request: improve string indexing error message

[steven807]

The following buggy code:

fn f() {
    let s = "hello";
    let c = s[0];
}

produces the error:

error[E0277]: the type `str` cannot be indexed by `{integer}`
 --> src/main.rs:3:13
  |
3 |     let c = s[0];
  |             ^^^^ `str` cannot be indexed by `{integer}`
  |
  = help: the trait `std::ops::Index<{integer}>` is not implemented for `str`

This is usefully documented in TRPL (ch08-02-strings), but that is not easily discoverable from the error. Meanwhile, the error itself is a natural one that programmers from many languages (C, C++, etc) might run into. Speaking as someone who spent some time assuming that the issue was related to slices/references/usize (i.e. various new, Rust-specific concepts), not strings and UTF-8 encoding, I wonder whether a special case might be produced, replacing error E0277, when the types involved are specifically strings/strs and integers.

Meta

rustc --version --verbose:

rustc 1.32.0-nightly (4a45578bc 2018-12-07)
binary: rustc
commit-hash: 4a45578bc58ff262864f72680cc02e83f5d2f5b3
commit-date: 2018-12-07
host: x86_64-apple-darwin
release: 1.32.0-nightly
LLVM version: 8.0

[estebank]

We should add a note pointing to https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings in the following code

rust/src/libcore/ops/index.rs

Lines 63 to 79 in bd47d68

	#[rustc_on_unimplemented(
	message="the type `{Self}` cannot be indexed by `{Idx}`",
	label="`{Self}` cannot be indexed by `{Idx}`",
	)]
	#[stable(feature = "rust1", since = "1.0.0")]
	#[doc(alias = "]")]
	#[doc(alias = "[")]
	#[doc(alias = "[]")]
	pub trait Index<Idx: ?Sized> {
	/// The returned type after indexing.
	#[stable(feature = "rust1", since = "1.0.0")]
	type Output: ?Sized;
	/// Performs the indexing (`container[index]`) operation.
	#[stable(feature = "rust1", since = "1.0.0")]
	fn index(&self, index: Idx) -> &Self::Output;
	}

rustc_on_unimplemented lets you customize the output of E0277 errors. The example has a similar case to this one, but the additional code should be something along the lines of

    on(
        _Self="&str",
        note="consider reading the appropriate chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
    ),
    on(
        _Self="str",
        note="consider reading the appropriate chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
    ),
    on(
        _Self="std::string::String",
        note="consider reading the appropriate chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
    ),

This will cause an extra note to be displayed for this case.

Beyond changing the code, you will also need to add a new test case with the repro case above and update all of the tests (running ./x.py test src/test/ui --stage 1 --bless and possibly updating the test comments for any remaining failing tests).

[jromer94]

I would like to work on this.

[estebank]

@jromer94 do not hesitate to reach out either here or on zulip if you need help!

[estebank]

Possible proposed output from #57055:

error[EXXXX]: cannot index `String` or string slice (`str`)
 --> main.rs:2:24
  |
  |     let _value = "test"[0];
  |                        ^^^ cannot index here
  |
  |
  = help: consider using `.chars()` or `.to_bytes()