add rust-src to the toolchain and update trybuild test output (#3440)

# Description of Changes

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here. -->

Adds `rust-src` to the toolchain and updates the expected output of the
`trybuild` tests.

`rust-src` places the entire source code of the rust stdlib under the
local toolchain directory. `rust-analyzer` uses it for "go to
definition" (among other things) in the stdlib. When included in the
toolchain, rustc can provide code frames that it can't otherwise.

The reason for its addition is that if anyone includes it in their local
toolchain, the error messages produced by rustc when running the `ui`
tests will be slightly different (have some more information) than when
run in CI. Hence those tests will fail locally but not in CI. Rather
than removing `rust-src` locally so that the tests will pass, I've opted
to just add it to the toolchain so that everyone runs with it by
default, including in CI.

# API and ABI breaking changes

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corresponding GitHub label. -->

None

# Expected complexity level and risk

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0

# Testing

<!-- Describe any testing you've done, and any testing you'd like your
reviewers to do,
so that you're confident that all the changes work as expected! -->

- [x] Updated rustc tests

crates/bindings/tests/ui/views.stderr

@@ -25,6 +25,9 @@ error[E0599]: `&test__ViewHandle` is not an iterator
            which is required by `&mut test__ViewHandle: Iterator`
 note: the trait `Iterator` must be implemented
   --> $RUST/core/src/iter/traits/iterator.rs
+   |
+   | pub trait Iterator {
+   | ^^^^^^^^^^^^^^^^^^
    = help: items from traits can only be used if the trait is implemented and in scope
    = note: the following traits define an item `count`, perhaps you need to implement one of them:
            candidate #1: `Table`
@@ -54,6 +57,9 @@ error[E0599]: no method named `try_insert` found for reference `&test__ViewHandl
            candidate #1: `Table`
 help: there is a method `try_into` with a similar name, but with different arguments
   --> $RUST/core/src/convert/mod.rs
+   |
+   |     fn try_into(self) -> Result<T, Self::Error>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0599]: no method named `delete` found for reference `&test__ViewHandle` in the current scope
   --> tests/ui/views.rs:43:25

rust-toolchain.toml

@@ -5,3 +5,4 @@
 channel = "1.90.0"
 profile = "default"
 targets = ["wasm32-unknown-unknown"]
+components = ["rust-src"]