This crate provides Rust's quasi-quoting macro, intended to use in your proc-macro, to generate `TokenStream`
expanding variable interporation and expanding templates.

This macro is constructed based on `proc_macro` crate.

# Interporation

Original `quote!` macro syntax is fully supported. See [quote's doc](https://docs.rs/quote/1.0.23/quote/).

For backward compatibility, interporation rule is same as traditional
`quote!` macro. The interporation is done with `#var` (similar to the
variable `$var` in `macro_rules!`). Most variables in `Syn` crate are
interporated using [`::proc_quote::ToTokens`] trait.

## Rules

Repetition is done using syntax like `#(...)*` or `#(...),*`. It repeats the
variables (`#var`) inside this syntax, which implements
[`::proc_quote::Repeat`].

- `#(...)*` - repeat ... with no separators. at least one variable should be
  included in ...
- `#(...),*` - same as before, but interporates with separator ','.

## Problem

The interporation rule is **rough**, so I implemented new 'template' syntax.
For example, the following code will not allowed, because `#var1` cannot be
iterated double.

```
# use template_quote::quote;
let var1 = vec!['a', 'b'];
let var2 = vec![vec![1, 2], vec![3, 4]];
let tokens = quote!{
	#(#(#var1 #var2)*)*
};
assert_eq!("'a' 1i32 'a' 2i32 'b' 3i32 'b' 4i32", tokens.to_string());
```

# Template syntax

Template syntax is proceedual-like syntax, which allows you to use structual
statementsinside the macro.

## If syntax

This code iterates around `#i` (with interporation), and emits `i32` into
`TokenStream` while the number meets the condition.

```
# use template_quote::quote;
let i = vec![1, 2, 3];
let tokens = quote!{
	#(
		#(if i > &2) {
			#i
		}
	)*
};
assert_eq!("3i32", tokens.to_string());
```

The if-else and if-else-if is also allowed.

```
# use template_quote::quote;
let i = vec![1, 2, 3];
let tokens = quote!{
	#(
		#(if i > &2) {
			+ #i
		}
		#(else) {
			- #i
		}
	)*
};
assert_eq!("- 1i32 - 2i32 + 3i32", tokens.to_string());
```

```
# use template_quote::quote;
let i = vec![1, 2, 3, 4, 5];
let tokens = quote!{
	#(
		#(if i % &2 == 0) {
			+ #i
		}
		#(else if i % &3 == 0) {
			- #i
		}
		#(else) {
			#i
		}
	)*
};
assert_eq!("1i32 + 2i32 - 3i32 + 4i32 5i32", tokens.to_string());
```

## For syntax

For syntax iterates around the variable (like interporation), but it
specifies which variable to iterate.

```
# use template_quote::quote;
let v1 = vec![1, 2];
let v2 = vec!['a', 'b'];
let tokens = quote!{
	#(for i1 in &v1) {
		#(for i2 in &v2) {
			#i1 -> #i2
		}
	}
};
assert_eq!("1i32 -> 'a' 1i32 -> 'b' 2i32 -> 'a' 2i32 -> 'b'", tokens.to_string());
```

Internal loop can be replaced with interporation:

```
# use template_quote::quote;
let v1 = vec![1, 2];
let v2 = vec!['a', 'b'];
let tokens = quote!{
	#(for i1 in &v1) {
		#(
			#i1 -> #v2
		)*
	}
};
assert_eq!("1i32 -> 'a' 1i32 -> 'b' 2i32 -> 'a' 2i32 -> 'b'", tokens.to_string());
```

You can also specify separator with for statement.

```
# use template_quote::quote;
let v = vec![1, 2];
let tokens = quote!{
	#(for i in v) | { #i }
};
assert_eq!("1i32 | 2i32", tokens.to_string());
```

Interporation is not usable with variables binded in for syntax. For
example,

```compile_fail
# use template_quote::quote;
let v = vec![vec![1, 2], vec![3]];
let tokens = quote!{
	#(
		#(for i in v) { #i }
	),*
};
assert_eq!("1i32 2i32 , 3i32", tokens.to_string());
```

will fail into error because no variables is available in the interporation
syntax.

```text
error: proc macro panicked
  --> ***
   |
6  |   let tokens = quote!{
   |  ______________^
7  | |     #(
8  | |         #(for i in v) { #i }
9  | |     )*
10 | | };
   | |_^
   |
   = help: message: Iterative vals not found
```

In this case, you can use `#(for i in #v)` syntax to specify which variable
to iterate with interporation:

```
# use template_quote::quote;
let v = vec![vec![1, 2], vec![3]];
let tokens = quote!{
	#(
		#(for i in #v) { #i }
	),*
};
assert_eq!("1i32 2i32 , 3i32", tokens.to_string());
```

## While syntax

```
# use template_quote::quote;
let mut v = vec![1, 2].into_iter();
let tokens = quote!{
	#(while v.next().is_some()) { hello }
};
assert_eq!("hello hello", tokens.to_string());
```

## While-Let syntax

```
# use template_quote::quote;
let mut v = vec![1, 2].into_iter();
let tokens = quote!{
	#(while let Some(i) = v.next()) { #i }
};
assert_eq!("1i32 2i32", tokens.to_string());
```

Same as 'for' syntax, the binded valiables in 'while' is not iteratable with
interporation syntax. For example,

```compile_fail
# use template_quote::quote;
let mut v = vec![1, 2].into_iter();
quote!{
	#(
		#(while let Some(i) = v.next()) { #i }
	)*
};
```

will fail.

## Let syntax

Let syntax bind new variables usable inside the block.

```
# use template_quote::quote;
let v = vec![(1, 'a'), (2, 'b')];
let tokens = quote!{
	#(for i in v), {
		#(let (n, c) = i) {
			#n -> #c
		}
	}
};
assert_eq!("1i32 -> 'a' , 2i32 -> 'b'", tokens.to_string());
```

Here, `#n` and `#c` is not iteratable with interporation syntax.

## Inline expression

You can place inline expression in `quote!` macro.

```
# use template_quote::quote;
let v = vec![1, 2];
let tokens = quote!{
	#(for i in v){
		#i -> #{ i.to_string() }
	}
};
assert_eq!("1i32 -> \"1\" 2i32 -> \"2\"", tokens.to_string());
```

The following example will fail to compile because it does not understand
which variable to be interpolated:

```compile_fail
# use template_quote::quote;
let v = vec![1, 2];
let tokens = quote!{
	#(
		#{ v.to_string() }
	)*
};
assert_eq!("\"1\" \"2\"", tokens.to_string());
```

In this case, you can use `#i` syntax in inline expression to specify which
variable to iterate with interporation syntax.

```
# use template_quote::quote;
let v = vec![1, 2];
let tokens = quote!{
	#(
		#{ #v.to_string() }
	)*
};
assert_eq!("\"1\" \"2\"", tokens.to_string());
```

## Inline statement

You can place arbitrary statement inside this macro. For example,

```
# use template_quote::quote;
let v = vec![1, 2, 3];
let tokens = quote!{
	#(
		#v
		#{ eprintln!("debug: {}", &v); }
	)*
};
assert_eq!("1i32 2i32 3i32", tokens.to_string());
```

will print:

```text
debug: 1
debug: 2
debug: 3
```

To be distinguishable, all statements have to end with ';'. For example,
'if' statement in inline statement syntax should placed with extra ';'.

```
# use template_quote::quote;
let v = vec![1, 2, 3];
quote!{
	#(
		#v
		#{ if v >= &2 { eprintln!("debug: {}", &v); } ; }
	)*
};
```

## Break, Continue

You can put control statement like `break` or `continue` in inline
statement, but it is a bit danger.

If you use `break;` inside block (like `{ ... }` or `( ... )`), `break` will
suddenly give up emitting whole group, and nothing will be emitted. For
example, the following code does not emit any group:

```
# use template_quote::quote;
let v = vec![1, 2, 3];
let tokens = quote!{
	#(for i in v) {
		#i // this is emitted once
		// The block is not emitted
		{
			#i
			#{ break; }
		}
	}
};
assert_eq!("1i32", tokens.to_string());
```

`break` also affects on interporation syntax like:

```
# use template_quote::quote;
let v = vec![1, 2, 3];
let tokens = quote!{
	#(
		#v
		#{ break; }
	),*
};
assert_eq!("1i32", tokens.to_string());
```

Unfortunately, `break` will leak outside of `quote!` macro. This is example
which the internal `break` affects on 'for' loop, which is placed outer of
the `quote!` macro.

```
# use template_quote::quote;
let mut v = Vec::new();
for _ in 0..3 {
	let tokens = quote!{
		#{ break; }
	};
	v.push(tokens);
}
assert_eq!(v.len(), 0);
```

This crate provides quasi-quoting macros like [quote](https://github.com/dtolnay/quote).
This crate has backward-compatibility with original `quote!` macro and also provides
new template-engine like syntax.

This crate is get some inspiration from [proc-quote](https://crates.io/crates/proc-quote).

# Using this crate

This crate is useful for developing proc-macro. Usually an proc-macro crate using template_quote is placed with following `Cargo.toml`:

```Cargo.toml
[package]
name = "your_crate_name"
version = "0.0.0"
edition = "2021"

[lib]
proc-macro = true

[dependencies]
template-quote = "0.2"
proc-macro2 = "1.0"
```

and with following `src/lib.rs` code:

```lib.rs
extern crate proc_macro;
extern crate proc_macro2;
extern crate template_quote;

use template_quote::quote;
use proc_macro::TokenStream;
use proc_macro2::TokenStream as TokenStream2;

#[proc_macro]
pub fn my_macro(_: TokenStream) -> TokenStream {
	quote! { /* something here */ }.into()
}
```

then you will be able to use it like:

```rust
extern crate your_crate_name;
use your_crate_name::my_macro;

my_macro!()
```


# Limitation

- If the punct token before '#' in the macro body has `Spacing::Join`, then the emitting punct also has same spacing, whether the '#' token is processed by the macro or not.