1use std::convert::TryFrom;
2
3use proc_macro2::TokenStream;
4use quote::ToTokens;
5use syn::{self, spanned::Spanned, Block, LitStr};
6
7/// A wrapper for expressions/blocks which automatically adds the start and end
8/// braces.
9///
10/// - **full access** to variables environment.
11/// - **full access** to control-flow of the environment via `return`, `?` etc.
12#[derive(Debug, Clone)]
13pub struct BlockContents(Block);
14
15impl BlockContents {
16 pub fn is_empty(&self) -> bool {
17 self.0.stmts.is_empty()
18 }
19}
20
21impl ToTokens for BlockContents {
22 fn to_tokens(&self, tokens: &mut TokenStream) {
23 self.0.to_tokens(tokens)
24 }
25}
26
27impl TryFrom<&'_ LitStr> for BlockContents {
28 type Error = syn::Error;
29
30 fn try_from(s: &LitStr) -> Result<Self, Self::Error> {
31 let mut block_str: String = s.value();
32 block_str.insert(idx:0, ch:'{');
33 block_str.push(ch:'}');
34 LitStr::new(&block_str, s.span()).parse().map(Self)
35 }
36}
37
38impl From<syn::Expr> for BlockContents {
39 fn from(v: syn::Expr) -> Self {
40 Self(Block {
41 brace_token: syn::token::Brace(v.span()),
42 stmts: vec![syn::Stmt::Expr(v)],
43 })
44 }
45}
46
47impl darling::FromMeta for BlockContents {
48 fn from_value(value: &syn::Lit) -> darling::Result<Self> {
49 if let syn::Lit::Str(s: &LitStr) = value {
50 let contents: BlockContents = BlockContents::try_from(s)?;
51 if contents.is_empty() {
52 Err(darling::Error::unknown_value("").with_span(node:s))
53 } else {
54 Ok(contents)
55 }
56 } else {
57 Err(darling::Error::unexpected_lit_type(lit:value))
58 }
59 }
60}
61
62#[cfg(test)]
63mod test {
64 use std::convert::TryInto;
65
66 use super::*;
67 use proc_macro2::Span;
68
69 fn parse(s: &str) -> Result<BlockContents, syn::Error> {
70 (&LitStr::new(s, Span::call_site())).try_into()
71 }
72
73 #[test]
74 #[should_panic(expected = r#"lex error"#)]
75 fn block_invalid_token_trees() {
76 parse("let x = 2; { x+1").unwrap();
77 }
78
79 #[test]
80 fn block_delimited_token_tree() {
81 let expr = parse("let x = 2; { x+1 }").unwrap();
82 assert_eq!(
83 quote!(#expr).to_string(),
84 quote!({
85 let x = 2;
86 {
87 x + 1
88 }
89 })
90 .to_string()
91 );
92 }
93
94 #[test]
95 fn block_single_token_tree() {
96 let expr = parse("42").unwrap();
97 assert_eq!(quote!(#expr).to_string(), quote!({ 42 }).to_string());
98 }
99}
100