suggest adding an array length if possible

compiler/rustc_ast_lowering/src/lib.rs

@@ -49,7 +49,7 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{struct_span_err, Applicability, Handler};
+use rustc_errors::{struct_span_err, Applicability, Handler, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
@@ -2236,7 +2236,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         c.value.span,
                         "using `_` for array lengths is unstable",
                     )
-                    .emit();
+                    .stash(c.value.span, StashKey::UnderscoreForArrayLengths);
                     hir::ArrayLen::Body(self.lower_anon_const(c))
                 }
             }

compiler/rustc_errors/src/lib.rs

@@ -457,6 +457,7 @@ struct HandlerInner {
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
 pub enum StashKey {
     ItemNoType,
+    UnderscoreForArrayLengths,
 }
 
 fn default_track_diagnostic(_: &Diagnostic) {}

compiler/rustc_typeck/src/check/expr.rs

@@ -28,7 +28,7 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::{
     pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, DiagnosticId,
-    ErrorGuaranteed,
+    ErrorGuaranteed, StashKey,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
@@ -1307,7 +1307,39 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 span: expr.span,
             })
         };
-        self.tcx.mk_array(element_ty, args.len() as u64)
+        let array_len = args.len() as u64;
+        self.suggest_array_len(expr, array_len);
+        self.tcx.mk_array(element_ty, array_len)
+    }
+
+    fn suggest_array_len(&self, expr: &'tcx hir::Expr<'tcx>, array_len: u64) {
+        if let Some(parent_hir_id) = self.tcx.hir().find_parent_node(expr.hir_id) {
+            let ty = match self.tcx.hir().find(parent_hir_id) {
+                Some(
+                    hir::Node::Local(hir::Local { ty: Some(ty), .. })
+                    | hir::Node::Item(hir::Item { kind: hir::ItemKind::Const(ty, _), .. }),
+                ) => Some(ty),
+                _ => None,
+            };
+            if let Some(ty) = ty
+                && let hir::TyKind::Array(_, length) = ty.kind
+                && let hir::ArrayLen::Body(hir::AnonConst { hir_id, .. }) = length
+                && let Some(span) = self.tcx.hir().opt_span(hir_id)
+            {
+                match self.tcx.sess.diagnostic().steal_diagnostic(span, StashKey::UnderscoreForArrayLengths) {
+                    Some(mut err) => {
+                        err.span_suggestion_verbose(
+                            span,
+                            "consider adding an array length",
+                            array_len,
+                            Applicability::MaybeIncorrect,
+                        );
+                        err.emit();
+                    }
+                    None => ()
+                }
+            }
+        }
     }
 
     fn check_expr_const_block(
@@ -1333,10 +1365,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         element: &'tcx hir::Expr<'tcx>,
         count: &'tcx hir::ArrayLen,
         expected: Expectation<'tcx>,
-        _expr: &'tcx hir::Expr<'tcx>,
+        expr: &'tcx hir::Expr<'tcx>,
     ) -> Ty<'tcx> {
         let tcx = self.tcx;
         let count = self.array_length_to_const(count);
+        if let Some(count) = count.try_eval_usize(tcx, self.param_env) {
+            self.suggest_array_len(expr, count);
+        }
 
         let uty = match expected {
             ExpectHasType(uty) => match *uty.kind() {

src/test/ui/array-slice-vec/suggest-array-length.fixed

@@ -0,0 +1,14 @@
+// run-rustfix
+#![allow(unused_variables, dead_code, non_upper_case_globals)]
+
+fn main() {
+    const Foo: [i32; 3] = [1, 2, 3];
+    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
+    //~| ERROR using `_` for array lengths is unstable
+    let foo: [i32; 3] = [1, 2, 3];
+    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
+    //~| ERROR using `_` for array lengths is unstable
+    let bar: [i32; 3] = [0; 3];
+    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
+    //~| ERROR using `_` for array lengths is unstable
+}

src/test/ui/array-slice-vec/suggest-array-length.rs

@@ -0,0 +1,14 @@
+// run-rustfix
+#![allow(unused_variables, dead_code, non_upper_case_globals)]
+
+fn main() {
+    const Foo: [i32; _] = [1, 2, 3];
+    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
+    //~| ERROR using `_` for array lengths is unstable
+    let foo: [i32; _] = [1, 2, 3];
+    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
+    //~| ERROR using `_` for array lengths is unstable
+    let bar: [i32; _] = [0; 3];
+    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
+    //~| ERROR using `_` for array lengths is unstable
+}

src/test/ui/array-slice-vec/suggest-array-length.stderr

@@ -0,0 +1,60 @@
+error: in expressions, `_` can only be used on the left-hand side of an assignment
+  --> $DIR/suggest-array-length.rs:8:20
+   |
+LL |     let foo: [i32; _] = [1, 2, 3];
+   |                    ^ `_` not allowed here
+
+error: in expressions, `_` can only be used on the left-hand side of an assignment
+  --> $DIR/suggest-array-length.rs:11:20
+   |
+LL |     let bar: [i32; _] = [0; 3];
+   |                    ^ `_` not allowed here
+
+error: in expressions, `_` can only be used on the left-hand side of an assignment
+  --> $DIR/suggest-array-length.rs:5:22
+   |
+LL |     const Foo: [i32; _] = [1, 2, 3];
+   |                      ^ `_` not allowed here
+
+error[E0658]: using `_` for array lengths is unstable
+  --> $DIR/suggest-array-length.rs:5:22
+   |
+LL |     const Foo: [i32; _] = [1, 2, 3];
+   |                      ^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+help: consider adding an array length
+   |
+LL |     const Foo: [i32; 3] = [1, 2, 3];
+   |                      ~
+
+error[E0658]: using `_` for array lengths is unstable
+  --> $DIR/suggest-array-length.rs:8:20
+   |
+LL |     let foo: [i32; _] = [1, 2, 3];
+   |                    ^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+help: consider adding an array length
+   |
+LL |     let foo: [i32; 3] = [1, 2, 3];
+   |                    ~
+
+error[E0658]: using `_` for array lengths is unstable
+  --> $DIR/suggest-array-length.rs:11:20
+   |
+LL |     let bar: [i32; _] = [0; 3];
+   |                    ^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+help: consider adding an array length
+   |
+LL |     let bar: [i32; 3] = [0; 3];
+   |                    ~
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/async-await/issues/issue-95307.stderr

@@ -9,6 +9,12 @@ LL |     async fn new() -> [u8; _];
    = note: `async` trait functions are not currently supported
    = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
 
+error: in expressions, `_` can only be used on the left-hand side of an assignment
+  --> $DIR/issue-95307.rs:7:28
+   |
+LL |     async fn new() -> [u8; _];
+   |                            ^ `_` not allowed here
+
 error[E0658]: using `_` for array lengths is unstable
   --> $DIR/issue-95307.rs:7:28
    |
@@ -18,12 +24,6 @@ LL |     async fn new() -> [u8; _];
    = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
    = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
 
-error: in expressions, `_` can only be used on the left-hand side of an assignment
-  --> $DIR/issue-95307.rs:7:28
-   |
-LL |     async fn new() -> [u8; _];
-   |                            ^ `_` not allowed here
-
 error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0658, E0706.

src/test/ui/feature-gates/feature-gate-generic_arg_infer.normal.stderr

@@ -1,18 +1,15 @@
-error[E0658]: using `_` for array lengths is unstable
-  --> $DIR/feature-gate-generic_arg_infer.rs:11:27
-   |
-LL |     let _x: [u8; 3] = [0; _];
-   |                           ^
-   |
-   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
-   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
-
 error: in expressions, `_` can only be used on the left-hand side of an assignment
   --> $DIR/feature-gate-generic_arg_infer.rs:11:27
    |
 LL |     let _x: [u8; 3] = [0; _];
    |                           ^ `_` not allowed here
 
+error: in expressions, `_` can only be used on the left-hand side of an assignment
+  --> $DIR/feature-gate-generic_arg_infer.rs:14:18
+   |
+LL |     let _y: [u8; _] = [0; 3];
+   |                  ^ `_` not allowed here
+
 error[E0658]: using `_` for array lengths is unstable
   --> $DIR/feature-gate-generic_arg_infer.rs:14:18
    |
@@ -21,12 +18,10 @@ LL |     let _y: [u8; _] = [0; 3];
    |
    = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
    = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
-
-error: in expressions, `_` can only be used on the left-hand side of an assignment
-  --> $DIR/feature-gate-generic_arg_infer.rs:14:18
+help: consider adding an array length
    |
-LL |     let _y: [u8; _] = [0; 3];
-   |                  ^ `_` not allowed here
+LL |     let _y: [u8; 3] = [0; 3];
+   |                  ~
 
 error[E0747]: type provided when a constant was expected
   --> $DIR/feature-gate-generic_arg_infer.rs:20:20
@@ -37,6 +32,15 @@ LL |     let _x = foo::<_>([1,2]);
    = help: const arguments cannot yet be inferred with `_`
    = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
 
+error[E0658]: using `_` for array lengths is unstable
+  --> $DIR/feature-gate-generic_arg_infer.rs:11:27
+   |
+LL |     let _x: [u8; 3] = [0; _];
+   |                           ^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+
 error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0658, E0747.