diff --git a/src/dreammaker/constants.rs b/src/dreammaker/constants.rs
index 13739469..245325ac 100644
--- a/src/dreammaker/constants.rs
+++ b/src/dreammaker/constants.rs
@@ -394,11 +394,12 @@ impl fmt::Display for ConstFn {
 pub fn evaluate_str(location: Location, input: &[u8]) -> Result<Constant, DMError> {
     use super::lexer::{Lexer, from_utf8_or_latin1_borrowed};
 
-    let mut bytes = input.iter().map(|&x| Ok(x));
     let ctx = Context::default();
-    let expr = crate::parser::parse_expression(&ctx, location, Lexer::new(&ctx, location.file, &mut bytes))?;
-    if bytes.next().is_some() {
-        return Err(DMError::new(location, format!("leftover: {:?} {}", from_utf8_or_latin1_borrowed(&input), bytes.len())));
+    let mut lexer = Lexer::new(&ctx, location.file, input);
+    let expr = crate::parser::parse_expression(&ctx, location, &mut lexer)?;
+    let leftover = lexer.remaining();
+    if !leftover.is_empty() {
+        return Err(DMError::new(location, format!("leftover: {:?} {}", from_utf8_or_latin1_borrowed(&input), leftover.len())));
     }
     expr.simple_evaluate(location)
 }
diff --git a/src/dreammaker/lexer.rs b/src/dreammaker/lexer.rs
index c28cff52..62f30820 100644
--- a/src/dreammaker/lexer.rs
+++ b/src/dreammaker/lexer.rs
@@ -1,5 +1,5 @@
 //! The lexer/tokenizer.
-use std::io;
+use std::io::Read;
 use std::str::FromStr;
 use std::fmt;
 use std::borrow::Cow;
@@ -469,25 +469,61 @@ enum Directive {
     Stringy,
 }
 
+fn buffer_read<R: Read>(file: FileId, mut read: R) -> Result<Vec<u8>, DMError> {
+    let mut buffer = Vec::new();
+
+    if let Err(error) = read.read_to_end(&mut buffer) {
+        let mut tracker = LocationTracker::new(file, buffer.as_slice().into());
+        tracker.by_ref().count();
+        return Err(DMError::new(tracker.location(), "i/o error reading file").with_cause(error));
+    }
+
+    Ok(buffer)
+}
+
+/// Attempt to read an entire file into memory, returning a line and column if
+/// an I/O error occurs.
+pub fn buffer_file(file: FileId, path: &std::path::Path) -> Result<Vec<u8>, DMError> {
+    let mut buffer = match std::fs::metadata(path) {
+        Ok(metadata) => Vec::with_capacity(metadata.len() as usize),
+        Err(_) => Vec::new(),
+    };
+
+    let mut read = match std::fs::File::open(path) {
+        Ok(read) => read,
+        Err(error) => return Err(DMError::new(Location { file, line: 1, column: 1 }, "i/o error opening file").with_cause(error)),
+    };
+
+    if let Err(error) = read.read_to_end(&mut buffer) {
+        let mut tracker = LocationTracker::new(file, buffer.as_slice().into());
+        tracker.by_ref().count();
+        return Err(DMError::new(tracker.location(), "i/o error reading file").with_cause(error));
+    }
+
+    Ok(buffer)
+}
+
 /// A wrapper for an input stream which tracks line and column numbers.
 ///
 /// All characters, including tabs, are considered to occupy one column
 /// regardless of position.
 ///
 /// `io::Error`s are converted to `DMError`s which include the location.
-pub struct LocationTracker<I> {
-    inner: I,
+pub struct LocationTracker<'a> {
+    inner: Cow<'a, [u8]>,
+    offset: usize,
     /// The location of the last character returned by `next()`.
     location: Location,
     at_line_end: bool,
 }
 
-impl<I> LocationTracker<I> {
-    pub fn new(file_number: FileId, inner: I) -> LocationTracker<I> {
+impl<'a> LocationTracker<'a> {
+    pub fn new(file: FileId, inner: Cow<'a, [u8]>) -> LocationTracker<'a> {
         LocationTracker {
             inner,
+            offset: 0,
             location: Location {
-                file: file_number,
+                file,
                 line: 0,
                 column: 0,
             },
@@ -498,22 +534,27 @@ impl<I> LocationTracker<I> {
     pub fn location(&self) -> Location {
         self.location
     }
+
+    pub fn remaining(&self) -> &[u8] {
+        &self.inner[self.offset..]
+    }
 }
 
-impl<I> fmt::Debug for LocationTracker<I> {
+impl<'a> fmt::Debug for LocationTracker<'a> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         fmt.debug_struct("LocationTracker")
             // inner omitted
+            .field("offset", &self.offset)
             .field("location", &self.location)
             .field("at_line_end", &self.at_line_end)
             .finish()
     }
 }
 
-impl<I: Iterator<Item=io::Result<u8>>> Iterator for LocationTracker<I> {
-    type Item = Result<u8, DMError>;
+impl<'a> Iterator for LocationTracker<'a> {
+    type Item = u8;
 
-    fn next(&mut self) -> Option<Result<u8, DMError>> {
+    fn next(&mut self) -> Option<u8> {
         if self.at_line_end {
             self.at_line_end = false;
             match self.location.line.checked_add(1) {
@@ -523,27 +564,27 @@ impl<I: Iterator<Item=io::Result<u8>>> Iterator for LocationTracker<I> {
             self.location.column = 0;
         }
 
-        match self.inner.next() {
-            None => None,
-            Some(Ok(ch)) => {
-                if ch == b'\n' {
-                    self.at_line_end = true;
-                }
-                match self.location.column.checked_add(1) {
-                    Some(new) => self.location.column = new,
-                    None => panic!("per-line column limit of {} exceeded", self.location.column),
-                }
-                Some(Ok(ch))
-            }
-            Some(Err(e)) => Some(Err(DMError::new(self.location, "i/o error").with_cause(e))),
+        let ch = match self.inner.get(self.offset) {
+            Some(&ch) => ch,
+            None => return None,
+        };
+        self.offset += 1;
+
+        if ch == b'\n' {
+            self.at_line_end = true;
         }
+        match self.location.column.checked_add(1) {
+            Some(new) => self.location.column = new,
+            None => panic!("per-line column limit of {} exceeded", self.location.column),
+        }
+        Some(ch)
     }
 }
 
 /// The lexer, which serves as a source of tokens through iteration.
-pub struct Lexer<'ctx, I> {
+pub struct Lexer<'ctx> {
     context: &'ctx Context,
-    input: LocationTracker<I>,
+    input: LocationTracker<'ctx>,
     next: Option<u8>,
     final_newline: bool,
     at_line_head: bool,
@@ -552,7 +593,7 @@ pub struct Lexer<'ctx, I> {
     interp_stack: Vec<Interpolation>,
 }
 
-impl<'ctx, I> fmt::Debug for Lexer<'ctx, I> {
+impl<'ctx> fmt::Debug for Lexer<'ctx> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         fmt.debug_struct("Lexer")
             .field("context", self.context)
@@ -566,26 +607,19 @@ impl<'ctx, I> fmt::Debug for Lexer<'ctx, I> {
     }
 }
 
-impl<'ctx, I: Iterator<Item=io::Result<u8>>> HasLocation for Lexer<'ctx, I> {
+impl<'ctx> HasLocation for Lexer<'ctx> {
     #[inline]
     fn location(&self) -> Location {
         self.input.location
     }
 }
 
-impl<'ctx, R: io::Read> Lexer<'ctx, io::Bytes<R>> {
-    /// Create a new lexer from a reader.
-    pub fn from_read(context: &'ctx Context, file_number: FileId, source: R) -> Lexer<io::Bytes<R>> {
-        Lexer::new(context, file_number, source.bytes())
-    }
-}
-
-impl<'ctx, I: Iterator<Item=io::Result<u8>>> Lexer<'ctx, I> {
+impl<'ctx> Lexer<'ctx> {
     /// Create a new lexer from a byte stream.
-    pub fn new(context: &'ctx Context, file_number: FileId, input: I) -> Lexer<I> {
+    pub fn new<I: Into<Cow<'ctx, [u8]>>>(context: &'ctx Context, file_number: FileId, input: I) -> Self {
         Lexer {
             context,
-            input: LocationTracker::new(file_number, input),
+            input: LocationTracker::new(file_number, input.into()),
             next: None,
             final_newline: false,
             at_line_head: true,
@@ -595,6 +629,20 @@ impl<'ctx, I: Iterator<Item=io::Result<u8>>> Lexer<'ctx, I> {
         }
     }
 
+    /// Create a new lexer from a reader.
+    pub fn from_read<R: Read>(context: &'ctx Context, file: FileId, read: R) -> Result<Self, DMError> {
+        Ok(Lexer::new(context, file, buffer_read(file, read)?))
+    }
+
+    /// Create a new lexer from a reader.
+    pub fn from_file(context: &'ctx Context, file: FileId, path: &std::path::Path) -> Result<Self, DMError> {
+        Ok(Lexer::new(context, file, buffer_file(file, path)?))
+    }
+
+    pub fn remaining(&self) -> &[u8] {
+        self.input.remaining()
+    }
+
     fn next(&mut self) -> Option<u8> {
         if let Some(next) = self.next.take() {
             return Some(next);
@@ -608,17 +656,12 @@ impl<'ctx, I: Iterator<Item=io::Result<u8>>> Lexer<'ctx, I> {
         }
         match result {
             None => None,
-            Some(Ok(ch)) => {
+            Some(ch) => {
                 if ch != b'\t' && ch != b' ' {
                     self.at_line_head = false;
                 }
                 Some(ch)
             }
-            Some(Err(err)) => {
-                // I/O error is effectively EOF.
-                self.context.register_error(err);
-                None
-            }
         }
     }
 
@@ -1049,7 +1092,7 @@ impl<'ctx, I: Iterator<Item=io::Result<u8>>> Lexer<'ctx, I> {
     }
 }
 
-impl<'ctx, I: Iterator<Item=io::Result<u8>>> Iterator for Lexer<'ctx, I> {
+impl<'ctx> Iterator for Lexer<'ctx> {
     type Item = LocatedToken;
 
     fn next(&mut self) -> Option<LocatedToken> {
diff --git a/src/dreammaker/lib.rs b/src/dreammaker/lib.rs
index 37043ca3..99286bf1 100644
--- a/src/dreammaker/lib.rs
+++ b/src/dreammaker/lib.rs
@@ -50,7 +50,7 @@ impl Context {
     /// Will only return failure on an `io::Error`. Compilation failures will
     /// return a best-effort parse. Call `print_all_errors` to pretty-print
     /// errors to standard error.
-    pub fn parse_environment(&self, dme: &Path) -> io::Result<objtree::ObjectTree> {
+    pub fn parse_environment(&self, dme: &Path) -> Result<objtree::ObjectTree, DMError> {
         Ok(parser::parse(self,
             indents::IndentProcessor::new(self,
                 preprocessor::Preprocessor::new(self, dme.to_owned())?
diff --git a/src/dreammaker/preprocessor.rs b/src/dreammaker/preprocessor.rs
index 48f79451..5907d57b 100644
--- a/src/dreammaker/preprocessor.rs
+++ b/src/dreammaker/preprocessor.rs
@@ -246,7 +246,7 @@ enum Include<'ctx> {
     File {
         path: PathBuf,
         file: FileId,
-        lexer: Lexer<'ctx, io::Bytes<Box<dyn io::Read>>>,
+        lexer: Lexer<'ctx>,
     },
     Expansion {
         name: String,
@@ -256,11 +256,20 @@ enum Include<'ctx> {
 }
 
 impl<'ctx> Include<'ctx> {
-    fn from_read(context: &'ctx Context, path: PathBuf, read: Box<dyn io::Read>) -> Include {
+    fn from_path(context: &'ctx Context, path: PathBuf) -> Result<Include<'ctx>, DMError> {
+        let idx = context.register_file(&path);
+        Ok(Include::File {
+            file: idx,
+            lexer: Lexer::from_file(context, idx, &path)?,
+            path,
+        })
+    }
+
+    fn from_buffer(context: &'ctx Context, path: PathBuf, buffer: Cow<'ctx, [u8]>) -> Include<'ctx> {
         let idx = context.register_file(&path);
         Include::File {
             file: idx,
-            lexer: Lexer::from_read(context, idx, read),
+            lexer: Lexer::new(context, idx, buffer),
             path,
         }
     }
@@ -387,26 +396,12 @@ impl<'ctx> HasLocation for Preprocessor<'ctx> {
     }
 }
 
-fn buffer_file(path: &Path) -> io::Result<Vec<u8>> {
-    use std::io::Read;
-
-    let mut buffer = if let Ok(metadata) = std::fs::metadata(path) {
-        Vec::with_capacity(metadata.len() as usize)
-    } else {
-        Vec::new()
-    };
-    let mut file = File::open(path)?;
-    file.read_to_end(&mut buffer)?;
-    Ok(buffer)
-}
-
 impl<'ctx> Preprocessor<'ctx> {
     /// Create a new preprocessor from the given Context and environment file.
-    pub fn new(context: &'ctx Context, env_file: PathBuf) -> io::Result<Self> {
+    pub fn new(context: &'ctx Context, env_file: PathBuf) -> Result<Self, DMError> {
         // Buffer the entire environment file. Large environments take a while
         // to load and locking it for the whole time is somewhat inconvenient.
-        let buffer = buffer_file(&env_file)?;
-        let include = Include::from_read(context, env_file.clone(), Box::new(io::Cursor::new(buffer)));
+        let include = Include::from_path(context, env_file.clone())?;
 
         Ok(Preprocessor {
             context,
@@ -436,7 +431,7 @@ impl<'ctx> Preprocessor<'ctx> {
             Cow::Borrowed(s) => Cow::Borrowed(s.as_bytes()),
             Cow::Owned(s) => Cow::Owned(s.into_bytes()),
         };
-        let include = Include::from_read(context, env_file.clone(), Box::new(io::Cursor::new(cow_u8)));
+        let include = Include::from_buffer(context, env_file.clone(), cow_u8);
         Preprocessor {
             context,
             env_file,
@@ -499,14 +494,14 @@ impl<'ctx> Preprocessor<'ctx> {
     */
 
     /// Push a DM file to the top of this preprocessor's stack.
-    pub fn push_file<R: io::Read + 'static>(&mut self, path: PathBuf, read: R) -> FileId {
+    pub fn push_file<R: io::Read + 'static>(&mut self, path: PathBuf, read: R) -> Result<FileId, DMError> {
         let idx = self.context.register_file(&path);
         self.include_stack.stack.push(Include::File {
-            lexer: Lexer::from_read(self.context, idx, Box::new(read)),
+            lexer: Lexer::from_read(self.context, idx, read)?,
             file: idx,
             path,
         });
-        idx
+        Ok(idx)
     }
 
     /// Enable source file annotations.
@@ -642,7 +637,7 @@ impl<'ctx> Preprocessor<'ctx> {
             Ok(Include::File {
                 path,
                 file: file_id,
-                lexer: Lexer::from_read(&self.context, file_id, Box::new(read)),
+                lexer: Lexer::from_read(&self.context, file_id, read)?,
             })
         }
     }
diff --git a/src/dreammaker/tests/annotation_tests.rs b/src/dreammaker/tests/annotation_tests.rs
index 12a08bbc..f26a1ddf 100644
--- a/src/dreammaker/tests/annotation_tests.rs
+++ b/src/dreammaker/tests/annotation_tests.rs
@@ -22,7 +22,7 @@ fn annotation_basic() {
 "##.trim();
 
     let context = Default::default();
-    let lexer = Lexer::new(&context, Default::default(), code.bytes().map(Ok));
+    let lexer = Lexer::new(&context, Default::default(), code.as_bytes());
     let indent = IndentProcessor::new(&context, lexer);
     let mut annotations = AnnotationTree::default();
     Parser::new(&context, indent).parse_annotations_only(&mut annotations);
diff --git a/src/dreammaker/tests/expression_tests.rs b/src/dreammaker/tests/expression_tests.rs
index e6001058..644f079c 100644
--- a/src/dreammaker/tests/expression_tests.rs
+++ b/src/dreammaker/tests/expression_tests.rs
@@ -6,7 +6,7 @@ use dm::ast::*;
 
 fn parse_expr(f: &str) -> Expression {
     let context = Default::default();
-    let lexer = Lexer::new(&context, Default::default(), f.bytes().map(Ok));
+    let lexer = Lexer::new(&context, Default::default(), f.as_bytes());
     let result = parse_expression(&context, Default::default(), lexer).expect("failed to parse expression");
     context.assert_success();
     result
diff --git a/src/dreammaker/tests/lexer_tests.rs b/src/dreammaker/tests/lexer_tests.rs
index 7937381b..59232298 100644
--- a/src/dreammaker/tests/lexer_tests.rs
+++ b/src/dreammaker/tests/lexer_tests.rs
@@ -6,7 +6,7 @@ use dm::lexer::Punctuation::*;
 
 fn lex(f: &str) -> Vec<Token> {
     let context = Default::default();
-    let result = Lexer::new(&context, Default::default(), f.bytes().map(Ok))
+    let result = Lexer::new(&context, Default::default(), f.as_bytes())
         .map(|t| t.token)
         .collect();
     context.assert_success();
diff --git a/src/dreammaker/tests/location_tests.rs b/src/dreammaker/tests/location_tests.rs
index abcf6533..912da1b6 100644
--- a/src/dreammaker/tests/location_tests.rs
+++ b/src/dreammaker/tests/location_tests.rs
@@ -18,7 +18,7 @@ fn simple_location_test() {
 "##.trim();
 
     let context = Default::default();
-    let located_tokens: Vec<_> = Lexer::new(&context, Default::default(), code.bytes().map(Ok)).collect();
+    let located_tokens: Vec<_> = Lexer::new(&context, Default::default(), code.as_bytes()).collect();
     context.assert_success();
 
     assert_eq!(located_tokens[0].location.line, 1);
diff --git a/src/langserver/main.rs b/src/langserver/main.rs
index fd0d7f87..69ca6bee 100644
--- a/src/langserver/main.rs
+++ b/src/langserver/main.rs
@@ -471,7 +471,7 @@ impl<'a> Engine<'a> {
                         None => (FileId::default(), defines.branch_at_end(&self.context)),
                     };
                     let contents = self.docs.read(url).map_err(invalid_request)?;
-                    let file_id = preprocessor.push_file(stripped.to_owned(), contents);
+                    let file_id = preprocessor.push_file(stripped.to_owned(), contents).map_err(invalid_request)?;
                     preprocessor.enable_annotations();
                     let mut annotations = AnnotationTree::default();
                     {
diff --git a/src/tools/dmm.rs b/src/tools/dmm.rs
index 18ea6693..cb965421 100644
--- a/src/tools/dmm.rs
+++ b/src/tools/dmm.rs
@@ -7,7 +7,7 @@ use std::fmt;
 use ndarray::{self, Array3, Axis};
 use linked_hash_map::LinkedHashMap;
 
-use dm::{DMError, Location};
+use dm::DMError;
 use dm::constants::Constant;
 use crate::dmi::Dir;
 
@@ -158,9 +158,7 @@ impl Map {
             dictionary: Default::default(),
             grid: Array3::default((1, 1, 1)),
         };
-        read::parse_map(&mut map, File::open(path).map_err(|e| {
-            DMError::new(Location::default(), "i/o error").with_cause(e)
-        })?)?;
+        read::parse_map(&mut map, path)?;
         Ok(map)
     }
 
diff --git a/src/tools/dmm/read.rs b/src/tools/dmm/read.rs
index 928b23b1..771c7c51 100644
--- a/src/tools/dmm/read.rs
+++ b/src/tools/dmm/read.rs
@@ -1,7 +1,5 @@
 //! Map parser, supporting standard DMM or TGM-format files.
 use std::collections::BTreeMap;
-use std::fs::File;
-use std::io::{Read, BufReader};
 use std::cmp::max;
 
 use ndarray::Array3;
@@ -16,8 +14,9 @@ fn take<T: Default>(t: &mut T) -> T {
     std::mem::replace(t, T::default())
 }
 
-pub fn parse_map(map: &mut Map, f: File) -> Result<(), DMError> {
-    let mut chars = LocationTracker::new(Default::default(), BufReader::new(f).bytes());
+pub fn parse_map(map: &mut Map, path: &std::path::Path) -> Result<(), DMError> {
+    let file_id = Default::default();
+    let mut chars = LocationTracker::new(file_id, dm::lexer::buffer_file(file_id, path)?.into());
 
     let mut in_comment_line = false;
     let mut comment_trigger = false;
@@ -39,7 +38,6 @@ pub fn parse_map(map: &mut Map, f: File) -> Result<(), DMError> {
     let mut skip_whitespace = false;
 
     while let Some(ch) = chars.next() {
-        let ch = ch?;
         if ch == b'\n' || ch == b'\r' {
             in_comment_line = false;
             comment_trigger = false;
@@ -178,7 +176,6 @@ pub fn parse_map(map: &mut Map, f: File) -> Result<(), DMError> {
     let mut adjust_y = true;
 
     while let Some(ch) = chars.next() {
-        let ch = ch?;
         if in_coord_block {
             if ch == b',' {
                 if reading_coord == Coord::X {