turbo-tasks: Expand <T as TaskOutput>::Return values in macros (ver…

…cel/turborepo#8096)

Previously, we'd generate function return types like:

```rust
<Vc<AssetIdent> as TaskOutput>::Return
```

which is equivalent to the much simpler:

```rust
Vc<AssetIdent>
```

That approach is nice because it's easier to implement in the macros, and it's more correct, because it uses the type system to verify the type of `Vc` and `Result` are what we expect. However, it makes the rustdoc output much harder to read.

This attempts to the expansion of `TaskOutput::Return` inside the macro to generate more readable documentation, at the cost of some correctness.

<details>
<summary><strong>Rustdoc Before</strong></summary>

![Screenshot 2024-05-06 at 5.35.11 PM.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/HAZVitxRNnZz8QMiPn4a/6a64062d-9fa8-42b9-8788-0972735dfcc8.png)
</details>

<details>
<summary><strong>Rustdoc After</strong></summary>

![Screenshot 2024-05-06 at 5.35.01 PM.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/HAZVitxRNnZz8QMiPn4a/86995431-6e87-4205-ad23-54a2ddb7a535.png)
</details>

<details>
<summary><strong>Example compilation error</strong> with an invalid return type</summary>

![Screenshot 2024-05-06 at 5.48.00 PM.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/HAZVitxRNnZz8QMiPn4a/257cb449-4b8e-4fde-8ee0-0b2af1b7b435.png)
</details>

crates/turbo-tasks-macros/src/func.rs

@@ -1,7 +1,11 @@
 use proc_macro2::Ident;
 use syn::{
-    parse_quote, punctuated::Punctuated, spanned::Spanned, Block, Expr, ExprPath, FnArg, Pat,
-    PatIdent, PatType, Path, Receiver, ReturnType, Signature, Token, Type,
+    parse_quote,
+    punctuated::{Pair, Punctuated},
+    spanned::Spanned,
+    AngleBracketedGenericArguments, Block, Expr, ExprPath, FnArg, GenericArgument, Pat, PatIdent,
+    PatType, Path, PathArguments, PathSegment, Receiver, ReturnType, Signature, Token, Type,
+    TypeGroup, TypePath, TypeTuple,
 };
 
 #[derive(Debug)]
@@ -257,10 +261,11 @@ impl TurboFn {
             .collect();
 
         let ident = &self.ident;
-        let output = &self.output;
+        let orig_output = &self.output;
+        let new_output = expand_vc_return_type(orig_output);
 
         parse_quote! {
-            fn #ident(#exposed_inputs) -> <#output as turbo_tasks::task::TaskOutput>::Return
+            fn #ident(#exposed_inputs) -> #new_output
         }
     }
 
@@ -327,6 +332,100 @@ fn return_type_to_type(return_type: &ReturnType) -> Type {
     }
 }
 
+fn expand_vc_return_type(orig_output: &Type) -> Type {
+    // HACK: Approximate the expansion that we'd otherwise get from
+    // `<T as TaskOutput>::Return`, so that the return type shown in the rustdocs
+    // is as simple as possible. Break out as soon as we see something we don't
+    // recognize.
+    let mut new_output = orig_output.clone();
+    let mut found_vc = false;
+    loop {
+        new_output = match new_output {
+            Type::Group(TypeGroup { elem, .. }) => *elem,
+            Type::Tuple(TypeTuple { elems, .. }) if elems.is_empty() => {
+                Type::Path(parse_quote!(::turbo_tasks::Vc<()>))
+            }
+            Type::Path(TypePath {
+                qself: None,
+                path:
+                    Path {
+                        leading_colon,
+                        ref segments,
+                    },
+            }) => {
+                let mut pairs = segments.pairs();
+                let mut cur_pair = pairs.next();
+
+                enum PathPrefix {
+                    Anyhow,
+                    TurboTasks,
+                }
+
+                // try to strip a `turbo_tasks::` or `anyhow::` prefix
+                let prefix = if let Some(first) = cur_pair.as_ref().map(|p| p.value()) {
+                    if first.arguments.is_none() {
+                        if first.ident == "turbo_tasks" {
+                            Some(PathPrefix::TurboTasks)
+                        } else if first.ident == "anyhow" {
+                            Some(PathPrefix::Anyhow)
+                        } else {
+                            None
+                        }
+                    } else {
+                        None
+                    }
+                } else {
+                    None
+                };
+
+                if prefix.is_some() {
+                    cur_pair = pairs.next(); // strip the matched prefix
+                } else if leading_colon.is_some() {
+                    break; // something like `::Vc` isn't valid
+                }
+
+                // Look for a `Vc<...>` or `Result<...>` generic
+                let Some(Pair::End(PathSegment {
+                    ident,
+                    arguments:
+                        PathArguments::AngleBracketed(AngleBracketedGenericArguments { args, .. }),
+                })) = cur_pair
+                else {
+                    break;
+                };
+                if ident == "Vc" {
+                    found_vc = true;
+                    break; // Vc is the bottom-most level
+                }
+                if ident == "Result" && args.len() == 1 {
+                    let GenericArgument::Type(ty) =
+                        args.first().expect("Result<...> type has an argument")
+                    else {
+                        break;
+                    };
+                    ty.clone()
+                } else {
+                    break; // we only support expanding Result<...>
+                }
+            }
+            _ => break,
+        }
+    }
+
+    if !found_vc {
+        orig_output
+            .span()
+            .unwrap()
+            .error(
+                "Expected return type to be `turbo_tasks::Vc<T>` or `anyhow::Result<Vc<T>>`. \
+                 Unable to process type.",
+            )
+            .emit();
+    }
+
+    new_output
+}
+
 /// The context in which the function is being defined.
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub enum DefinitionContext {