Suggest use ; to construct array type when other token exists

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Author: xizheyin

compiler/rustc_parse/src/parser/ty.rs

@@ -575,14 +575,46 @@ impl<'a> Parser<'a> {
                 self.expect(exp!(CloseBracket))?;
             }
             TyKind::Array(elt_ty, length)
-        } else {
-            self.expect(exp!(CloseBracket))?;
+        } else if self.eat(exp!(CloseBracket)) {
             TyKind::Slice(elt_ty)
+        } else {
+            let span = self.token.span;
+            let token_descr = super::token_descr(&self.token);
+            self.suggest_array_ty(span, token_descr)?
         };
 
         Ok(ty)
     }
 
+    /// Suggest with malformed array type syntax.
+    ///
+    /// This method attempts to suggest with cases like:
+    /// - `[u8, 5]` → suggests using `;`
+    /// Consider to add more cases in the future.
+    fn suggest_array_ty(&mut self, span: Span, token_descr: String) -> PResult<'a, TyKind> {
+        let mut err =
+            self.dcx().struct_span_err(span, format!("expected `;` or `]`, found {}", token_descr));
+        err.span_label(span, "expected `;` or `]`");
+        err.note("you might need a array type or a slice type");
+
+        let mut snapshot = self.create_snapshot_for_diagnostic();
+        snapshot.bump();
+        if let Err(e) = snapshot.parse_expr_anon_const() {
+            e.cancel();
+            return Err(err);
+        }
+        err.span_suggestion_verbose(
+            span,
+            "you might have meant to use `;` as the separator",
+            ";",
+            Applicability::MaybeIncorrect,
+        );
+        if let Err(e) = snapshot.expect(exp!(CloseBracket)) {
+            e.cancel();
+        }
+        return Err(err);
+    }
+
     fn parse_borrowed_pointee(&mut self) -> PResult<'a, TyKind> {
         let and_span = self.prev_token.span;
         let mut opt_lifetime = self.check_lifetime().then(|| self.expect_lifetime());

tests/ui/parser/better-expected.rs

@@ -1,3 +1,3 @@
 fn main() {
-    let x: [isize 3]; //~ ERROR expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `3`
+    let x: [isize 3]; //~ ERROR expected `;` or `]`, found `3`
 }

tests/ui/parser/better-expected.stderr

@@ -1,10 +1,12 @@
-error: expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `3`
+error: expected `;` or `]`, found `3`
   --> $DIR/better-expected.rs:2:19
    |
 LL |     let x: [isize 3];
-   |          -        ^ expected one of 7 possible tokens
+   |          -        ^ expected `;` or `]`
    |          |
    |          while parsing the type for `x`
+   |
+   = note: you might need a array type or a slice type
 
 error: aborting due to 1 previous error
 

tests/ui/parser/recover/array-type-no-semi.rs

@@ -5,13 +5,13 @@
 fn main() {
     let x = 5;
     let b: [i32, 5];
-    //~^ ERROR expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `,`
+    //~^ ERROR expected `;` or `]`, found `,`
     //~| ERROR expected value, found builtin type `i32` [E0423]
     let a: [i32, ];
-    //~^ ERROR expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `,`
+    //~^ ERROR expected `;` or `]`, found `,`
     //~| ERROR expected value, found builtin type `i32` [E0423]
     let c: [i32, x];
-    //~^ ERROR expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `,`
+    //~^ ERROR expected `;` or `]`, found `,`
     //~| ERROR expected value, found builtin type `i32` [E0423]
 
 }

tests/ui/parser/recover/array-type-no-semi.stderr

@@ -1,29 +1,53 @@
-error: expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `,`
+error: expected `;` or `]`, found `,`
   --> $DIR/array-type-no-semi.rs:7:16
    |
 LL |     let b: [i32, 5];
-   |          -     ^ expected one of 7 possible tokens
+   |          -     ^ expected `;` or `]`
    |          |
    |          while parsing the type for `b`
-   |          help: use `=` if you meant to assign
+   |
+   = note: you might need a array type or a slice type
+help: you might have meant to use `;` as the separator
+   |
+LL -     let b: [i32, 5];
+LL +     let b: [i32; 5];
+   |
+help: use `=` if you meant to assign
+   |
+LL -     let b: [i32, 5];
+LL +     let b = [i32, 5];
+   |
 
-error: expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `,`
+error: expected `;` or `]`, found `,`
   --> $DIR/array-type-no-semi.rs:10:16
    |
 LL |     let a: [i32, ];
-   |          -     ^ expected one of 7 possible tokens
+   |          -     ^ expected `;` or `]`
    |          |
    |          while parsing the type for `a`
    |          help: use `=` if you meant to assign
+   |
+   = note: you might need a array type or a slice type
 
-error: expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `,`
+error: expected `;` or `]`, found `,`
   --> $DIR/array-type-no-semi.rs:13:16
    |
 LL |     let c: [i32, x];
-   |          -     ^ expected one of 7 possible tokens
+   |          -     ^ expected `;` or `]`
    |          |
    |          while parsing the type for `c`
-   |          help: use `=` if you meant to assign
+   |
+   = note: you might need a array type or a slice type
+help: you might have meant to use `;` as the separator
+   |
+LL -     let c: [i32, x];
+LL +     let c: [i32; x];
+   |
+help: use `=` if you meant to assign
+   |
+LL -     let c: [i32, x];
+LL +     let c = [i32, x];
+   |
 
 error[E0423]: expected value, found builtin type `i32`
   --> $DIR/array-type-no-semi.rs:7:13

tests/ui/parser/removed-syntax/removed-syntax-fixed-vec.rs

@@ -1 +1,3 @@
-type v = [isize * 3]; //~ ERROR expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `*`
+type v = [isize * 3]; //~ ERROR expected `;` or `]`, found `*`
+
+fn main() {}

tests/ui/parser/removed-syntax/removed-syntax-fixed-vec.stderr

@@ -1,8 +1,15 @@
-error: expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `*`
+error: expected `;` or `]`, found `*`
   --> $DIR/removed-syntax-fixed-vec.rs:1:17
    |
 LL | type v = [isize * 3];
-   |                 ^ expected one of 7 possible tokens
+   |                 ^ expected `;` or `]`
+   |
+   = note: you might need a array type or a slice type
+help: you might have meant to use `;` as the separator
+   |
+LL - type v = [isize * 3];
+LL + type v = [isize ; 3];
+   |
 
 error: aborting due to 1 previous error