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
--> ***
|
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.