wgsl-in: Improve assignment diagnostics

[SparkyPotato]

This improves the diagnostics on invalid assignment statements.
Fixes #2052.

Given:

fn f() {
    var v = vec2(0);
    v.xy = vec2(1);
}

Naga used to error with:

error: the left-hand side of an assignment must be a reference
  ┌─ test.wgsl:3:5
  │
3 │     v.xy = vec2(1);
  │     ^^^^ expression is not a reference

Now:

error: invalid left-hand side of assignment
  ┌─ test.wgsl:3:5
  │
3 │     v.xy = vec2(1);
  │     ^^^^ cannot assign to this expression
  │
  = note: WGSL does not support assignments to swizzles
  = note: consider assigning each component individually

Given:

fn f() {
    3 + 5;
}

Naga used to error with:

error: the left-hand side of an assignment must be a reference
  ┌─ test.wgsl:2:5
  │
2 │     3 + 5;
  │     ^ expression is not a reference

Now:

error: expected assignment or increment/decrement, found '3 + 5'
  ┌─ test.wgsl:2:5
  │
2 │     3 + 5;
  │     ^^^^^ expected assignment or increment/decrement

Given:

fn f() {
    3 + 5 = 10;
}

Naga used to error with:

error: the left-hand side of an assignment must be a reference
  ┌─ test.wgsl:2:5
  │
2 │     3 + 5 = 10;
  │     ^ expression is not a reference

Now:

error: invalid left-hand side of assignment
  ┌─ test.wgsl:2:5
  │
2 │     3 + 5 = 10;
  │     ^^^^^ cannot assign to this expression

Given:

fn f() {
    let a = 10;
	a = 20;
}

Naga used to error with:

error: invalid left-hand side of assignment
  ┌─ test.wgsl:3:2
  │
3 │     a = 20;
  │     ^ cannot assign to this expression

Now:

error: invalid left-hand side of assignment
  ┌─ test.wgsl:3:2
  │
3 │     a = 20;
  │     ^ cannot assign to this expression
  │
  = note: 'a' is an immutable binding
  = note: consider declaring it with `var` instead of `let`

Error has breaking changes made to it.

[jimblandy]

@SparkyPotato This needs a rebase.

[jimblandy]

Let's get the examples you put in the PR description into the test suite. It seems like they'd fit well in tests/wgsl-errors.rs, as a check-based test.

The simplification to Error::Unexpected is great. In the future, see if it's possible to separate out changes like that, that make a bunch of changes all over the place, into their own commits. This makes the series easier to review, because it makes it easier to see how simple the other changes are. But don't worry about that for this PR.

[jimblandy]

@SparkyPotato This looks fantastic, btw - thanks very much!

[jimblandy]

Looks good to me - thank you!

[jimblandy]

Okay, here's why cargo fmt -- --check wasn't complaining about the long lines in front/wgsl/mod.rs: rust-lang/rustfmt#3863

I'll push a workaround for that.