[WIP] Allow #[project] to be used on if let expressions

This PR would work correctly, except for the fact that
rust-lang/rust#68618 causes a compilation error to be emitted before we
even have a chance to run. That issue is independent of the
implementation of this PR, so this PR should start working automatically
once the issue is resolved.

pin-project-internal/src/project.rs

@@ -13,17 +13,33 @@ pub(crate) fn attribute(args: &TokenStream, input: Stmt, mutability: Mutability)
         .unwrap_or_else(|e| e.to_compile_error())
 }
 
-fn parse(mut stmt: Stmt, mutability: Mutability) -> Result<TokenStream> {
-    match &mut stmt {
+fn replace_stmt(stmt: &mut Stmt, mutability: Mutability) -> Result<bool> {
+    match stmt {
         Stmt::Expr(Expr::Match(expr)) | Stmt::Semi(Expr::Match(expr), _) => {
-            Context::new(mutability).replace_expr_match(expr)
+            Context::new(mutability).replace_expr_match(expr);
+            return Ok(true)
+        }
+        Stmt::Expr(Expr::If(expr_if)) => {
+            if let Expr::Let(ref mut expr) = &mut *expr_if.cond {
+                Context::new(mutability).replace_expr_let(expr);
+                return Ok(true);
+            }
         }
         Stmt::Local(local) => Context::new(mutability).replace_local(local)?,
-        Stmt::Item(Item::Fn(item)) => replace_item_fn(item, mutability)?,
-        Stmt::Item(Item::Impl(item)) => replace_item_impl(item, mutability),
-        Stmt::Item(Item::Use(item)) => replace_item_use(item, mutability)?,
         _ => {}
     }
+    Ok(false)
+}
+
+fn parse(mut stmt: Stmt, mutability: Mutability) -> Result<TokenStream> {
+    if !replace_stmt(&mut stmt, mutability)? {
+        match &mut stmt {
+            Stmt::Item(Item::Fn(item)) => replace_item_fn(item, mutability)?,
+            Stmt::Item(Item::Impl(item)) => replace_item_impl(item, mutability),
+            Stmt::Item(Item::Use(item)) => replace_item_use(item, mutability)?,
+            _ => {}
+        }
+    }
 
     Ok(stmt.into_token_stream())
 }
@@ -73,6 +89,10 @@ impl Context {
         Ok(())
     }
 
+    fn replace_expr_let(&mut self, expr: &mut ExprLet) {
+        self.replace_pat(&mut expr.pat, true)
+    }
+
     fn replace_expr_match(&mut self, expr: &mut ExprMatch) {
         expr.arms.iter_mut().for_each(|Arm { pat, .. }| self.replace_pat(pat, true))
     }
@@ -195,17 +215,18 @@ impl FnVisitor {
                 expr.attrs.find_remove(self.name())?
             }
             Stmt::Local(local) => local.attrs.find_remove(self.name())?,
+            Stmt::Expr(Expr::If(expr_if)) => {
+                if let Expr::Let(_) = &*expr_if.cond {
+                    expr_if.attrs.find_remove(self.name())?
+                } else {
+                    None
+                }
+            }
             _ => return Ok(()),
         };
         if let Some(attr) = attr {
             parse_as_empty(&attr.tokens)?;
-            match node {
-                Stmt::Expr(Expr::Match(expr)) | Stmt::Semi(Expr::Match(expr), _) => {
-                    Context::new(self.mutability).replace_expr_match(expr)
-                }
-                Stmt::Local(local) => Context::new(self.mutability).replace_local(local)?,
-                _ => unreachable!(),
-            }
+            replace_stmt(node, self.mutability)?;
         }
         Ok(())
     }