src/errors.rs

// Copyright (c) 2020 Google LLC All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

use {
    proc_macro2::{Span, TokenStream},
    quote::ToTokens,
    std::cell::RefCell,
};

/// A type for collecting procedural macro errors.
#[derive(Default)]
pub struct Errors {
    errors: RefCell<Vec<syn::Error>>,
}

/// Produce functions to expect particular variants of `syn::Lit`
macro_rules! expect_lit_fn {
    ($(($fn_name:ident, $syn_type:ident, $variant:ident, $lit_name:literal),)*) => {
        $(
            pub fn $fn_name<'a>(&self, lit: &'a syn::Lit) -> Option<&'a syn::$syn_type> {
                if let syn::Lit::$variant(inner) = lit {
                    Some(inner)
                } else {
                    self.unexpected_lit($lit_name, lit);
                    None
                }
            }
        )*
    }
}

/// Produce functions to expect particular variants of `syn::Meta`
macro_rules! expect_meta_fn {
    ($(($fn_name:ident, $syn_type:ident, $variant:ident, $meta_name:literal),)*) => {
        $(
            pub fn $fn_name<'a>(&self, meta: &'a syn::Meta) -> Option<&'a syn::$syn_type> {
                if let syn::Meta::$variant(inner) = meta {
                    Some(inner)
                } else {
                    self.unexpected_meta($meta_name, meta);
                    None
                }
            }
        )*
    }
}

impl Errors {
    /// Issue an error like:
    ///
    /// Duplicate foo attribute
    /// First foo attribute here
    pub fn duplicate_attrs(
        &self,
        attr_kind: &str,
        first: &impl syn::spanned::Spanned,
        second: &impl syn::spanned::Spanned,
    ) {
        self.duplicate_attrs_inner(attr_kind, first.span(), second.span())
    }

    fn duplicate_attrs_inner(&self, attr_kind: &str, first: Span, second: Span) {
        self.err_span(second, &["Duplicate ", attr_kind, " attribute"].concat());
        self.err_span(first, &["First ", attr_kind, " attribute here"].concat());
    }

    /// Error on literals, expecting attribute syntax.
    pub fn expect_nested_meta<'a>(&self, nm: &'a syn::NestedMeta) -> Option<&'a syn::Meta> {
        match nm {
            syn::NestedMeta::Lit(l) => {
                self.err(l, "Unexpected literal");
                None
            }
            syn::NestedMeta::Meta(m) => Some(m),
        }
    }

    /// Error on attribute syntax, expecting literals
    pub fn expect_nested_lit<'a>(&self, nm: &'a syn::NestedMeta) -> Option<&'a syn::Lit> {
        match nm {
            syn::NestedMeta::Meta(m) => {
                self.err(m, "Expected literal");
                None
            }
            syn::NestedMeta::Lit(l) => Some(l),
        }
    }

    expect_lit_fn![
        (expect_lit_str, LitStr, Str, "string"),
        (expect_lit_char, LitChar, Char, "character"),
        (expect_lit_int, LitInt, Int, "integer"),
    ];

    expect_meta_fn![
        (expect_meta_word, Path, Path, "path"),
        (expect_meta_list, MetaList, List, "list"),
        (expect_meta_name_value, MetaNameValue, NameValue, "name-value pair"),
    ];

    fn unexpected_lit(&self, expected: &str, found: &syn::Lit) {
        fn lit_kind(lit: &syn::Lit) -> &'static str {
            use syn::Lit::{Bool, Byte, ByteStr, Char, Float, Int, Str, Verbatim};
            match lit {
                Str(_) => "string",
                ByteStr(_) => "bytestring",
                Byte(_) => "byte",
                Char(_) => "character",
                Int(_) => "integer",
                Float(_) => "float",
                Bool(_) => "boolean",
                Verbatim(_) => "unknown (possibly extra-large integer)",
            }
        }

        self.err(
            found,
            &["Expected ", expected, " literal, found ", lit_kind(found), " literal"].concat(),
        )
    }

    fn unexpected_meta(&self, expected: &str, found: &syn::Meta) {
        fn meta_kind(meta: &syn::Meta) -> &'static str {
            use syn::Meta::{List, NameValue, Path};
            match meta {
                Path(_) => "path",
                List(_) => "list",
                NameValue(_) => "name-value pair",
            }
        }

        self.err(
            found,
            &["Expected ", expected, " attribute, found ", meta_kind(found), " attribute"].concat(),
        )
    }

    /// Issue an error relating to a particular `Spanned` structure.
    pub fn err(&self, spanned: &impl syn::spanned::Spanned, msg: &str) {
        self.err_span(spanned.span(), msg);
    }

    /// Issue an error relating to a particular `Span`.
    pub fn err_span(&self, span: Span, msg: &str) {
        self.push(syn::Error::new(span, msg));
    }

    /// Push a `syn::Error` onto the list of errors to issue.
    pub fn push(&self, err: syn::Error) {
        self.errors.borrow_mut().push(err);
    }
}

impl ToTokens for Errors {
    /// Convert the errors into tokens that, when emit, will cause
    /// the user of the macro to receive compiler errors.
    fn to_tokens(&self, tokens: &mut TokenStream) {
        tokens.extend(self.errors.borrow().iter().map(|e| e.to_compile_error()));
    }
}