1use std::convert::TryFrom;
2
3use proc_macro2::{Span, TokenStream};
4use quote::ToTokens;
5use syn::{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 pub fn span(&self) -> Span {
21 self.0.span()
22 }
23}
24
25impl ToTokens for BlockContents {
26 fn to_tokens(&self, tokens: &mut TokenStream) {
27 self.0.to_tokens(tokens)
28 }
29}
30
31impl TryFrom<&'_ LitStr> for BlockContents {
32 type Error = syn::Error;
33
34 fn try_from(s: &LitStr) -> Result<Self, Self::Error> {
35 let mut block_str: String = s.value();
36 block_str.insert(idx:0, ch:'{');
37 block_str.push(ch:'}');
38 LitStr::new(&block_str, s.span()).parse().map(Self)
39 }
40}
41
42impl From<syn::Expr> for BlockContents {
43 fn from(v: syn::Expr) -> Self {
44 Self(Block {
45 brace_token: syn::token::Brace(v.span()),
46 stmts: vec![syn::Stmt::Expr(v, None)],
47 })
48 }
49}
50
51impl darling::FromMeta for BlockContents {
52 fn from_value(value: &syn::Lit) -> darling::Result<Self> {
53 if let syn::Lit::Str(s: &LitStr) = value {
54 let contents: BlockContents = BlockContents::try_from(s)?;
55 if contents.is_empty() {
56 Err(darling::Error::unknown_value("").with_span(node:s))
57 } else {
58 Ok(contents)
59 }
60 } else {
61 Err(darling::Error::unexpected_lit_type(lit:value))
62 }
63 }
64
65 fn from_expr(expr: &syn::Expr) -> darling::Result<Self> {
66 if let syn::Expr::Lit(lit: &ExprLit) = expr {
67 Self::from_value(&lit.lit)
68 } else {
69 Ok(Self::from(expr.clone()))
70 }
71 }
72}
73
74#[cfg(test)]
75mod test {
76 use std::convert::TryInto;
77
78 use super::*;
79
80 fn parse(s: &str) -> Result<BlockContents, syn::Error> {
81 (&LitStr::new(s, Span::call_site())).try_into()
82 }
83
84 #[test]
85 #[should_panic(expected = r#"cannot parse"#)]
86 fn block_invalid_token_trees() {
87 parse("let x = 2; { x+1").unwrap();
88 }
89
90 #[test]
91 fn block_delimited_token_tree() {
92 let expr = parse("let x = 2; { x+1 }").unwrap();
93 assert_eq!(
94 quote!(#expr).to_string(),
95 quote!({
96 let x = 2;
97 {
98 x + 1
99 }
100 })
101 .to_string()
102 );
103 }
104
105 #[test]
106 fn block_single_token_tree() {
107 let expr = parse("42").unwrap();
108 assert_eq!(quote!(#expr).to_string(), quote!({ 42 }).to_string());
109 }
110}
111

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