feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/basic-toml/src/ser.rs

@@ -0,0 +1,833 @@
+use serde::ser::{self, Serialize};
+use std::cell::Cell;
+use std::error;
+use std::fmt::{self, Display, Write};
+
+/// Serialize the given data structure as a String of TOML.
+///
+/// Serialization can fail if `T`'s implementation of `Serialize` decides to
+/// fail, if `T` contains a map with non-string keys, or if `T` attempts to
+/// serialize an unsupported datatype such as an enum, tuple, or tuple struct.
+pub fn to_string<T>(value: &T) -> Result<String, crate::Error>
+where
+    T: ?Sized + Serialize,
+{
+    let mut dst = String::with_capacity(128);
+    value.serialize(&mut Serializer::new(&mut dst))?;
+    Ok(dst)
+}
+
+#[derive(Debug)]
+pub(crate) enum Error {
+    /// Indicates that a Rust type was requested to be serialized but it was not
+    /// supported.
+    ///
+    /// Currently the TOML format does not support serializing types such as
+    /// enums, tuples and tuple structs.
+    UnsupportedType,
+
+    /// The key of all TOML maps must be strings, but serialization was
+    /// attempted where the key of a map was not a string.
+    KeyNotString,
+
+    /// All values in a TOML table must be emitted before further tables are
+    /// emitted. If a value is emitted *after* a table then this error is
+    /// generated.
+    ValueAfterTable,
+
+    /// None was attempted to be serialized, but it's not supported.
+    UnsupportedNone,
+
+    /// A custom error which could be generated when serializing a particular
+    /// type.
+    Custom(String),
+}
+
+struct Serializer<'a> {
+    dst: &'a mut String,
+    state: State<'a>,
+}
+
+#[derive(Debug, Copy, Clone)]
+enum ArrayState {
+    Started,
+    StartedAsATable,
+}
+
+#[derive(Debug, Clone)]
+enum State<'a> {
+    Table {
+        key: &'a str,
+        parent: &'a State<'a>,
+        first: &'a Cell<bool>,
+        table_emitted: &'a Cell<bool>,
+    },
+    Array {
+        parent: &'a State<'a>,
+        first: &'a Cell<bool>,
+        type_: &'a Cell<Option<ArrayState>>,
+        len: Option<usize>,
+    },
+    End,
+}
+
+struct SerializeSeq<'a, 'b> {
+    ser: &'b mut Serializer<'a>,
+    first: Cell<bool>,
+    type_: Cell<Option<ArrayState>>,
+    len: Option<usize>,
+}
+
+struct SerializeTable<'a, 'b> {
+    ser: &'b mut Serializer<'a>,
+    key: String,
+    first: Cell<bool>,
+    table_emitted: Cell<bool>,
+}
+
+impl<'a> Serializer<'a> {
+    fn new(dst: &'a mut String) -> Serializer<'a> {
+        Serializer {
+            dst,
+            state: State::End,
+        }
+    }
+
+    fn display<T: Display>(&mut self, t: T, type_: ArrayState) -> Result<(), Error> {
+        self.emit_key(type_)?;
+        write!(self.dst, "{}", t).map_err(ser::Error::custom)?;
+        if let State::Table { .. } = self.state {
+            self.dst.push('\n');
+        }
+        Ok(())
+    }
+
+    fn emit_key(&mut self, type_: ArrayState) -> Result<(), Error> {
+        self.array_type(type_);
+        let state = self.state.clone();
+        self.do_emit_key(&state)
+    }
+
+    // recursive implementation of `emit_key` above
+    fn do_emit_key(&mut self, state: &State) -> Result<(), Error> {
+        match *state {
+            State::End => Ok(()),
+            State::Array {
+                parent,
+                first,
+                type_,
+                len,
+            } => {
+                assert!(type_.get().is_some());
+                if first.get() {
+                    self.do_emit_key(parent)?;
+                }
+                self.emit_array(first, len);
+                Ok(())
+            }
+            State::Table {
+                parent,
+                first,
+                table_emitted,
+                key,
+            } => {
+                if table_emitted.get() {
+                    return Err(Error::ValueAfterTable);
+                }
+                if first.get() {
+                    self.emit_table_header(parent)?;
+                    first.set(false);
+                }
+                self.escape_key(key)?;
+                self.dst.push_str(" = ");
+                Ok(())
+            }
+        }
+    }
+
+    fn emit_array(&mut self, first: &Cell<bool>, _len: Option<usize>) {
+        if first.get() {
+            self.dst.push('[');
+        } else {
+            self.dst.push_str(", ");
+        }
+    }
+
+    fn array_type(&mut self, type_: ArrayState) {
+        let prev = match self.state {
+            State::Array { type_, .. } => type_,
+            _ => return,
+        };
+        if prev.get().is_none() {
+            prev.set(Some(type_));
+        }
+    }
+
+    fn escape_key(&mut self, key: &str) -> Result<(), Error> {
+        let ok = !key.is_empty()
+            && key.chars().all(|c| match c {
+                'a'..='z' | 'A'..='Z' | '0'..='9' | '-' | '_' => true,
+                _ => false,
+            });
+        if ok {
+            write!(self.dst, "{}", key).map_err(ser::Error::custom)?;
+        } else {
+            self.emit_str(key)?;
+        }
+        Ok(())
+    }
+
+    fn emit_str(&mut self, value: &str) -> Result<(), Error> {
+        self.dst.push('"');
+        for ch in value.chars() {
+            match ch {
+                '\u{8}' => self.dst.push_str("\\b"),
+                '\u{9}' => self.dst.push_str("\\t"),
+                '\u{a}' => self.dst.push_str("\\n"),
+                '\u{c}' => self.dst.push_str("\\f"),
+                '\u{d}' => self.dst.push_str("\\r"),
+                '\u{22}' => self.dst.push_str("\\\""),
+                '\u{5c}' => self.dst.push_str("\\\\"),
+                c if c <= '\u{1f}' || c == '\u{7f}' => {
+                    write!(self.dst, "\\u{:04X}", ch as u32).map_err(ser::Error::custom)?;
+                }
+                ch => self.dst.push(ch),
+            }
+        }
+        self.dst.push('"');
+        Ok(())
+    }
+
+    fn emit_table_header(&mut self, state: &State) -> Result<(), Error> {
+        let array_of_tables = match *state {
+            State::End => return Ok(()),
+            State::Array { .. } => true,
+            State::Table { .. } => false,
+        };
+
+        // Unlike [..]s, we can't omit [[..]] ancestors, so be sure to emit
+        // table headers for them.
+        let mut p = state;
+        if let State::Array { first, parent, .. } = *state {
+            if first.get() {
+                p = parent;
+            }
+        }
+        while let State::Table { first, parent, .. } = *p {
+            p = parent;
+            if !first.get() {
+                break;
+            }
+            if let State::Array {
+                parent: &State::Table { .. },
+                ..
+            } = *parent
+            {
+                self.emit_table_header(parent)?;
+                break;
+            }
+        }
+
+        match *state {
+            State::Table { first, .. } => {
+                if !first.get() {
+                    // Newline if we are a table that is not the first table in
+                    // the document.
+                    self.dst.push('\n');
+                }
+            }
+            State::Array { parent, first, .. } => {
+                if !first.get() {
+                    // Always newline if we are not the first item in the
+                    // table-array
+                    self.dst.push('\n');
+                } else if let State::Table { first, .. } = *parent {
+                    if !first.get() {
+                        // Newline if we are not the first item in the document
+                        self.dst.push('\n');
+                    }
+                }
+            }
+            State::End => {}
+        }
+        self.dst.push('[');
+        if array_of_tables {
+            self.dst.push('[');
+        }
+        self.emit_key_part(state)?;
+        if array_of_tables {
+            self.dst.push(']');
+        }
+        self.dst.push_str("]\n");
+        Ok(())
+    }
+
+    fn emit_key_part(&mut self, key: &State) -> Result<bool, Error> {
+        match *key {
+            State::Array { parent, .. } => self.emit_key_part(parent),
+            State::End => Ok(true),
+            State::Table {
+                key,
+                parent,
+                table_emitted,
+                ..
+            } => {
+                table_emitted.set(true);
+                let first = self.emit_key_part(parent)?;
+                if !first {
+                    self.dst.push('.');
+                }
+                self.escape_key(key)?;
+                Ok(false)
+            }
+        }
+    }
+}
+
+macro_rules! serialize_float {
+    ($this:expr, $v:expr) => {{
+        $this.emit_key(ArrayState::Started)?;
+        match ($v.is_sign_negative(), $v.is_nan(), $v == 0.0) {
+            (_, true, _) => write!($this.dst, "nan"),
+            (true, false, true) => write!($this.dst, "-0.0"),
+            (false, false, true) => write!($this.dst, "0.0"),
+            (_, false, false) => write!($this.dst, "{}", $v).and_then(|()| {
+                if $v % 1.0 == 0.0 {
+                    write!($this.dst, ".0")
+                } else {
+                    Ok(())
+                }
+            }),
+        }
+        .map_err(ser::Error::custom)?;
+
+        if let State::Table { .. } = $this.state {
+            $this.dst.push_str("\n");
+        }
+        return Ok(());
+    }};
+}
+
+impl<'a, 'b> ser::Serializer for &'b mut Serializer<'a> {
+    type Ok = ();
+    type Error = Error;
+    type SerializeSeq = SerializeSeq<'a, 'b>;
+    type SerializeTuple = SerializeSeq<'a, 'b>;
+    type SerializeTupleStruct = SerializeSeq<'a, 'b>;
+    type SerializeTupleVariant = ser::Impossible<(), Error>;
+    type SerializeMap = SerializeTable<'a, 'b>;
+    type SerializeStruct = SerializeTable<'a, 'b>;
+    type SerializeStructVariant = ser::Impossible<(), Error>;
+
+    fn serialize_bool(self, v: bool) -> Result<(), Self::Error> {
+        self.display(v, ArrayState::Started)
+    }
+
+    fn serialize_i8(self, v: i8) -> Result<(), Self::Error> {
+        self.display(v, ArrayState::Started)
+    }
+
+    fn serialize_i16(self, v: i16) -> Result<(), Self::Error> {
+        self.display(v, ArrayState::Started)
+    }
+
+    fn serialize_i32(self, v: i32) -> Result<(), Self::Error> {
+        self.display(v, ArrayState::Started)
+    }
+
+    fn serialize_i64(self, v: i64) -> Result<(), Self::Error> {
+        self.display(v, ArrayState::Started)
+    }
+
+    fn serialize_u8(self, v: u8) -> Result<(), Self::Error> {
+        self.display(v, ArrayState::Started)
+    }
+
+    fn serialize_u16(self, v: u16) -> Result<(), Self::Error> {
+        self.display(v, ArrayState::Started)
+    }
+
+    fn serialize_u32(self, v: u32) -> Result<(), Self::Error> {
+        self.display(v, ArrayState::Started)
+    }
+
+    fn serialize_u64(self, v: u64) -> Result<(), Self::Error> {
+        self.display(v, ArrayState::Started)
+    }
+
+    fn serialize_f32(self, v: f32) -> Result<(), Self::Error> {
+        serialize_float!(self, v)
+    }
+
+    fn serialize_f64(self, v: f64) -> Result<(), Self::Error> {
+        serialize_float!(self, v)
+    }
+
+    fn serialize_char(self, v: char) -> Result<(), Self::Error> {
+        let mut buf = [0; 4];
+        self.serialize_str(v.encode_utf8(&mut buf))
+    }
+
+    fn serialize_str(self, value: &str) -> Result<(), Self::Error> {
+        self.emit_key(ArrayState::Started)?;
+        self.emit_str(value)?;
+        if let State::Table { .. } = self.state {
+            self.dst.push('\n');
+        }
+        Ok(())
+    }
+
+    fn serialize_bytes(self, value: &[u8]) -> Result<(), Self::Error> {
+        value.serialize(self)
+    }
+
+    fn serialize_none(self) -> Result<(), Self::Error> {
+        Err(Error::UnsupportedNone)
+    }
+
+    fn serialize_some<T>(self, value: &T) -> Result<(), Self::Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_unit(self) -> Result<(), Self::Error> {
+        Err(Error::UnsupportedType)
+    }
+
+    fn serialize_unit_struct(self, _name: &'static str) -> Result<(), Self::Error> {
+        Err(Error::UnsupportedType)
+    }
+
+    fn serialize_unit_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        variant: &'static str,
+    ) -> Result<(), Self::Error> {
+        self.serialize_str(variant)
+    }
+
+    fn serialize_newtype_struct<T>(self, _name: &'static str, value: &T) -> Result<(), Self::Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_newtype_variant<T>(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _value: &T,
+    ) -> Result<(), Self::Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        Err(Error::UnsupportedType)
+    }
+
+    fn serialize_seq(self, len: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {
+        self.array_type(ArrayState::Started);
+        Ok(SerializeSeq {
+            ser: self,
+            first: Cell::new(true),
+            type_: Cell::new(None),
+            len,
+        })
+    }
+
+    fn serialize_tuple(self, len: usize) -> Result<Self::SerializeTuple, Self::Error> {
+        self.serialize_seq(Some(len))
+    }
+
+    fn serialize_tuple_struct(
+        self,
+        _name: &'static str,
+        len: usize,
+    ) -> Result<Self::SerializeTupleStruct, Self::Error> {
+        self.serialize_seq(Some(len))
+    }
+
+    fn serialize_tuple_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeTupleVariant, Self::Error> {
+        Err(Error::UnsupportedType)
+    }
+
+    fn serialize_map(self, _len: Option<usize>) -> Result<Self::SerializeMap, Self::Error> {
+        self.array_type(ArrayState::StartedAsATable);
+        Ok(SerializeTable {
+            ser: self,
+            key: String::new(),
+            first: Cell::new(true),
+            table_emitted: Cell::new(false),
+        })
+    }
+
+    fn serialize_struct(
+        self,
+        _name: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStruct, Self::Error> {
+        self.array_type(ArrayState::StartedAsATable);
+        Ok(SerializeTable {
+            ser: self,
+            key: String::new(),
+            first: Cell::new(true),
+            table_emitted: Cell::new(false),
+        })
+    }
+
+    fn serialize_struct_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStructVariant, Self::Error> {
+        Err(Error::UnsupportedType)
+    }
+}
+
+impl<'a, 'b> ser::SerializeSeq for SerializeSeq<'a, 'b> {
+    type Ok = ();
+    type Error = Error;
+
+    fn serialize_element<T>(&mut self, value: &T) -> Result<(), Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        value.serialize(&mut Serializer {
+            dst: &mut *self.ser.dst,
+            state: State::Array {
+                parent: &self.ser.state,
+                first: &self.first,
+                type_: &self.type_,
+                len: self.len,
+            },
+        })?;
+        self.first.set(false);
+        Ok(())
+    }
+
+    fn end(self) -> Result<(), Error> {
+        match self.type_.get() {
+            Some(ArrayState::StartedAsATable) => return Ok(()),
+            Some(ArrayState::Started) => self.ser.dst.push(']'),
+            None => {
+                assert!(self.first.get());
+                self.ser.emit_key(ArrayState::Started)?;
+                self.ser.dst.push_str("[]");
+            }
+        }
+        if let State::Table { .. } = self.ser.state {
+            self.ser.dst.push('\n');
+        }
+        Ok(())
+    }
+}
+
+impl<'a, 'b> ser::SerializeTuple for SerializeSeq<'a, 'b> {
+    type Ok = ();
+    type Error = Error;
+
+    fn serialize_element<T>(&mut self, value: &T) -> Result<(), Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        ser::SerializeSeq::serialize_element(self, value)
+    }
+
+    fn end(self) -> Result<(), Error> {
+        ser::SerializeSeq::end(self)
+    }
+}
+
+impl<'a, 'b> ser::SerializeTupleStruct for SerializeSeq<'a, 'b> {
+    type Ok = ();
+    type Error = Error;
+
+    fn serialize_field<T>(&mut self, value: &T) -> Result<(), Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        ser::SerializeSeq::serialize_element(self, value)
+    }
+
+    fn end(self) -> Result<(), Error> {
+        ser::SerializeSeq::end(self)
+    }
+}
+
+impl<'a, 'b> ser::SerializeMap for SerializeTable<'a, 'b> {
+    type Ok = ();
+    type Error = Error;
+
+    fn serialize_key<T>(&mut self, input: &T) -> Result<(), Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        self.key = input.serialize(StringExtractor)?;
+        Ok(())
+    }
+
+    fn serialize_value<T>(&mut self, value: &T) -> Result<(), Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        let res = value.serialize(&mut Serializer {
+            dst: &mut *self.ser.dst,
+            state: State::Table {
+                key: &self.key,
+                parent: &self.ser.state,
+                first: &self.first,
+                table_emitted: &self.table_emitted,
+            },
+        });
+        match res {
+            Ok(()) => self.first.set(false),
+            Err(Error::UnsupportedNone) => {}
+            Err(e) => return Err(e),
+        }
+        Ok(())
+    }
+
+    fn end(self) -> Result<(), Error> {
+        if self.first.get() {
+            let state = self.ser.state.clone();
+            self.ser.emit_table_header(&state)?;
+        }
+        Ok(())
+    }
+}
+
+impl<'a, 'b> ser::SerializeStruct for SerializeTable<'a, 'b> {
+    type Ok = ();
+    type Error = Error;
+
+    fn serialize_field<T>(&mut self, key: &'static str, value: &T) -> Result<(), Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        let res = value.serialize(&mut Serializer {
+            dst: &mut *self.ser.dst,
+            state: State::Table {
+                key,
+                parent: &self.ser.state,
+                first: &self.first,
+                table_emitted: &self.table_emitted,
+            },
+        });
+        match res {
+            Ok(()) => self.first.set(false),
+            Err(Error::UnsupportedNone) => {}
+            Err(e) => return Err(e),
+        }
+        Ok(())
+    }
+
+    fn end(self) -> Result<(), Error> {
+        if self.first.get() {
+            let state = self.ser.state.clone();
+            self.ser.emit_table_header(&state)?;
+        }
+        Ok(())
+    }
+}
+
+struct StringExtractor;
+
+impl ser::Serializer for StringExtractor {
+    type Ok = String;
+    type Error = Error;
+    type SerializeSeq = ser::Impossible<String, Error>;
+    type SerializeTuple = ser::Impossible<String, Error>;
+    type SerializeTupleStruct = ser::Impossible<String, Error>;
+    type SerializeTupleVariant = ser::Impossible<String, Error>;
+    type SerializeMap = ser::Impossible<String, Error>;
+    type SerializeStruct = ser::Impossible<String, Error>;
+    type SerializeStructVariant = ser::Impossible<String, Error>;
+
+    fn serialize_bool(self, _v: bool) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_i8(self, _v: i8) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_i16(self, _v: i16) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_i32(self, _v: i32) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_i64(self, _v: i64) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_u8(self, _v: u8) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_u16(self, _v: u16) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_u32(self, _v: u32) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_u64(self, _v: u64) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_f32(self, _v: f32) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_f64(self, _v: f64) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_char(self, _v: char) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_str(self, value: &str) -> Result<String, Self::Error> {
+        Ok(value.to_string())
+    }
+
+    fn serialize_bytes(self, _value: &[u8]) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_none(self) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_some<T>(self, _value: &T) -> Result<String, Self::Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_unit(self) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_unit_struct(self, _name: &'static str) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_unit_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+    ) -> Result<String, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_newtype_struct<T>(
+        self,
+        _name: &'static str,
+        value: &T,
+    ) -> Result<String, Self::Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_newtype_variant<T>(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _value: &T,
+    ) -> Result<String, Self::Error>
+    where
+        T: ?Sized + Serialize,
+    {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_seq(self, _len: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_tuple(self, _len: usize) -> Result<Self::SerializeTuple, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_tuple_struct(
+        self,
+        _name: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeTupleStruct, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_tuple_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeTupleVariant, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_map(self, _len: Option<usize>) -> Result<Self::SerializeMap, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_struct(
+        self,
+        _name: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStruct, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+
+    fn serialize_struct_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStructVariant, Self::Error> {
+        Err(Error::KeyNotString)
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Error::UnsupportedType => "unsupported Rust type".fmt(f),
+            Error::KeyNotString => "map key was not a string".fmt(f),
+            Error::ValueAfterTable => "values must be emitted before tables".fmt(f),
+            Error::UnsupportedNone => "unsupported None value".fmt(f),
+            Error::Custom(ref s) => s.fmt(f),
+        }
+    }
+}
+
+impl error::Error for Error {}
+
+impl ser::Error for Error {
+    fn custom<T: Display>(msg: T) -> Error {
+        Error::Custom(msg.to_string())
+    }
+}