Rollup merge of #146585 - hkBst:indexing-1, r=jdonszelmann

indexing: reword help

After looking at https://github.com/rust-lang/rust/issues/40850, I thought I'd try to improve wording around error E0608 a bit. Hopefully I've succeeded.

1 files changed, 11 insertions, 4 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0608.md b/compiler/rustc_error_codes/src/error_codes/E0608.md
index d0ebc3a26f0..3c29484f575 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0608.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0608.md
@@ -1,5 +1,5 @@
-An attempt to use index on a type which doesn't implement the `std::ops::Index`
-trait was performed.
+Attempted to index a value whose type doesn't implement the
+`std::ops::Index` trait.
 
 Erroneous code example:
 
@@ -7,8 +7,8 @@ Erroneous code example:
 0u8[2]; // error: cannot index into a value of type `u8`
 ```
 
-To be able to index into a type it needs to implement the `std::ops::Index`
-trait. Example:
+Only values with types that implement the `std::ops::Index` trait
+can be indexed with square brackets. Example:
 
 ```
 let v: Vec<u8> = vec![0, 1, 2, 3];
@@ -16,3 +16,10 @@ let v: Vec<u8> = vec![0, 1, 2, 3];
 // The `Vec` type implements the `Index` trait so you can do:
 println!("{}", v[2]);
 ```
+
+Tuples and structs are indexed with dot (`.`), not with brackets (`[]`),
+and tuple element names are their positions:
+```ignore(pseudo code)
+// this (pseudo code) expression is true for any tuple:
+tuple == (tuple.0, tuple.1, ...)
+```