generics/infer_arr_len_from_pat.stderr

@@ -0,0 +1,8 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/infer_arr_len_from_pat.rs:4:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+