feat(core): support generics (especially Param) in #[command] (#1622)

* wip: param argument proof of concept for #[command]

* use macros for automatic type inference in commands

* refactor command for better error handling

* remove redundant ToTokens impl for Wrapper and Handler

* create `StateP` to allow state to use type inference during commands

* wrap State instead of T

* remove accidental edit of attribute

* remove StateP

because we recommend `_: Window<P>` for type inference, the following
function types are now supported:
* Pat::Wild (arg: "_")
* Pat::Struct (arg: final path segment)
* Pat::TupleStruct (arg: final path segment)

* add wildcard, struct, and tuple struct commands to examples

* better unsupported command argument message

* feat(examples): move some commands to a separate module

* add change file

Co-authored-by: Lucas Nogueira <lucas@tauri.studio>

.changes/command-generics.md

@@ -0,0 +1,6 @@
+---
+"tauri-macros": patch
+---
+
+`#[command]` now generates a macro instead of a function to allow passing through `Params` and other generics.
+`generate_handler!` has been changed to consume the generated `#[command]` macro

core/tauri-macros/src/command/handler.rs

@@ -0,0 +1,65 @@
+// Copyright 2019-2021 Tauri Programme within The Commons Conservancy
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: MIT
+
+use syn::{
+  parse::{Parse, ParseBuffer},
+  Ident, Path, Token,
+};
+
+/// The items parsed from [`generate_handle!`](crate::generate_handle).
+pub struct Handler {
+  paths: Vec<Path>,
+  commands: Vec<Ident>,
+  wrappers: Vec<Path>,
+}
+
+impl Parse for Handler {
+  fn parse(input: &ParseBuffer) -> syn::Result<Self> {
+    let paths = input.parse_terminated::<Path, Token![,]>(Path::parse)?;
+
+    // parse the command names and wrappers from the passed paths
+    let (commands, wrappers) = paths
+      .iter()
+      .map(|path| {
+        let mut wrapper = path.clone();
+        let last = super::path_to_command(&mut wrapper);
+
+        // the name of the actual command function
+        let command = last.ident.clone();
+
+        // set the path to the command function wrapper
+        last.ident = super::format_command_wrapper(&command);
+
+        (command, wrapper)
+      })
+      .unzip();
+
+    Ok(Self {
+      paths: paths.into_iter().collect(), // remove punctuation separators
+      commands,
+      wrappers,
+    })
+  }
+}
+
+impl From<Handler> for proc_macro::TokenStream {
+  fn from(
+    Handler {
+      paths,
+      commands,
+      wrappers,
+    }: Handler,
+  ) -> Self {
+    quote::quote!(move |invoke| {
+      let cmd = invoke.message.command();
+      match cmd {
+        #(stringify!(#commands) => #wrappers!(#paths, invoke),)*
+        _ => {
+          invoke.resolver.reject(format!("command {} not found", cmd))
+        },
+      }
+    })
+    .into()
+  }
+}

core/tauri-macros/src/command/mod.rs

@@ -0,0 +1,27 @@
+// Copyright 2019-2021 Tauri Programme within The Commons Conservancy
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: MIT
+
+use proc_macro2::Ident;
+use syn::{Path, PathSegment};
+
+pub use self::{
+  handler::Handler,
+  wrapper::{Wrapper, WrapperBody},
+};
+
+mod handler;
+mod wrapper;
+
+/// The autogenerated wrapper ident.
+fn format_command_wrapper(function: &Ident) -> Ident {
+  quote::format_ident!("__cmd__{}", function)
+}
+
+/// This function will panic if the passed [`syn::Path`] does not have any segments.
+fn path_to_command(path: &mut Path) -> &mut PathSegment {
+  path
+    .segments
+    .last_mut()
+    .expect("parsed syn::Path has no segment")
+}