1 | use std::convert::TryFrom; |
2 | |
3 | use proc_macro2::TokenStream; |
4 | use quote::ToTokens; |
5 | use 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)] |
13 | pub struct BlockContents(Block); |
14 | |
15 | impl BlockContents { |
16 | pub fn is_empty(&self) -> bool { |
17 | self.0.stmts.is_empty() |
18 | } |
19 | } |
20 | |
21 | impl ToTokens for BlockContents { |
22 | fn to_tokens(&self, tokens: &mut TokenStream) { |
23 | self.0.to_tokens(tokens) |
24 | } |
25 | } |
26 | |
27 | impl 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 | |
38 | impl 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 | |
47 | impl 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)] |
63 | mod 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 | |