aboutsummaryrefslogtreecommitdiff
path: root/syn/src/error.rs
blob: 146d6522992a8377406505bbe79ec592bc293fb9 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
use std;
use std::fmt::{self, Debug, Display};
use std::iter::FromIterator;
use std::slice;
use std::vec;

use proc_macro2::{
    Delimiter, Group, Ident, LexError, Literal, Punct, Spacing, Span, TokenStream, TokenTree,
};
#[cfg(feature = "printing")]
use quote::ToTokens;

#[cfg(feature = "parsing")]
use crate::buffer::Cursor;
use crate::thread::ThreadBound;

/// The result of a Syn parser.
pub type Result<T> = std::result::Result<T, Error>;

/// Error returned when a Syn parser cannot parse the input tokens.
///
/// # Error reporting in proc macros
///
/// The correct way to report errors back to the compiler from a procedural
/// macro is by emitting an appropriately spanned invocation of
/// [`compile_error!`] in the generated code. This produces a better diagnostic
/// message than simply panicking the macro.
///
/// [`compile_error!`]: https://doc.rust-lang.org/std/macro.compile_error.html
///
/// When parsing macro input, the [`parse_macro_input!`] macro handles the
/// conversion to `compile_error!` automatically.
///
/// ```
/// extern crate proc_macro;
///
/// use proc_macro::TokenStream;
/// use syn::{parse_macro_input, AttributeArgs, ItemFn};
///
/// # const IGNORE: &str = stringify! {
/// #[proc_macro_attribute]
/// # };
/// pub fn my_attr(args: TokenStream, input: TokenStream) -> TokenStream {
///     let args = parse_macro_input!(args as AttributeArgs);
///     let input = parse_macro_input!(input as ItemFn);
///
///     /* ... */
///     # TokenStream::new()
/// }
/// ```
///
/// For errors that arise later than the initial parsing stage, the
/// [`.to_compile_error()`] method can be used to perform an explicit conversion
/// to `compile_error!`.
///
/// [`.to_compile_error()`]: Error::to_compile_error
///
/// ```
/// # extern crate proc_macro;
/// #
/// # use proc_macro::TokenStream;
/// # use syn::{parse_macro_input, DeriveInput};
/// #
/// # const IGNORE: &str = stringify! {
/// #[proc_macro_derive(MyDerive)]
/// # };
/// pub fn my_derive(input: TokenStream) -> TokenStream {
///     let input = parse_macro_input!(input as DeriveInput);
///
///     // fn(DeriveInput) -> syn::Result<proc_macro2::TokenStream>
///     expand::my_derive(input)
///         .unwrap_or_else(|err| err.to_compile_error())
///         .into()
/// }
/// #
/// # mod expand {
/// #     use proc_macro2::TokenStream;
/// #     use syn::{DeriveInput, Result};
/// #
/// #     pub fn my_derive(input: DeriveInput) -> Result<TokenStream> {
/// #         unimplemented!()
/// #     }
/// # }
/// ```
#[derive(Clone)]
pub struct Error {
    messages: Vec<ErrorMessage>,
}

struct ErrorMessage {
    // Span is implemented as an index into a thread-local interner to keep the
    // size small. It is not safe to access from a different thread. We want
    // errors to be Send and Sync to play nicely with the Failure crate, so pin
    // the span we're given to its original thread and assume it is
    // Span::call_site if accessed from any other thread.
    start_span: ThreadBound<Span>,
    end_span: ThreadBound<Span>,
    message: String,
}

#[cfg(test)]
struct _Test
where
    Error: Send + Sync;

impl Error {
    /// Usually the [`ParseStream::error`] method will be used instead, which
    /// automatically uses the correct span from the current position of the
    /// parse stream.
    ///
    /// Use `Error::new` when the error needs to be triggered on some span other
    /// than where the parse stream is currently positioned.
    ///
    /// [`ParseStream::error`]: crate::parse::ParseBuffer::error
    ///
    /// # Example
    ///
    /// ```
    /// use syn::{Error, Ident, LitStr, Result, Token};
    /// use syn::parse::ParseStream;
    ///
    /// // Parses input that looks like `name = "string"` where the key must be
    /// // the identifier `name` and the value may be any string literal.
    /// // Returns the string literal.
    /// fn parse_name(input: ParseStream) -> Result<LitStr> {
    ///     let name_token: Ident = input.parse()?;
    ///     if name_token != "name" {
    ///         // Trigger an error not on the current position of the stream,
    ///         // but on the position of the unexpected identifier.
    ///         return Err(Error::new(name_token.span(), "expected `name`"));
    ///     }
    ///     input.parse::<Token![=]>()?;
    ///     let s: LitStr = input.parse()?;
    ///     Ok(s)
    /// }
    /// ```
    pub fn new<T: Display>(span: Span, message: T) -> Self {
        Error {
            messages: vec![ErrorMessage {
                start_span: ThreadBound::new(span),
                end_span: ThreadBound::new(span),
                message: message.to_string(),
            }],
        }
    }

    /// Creates an error with the specified message spanning the given syntax
    /// tree node.
    ///
    /// Unlike the `Error::new` constructor, this constructor takes an argument
    /// `tokens` which is a syntax tree node. This allows the resulting `Error`
    /// to attempt to span all tokens inside of `tokens`. While you would
    /// typically be able to use the `Spanned` trait with the above `Error::new`
    /// constructor, implementation limitations today mean that
    /// `Error::new_spanned` may provide a higher-quality error message on
    /// stable Rust.
    ///
    /// When in doubt it's recommended to stick to `Error::new` (or
    /// `ParseStream::error`)!
    #[cfg(feature = "printing")]
    pub fn new_spanned<T: ToTokens, U: Display>(tokens: T, message: U) -> Self {
        let mut iter = tokens.into_token_stream().into_iter();
        let start = iter.next().map_or_else(Span::call_site, |t| t.span());
        let end = iter.last().map_or(start, |t| t.span());
        Error {
            messages: vec![ErrorMessage {
                start_span: ThreadBound::new(start),
                end_span: ThreadBound::new(end),
                message: message.to_string(),
            }],
        }
    }

    /// The source location of the error.
    ///
    /// Spans are not thread-safe so this function returns `Span::call_site()`
    /// if called from a different thread than the one on which the `Error` was
    /// originally created.
    pub fn span(&self) -> Span {
        let start = match self.messages[0].start_span.get() {
            Some(span) => *span,
            None => return Span::call_site(),
        };
        let end = match self.messages[0].end_span.get() {
            Some(span) => *span,
            None => return Span::call_site(),
        };
        start.join(end).unwrap_or(start)
    }

    /// Render the error as an invocation of [`compile_error!`].
    ///
    /// The [`parse_macro_input!`] macro provides a convenient way to invoke
    /// this method correctly in a procedural macro.
    ///
    /// [`compile_error!`]: https://doc.rust-lang.org/std/macro.compile_error.html
    pub fn to_compile_error(&self) -> TokenStream {
        self.messages
            .iter()
            .map(ErrorMessage::to_compile_error)
            .collect()
    }

    /// Add another error message to self such that when `to_compile_error()` is
    /// called, both errors will be emitted together.
    pub fn combine(&mut self, another: Error) {
        self.messages.extend(another.messages)
    }
}

impl ErrorMessage {
    fn to_compile_error(&self) -> TokenStream {
        let start = self
            .start_span
            .get()
            .cloned()
            .unwrap_or_else(Span::call_site);
        let end = self.end_span.get().cloned().unwrap_or_else(Span::call_site);

        // compile_error!($message)
        TokenStream::from_iter(vec![
            TokenTree::Ident(Ident::new("compile_error", start)),
            TokenTree::Punct({
                let mut punct = Punct::new('!', Spacing::Alone);
                punct.set_span(start);
                punct
            }),
            TokenTree::Group({
                let mut group = Group::new(Delimiter::Brace, {
                    TokenStream::from_iter(vec![TokenTree::Literal({
                        let mut string = Literal::string(&self.message);
                        string.set_span(end);
                        string
                    })])
                });
                group.set_span(end);
                group
            }),
        ])
    }
}

#[cfg(feature = "parsing")]
pub fn new_at<T: Display>(scope: Span, cursor: Cursor, message: T) -> Error {
    if cursor.eof() {
        Error::new(scope, format!("unexpected end of input, {}", message))
    } else {
        let span = crate::buffer::open_span_of_group(cursor);
        Error::new(span, message)
    }
}

impl Debug for Error {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        if self.messages.len() == 1 {
            formatter
                .debug_tuple("Error")
                .field(&self.messages[0])
                .finish()
        } else {
            formatter
                .debug_tuple("Error")
                .field(&self.messages)
                .finish()
        }
    }
}

impl Debug for ErrorMessage {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        Debug::fmt(&self.message, formatter)
    }
}

impl Display for Error {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        formatter.write_str(&self.messages[0].message)
    }
}

impl Clone for ErrorMessage {
    fn clone(&self) -> Self {
        let start = self
            .start_span
            .get()
            .cloned()
            .unwrap_or_else(Span::call_site);
        let end = self.end_span.get().cloned().unwrap_or_else(Span::call_site);
        ErrorMessage {
            start_span: ThreadBound::new(start),
            end_span: ThreadBound::new(end),
            message: self.message.clone(),
        }
    }
}

impl std::error::Error for Error {
    fn description(&self) -> &str {
        "parse error"
    }
}

impl From<LexError> for Error {
    fn from(err: LexError) -> Self {
        Error::new(Span::call_site(), format!("{:?}", err))
    }
}

impl IntoIterator for Error {
    type Item = Error;
    type IntoIter = IntoIter;

    fn into_iter(self) -> Self::IntoIter {
        IntoIter {
            messages: self.messages.into_iter(),
        }
    }
}

pub struct IntoIter {
    messages: vec::IntoIter<ErrorMessage>,
}

impl Iterator for IntoIter {
    type Item = Error;

    fn next(&mut self) -> Option<Self::Item> {
        Some(Error {
            messages: vec![self.messages.next()?],
        })
    }
}

impl<'a> IntoIterator for &'a Error {
    type Item = Error;
    type IntoIter = Iter<'a>;

    fn into_iter(self) -> Self::IntoIter {
        Iter {
            messages: self.messages.iter(),
        }
    }
}

pub struct Iter<'a> {
    messages: slice::Iter<'a, ErrorMessage>,
}

impl<'a> Iterator for Iter<'a> {
    type Item = Error;

    fn next(&mut self) -> Option<Self::Item> {
        Some(Error {
            messages: vec![self.messages.next()?.clone()],
        })
    }
}