ModuleSet: store a set rather than a map

Previously, a `ModuleSet` wrapped a `HashMap<NormalPath, TargetKind>`.
This had a number of undesirable consequences:
* The data about a module's name (as its path) and how it was loaded (as
  its `TargetKind`) were split from each other and difficult to
  reference.
* The module's import name wasn't explicitly stored anywhere, so we
  needed to convert between paths and dotted names when those were
  needed, which required hitting the disk.
* There wasn't a type for the module's import name, so when we (e.g.)
  `:unadd`ed modules we needed to format them as strings.

Now, a `ModuleSet` wraps a `HashSet<LoadedModule>`.

* A `LoadedModule` wraps a path but optionally contains the module's
  dotted name, if the module is loaded by name (and needs to be referred
  to by name to avoid the "module defined in multiple files" error).
* The `LoadedModule` `Display` instance formats the module's import name
  correctly (with a dotted name if needed) and avoids hitting the disk
  or any string processing.

src/ghci/loaded_module.rs

@@ -0,0 +1,122 @@
+use std::borrow::Borrow;
+use std::fmt::Display;
+use std::hash::Hash;
+use std::hash::Hasher;
+
+use camino::Utf8Path;
+
+use crate::normal_path::NormalPath;
+
+/// Information about a module loaded into a `ghci` session.
+///
+/// Hashing and equality are determined by the module's path alone.
+#[derive(Debug, Clone, Eq)]
+pub struct LoadedModule {
+    /// The module's source file.
+    path: NormalPath,
+
+    /// The module's name.
+    ///
+    /// This is present if and only if the module is loaded by name.
+    ///
+    /// Entries in `:show targets` can be one of two types: module paths or module names (with `.` in
+    /// place of path separators). Due to a `ghci` bug, the module can only be referred to as whichever
+    /// form it was originally added as (see below), so we use this to track how we refer to modules.
+    ///
+    /// See: <https://gitlab.haskell.org/ghc/ghc/-/issues/13254#note_525037>
+    name: Option<String>,
+}
+
+impl LoadedModule {
+    /// Create a new module, loaded by path.
+    pub fn new(path: NormalPath) -> Self {
+        Self { path, name: None }
+    }
+
+    /// Create a new module, loaded by name.
+    pub fn with_name(path: NormalPath, name: String) -> Self {
+        Self {
+            path,
+            name: Some(name),
+        }
+    }
+
+    /// Get the name to use to refer to this module.
+    pub fn name(&self) -> LoadedModuleName {
+        match self.name.as_deref() {
+            Some(name) => LoadedModuleName::Name(name),
+            None => LoadedModuleName::Path(&self.path),
+        }
+    }
+
+    /// Get the module's source path.
+    pub fn path(&self) -> &NormalPath {
+        &self.path
+    }
+}
+
+impl Display for LoadedModule {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.name())
+    }
+}
+
+impl Hash for LoadedModule {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.path.hash(state)
+    }
+}
+
+impl PartialEq for LoadedModule {
+    fn eq(&self, other: &Self) -> bool {
+        self.path.eq(&other.path)
+    }
+}
+
+impl PartialOrd for LoadedModule {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.path.partial_cmp(&other.path)
+    }
+}
+
+impl Ord for LoadedModule {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.path.cmp(&other.path)
+    }
+}
+
+impl Borrow<NormalPath> for LoadedModule {
+    fn borrow(&self) -> &NormalPath {
+        &self.path
+    }
+}
+
+impl Borrow<Utf8Path> for LoadedModule {
+    fn borrow(&self) -> &Utf8Path {
+        &self.path
+    }
+}
+
+/// The name to use to refer to a module loaded into a GHCi session.
+///
+/// Entries in `:show targets` can be one of two types: module paths or module names (with `.` in
+/// place of path separators). Due to a `ghci` bug, the module can only be referred to as whichever
+/// form it was originally added as (see below), so we use this to track how we refer to modules.
+///
+/// See: <https://gitlab.haskell.org/ghc/ghc/-/issues/13254#note_525037>
+#[derive(Debug)]
+pub enum LoadedModuleName<'a> {
+    /// A path to a Haskell source file, like `src/My/Cool/Module.hs`.
+    Path(&'a Utf8Path),
+    /// A dotted module name, like `My.Cool.Module`.
+    Name(&'a str),
+}
+
+impl<'a> Display for LoadedModuleName<'a> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            LoadedModuleName::Path(path) => write!(f, "{path}"),
+            LoadedModuleName::Name(name) => write!(f, "{name}"),
+        }
+    }
+}

src/ghci/mod.rs

@@ -6,6 +6,7 @@ use nix::sys::signal::Signal;
 use owo_colors::OwoColorize;
 use owo_colors::Stream::Stdout;
 use std::borrow::Borrow;
+use std::borrow::Cow;
 use std::collections::BTreeMap;
 use std::collections::BTreeSet;
 use std::fmt::Debug;
@@ -50,7 +51,6 @@ pub mod parse;
 use parse::parse_eval_commands;
 use parse::CompilationResult;
 use parse::EvalCommand;
-use parse::ModuleSet;
 use parse::ShowPaths;
 
 mod ghci_command;
@@ -63,6 +63,12 @@ mod writer;
 use crate::buffers::GHCI_BUFFER_CAPACITY;
 pub use crate::ghci::writer::GhciWriter;
 
+mod module_set;
+pub use module_set::ModuleSet;
+
+mod loaded_module;
+use loaded_module::LoadedModule;
+
 use crate::aho_corasick::AhoCorasickExt;
 use crate::buffers::LINE_BUFFER_CAPACITY;
 use crate::cli::Opts;
@@ -80,8 +86,6 @@ use crate::shutdown::ShutdownHandle;
 use crate::CommandExt;
 use crate::StringCase;
 
-use self::parse::TargetKind;
-
 /// The `ghci` prompt we use. Should be unique enough, but maybe we can make it better with Unicode
 /// private-use-area codepoints or something in the future.
 pub const PROMPT: &str = "###~GHCIWATCH-PROMPT~###";
@@ -613,10 +617,10 @@ impl Ghci {
 
         let mut eval_commands = BTreeMap::new();
 
-        for path in self.targets.iter() {
-            let commands = Self::parse_eval_commands(path).await?;
+        for target in self.targets.iter() {
+            let commands = Self::parse_eval_commands(target.path()).await?;
             if !commands.is_empty() {
-                eval_commands.insert(path.clone(), commands);
+                eval_commands.insert(target.path().clone(), commands);
             }
         }
 
@@ -670,23 +674,79 @@ impl Ghci {
         Ok(commands)
     }
 
-    /// `:add` a module or modules to the `ghci` session by path.
+    /// `:add` a module or modules to the GHCi session.
     #[instrument(skip(self), level = "debug")]
     async fn add_modules(
         &mut self,
         paths: &[NormalPath],
         log: &mut CompilationLog,
     ) -> miette::Result<()> {
-        let modules = self.targets.format_modules(&self.search_paths, paths)?;
+        let mut modules = Vec::with_capacity(paths.len());
+        for path in paths {
+            if self.targets.contains_source_path(path) {
+                return Err(miette!(
+                    "Attempting to add already-loaded module: {path}\n\
+                    This is a ghciwatch bug; please report it upstream"
+                ));
+            } else {
+                modules.push(LoadedModule::new(path.clone()));
+            }
+        }
 
         self.stdin
             .add_modules(&mut self.stdout, &modules, log)
             .await?;
 
-        for path in paths {
-            self.targets
-                .insert_source_path(path.clone(), TargetKind::Path);
-        }
+        // TODO: This could lead to the module set getting out of sync with the underlying GHCi
+        // session.
+        //
+        // If there's a TOATOU bug here (e.g. we're attempting to add a module but the file no
+        // longer exists), then we can get into a situation like this:
+        //
+        //     ghci> :add src/DoesntExist.hs src/MyLib.hs
+        //     File src/DoesntExist.hs not found
+        //     [4 of 4] Compiling MyLib        ( src/MyLib.hs, interpreted )
+        //     Ok, four modules loaded.
+        //
+        //     ghci> :show targets
+        //     src/MyLib.hs
+        //     ...
+        //
+        // We've requested to load two modules, only one has been loaded, but GHCi has reported
+        // that compilation was successful and hasn't added the failing module to the target set.
+        // Note that if the file is found but compilation fails, the file _is_ added to the target
+        // set:
+        //
+        //     ghci> :add src/MyCoolLib.hs
+        //     [4 of 4] Compiling MyCoolLib        ( src/MyCoolLib.hs, interpreted )
+        //
+        //     src/MyCoolLib.hs:4:12: error:
+        //         • Couldn't match expected type ‘IO ()’ with actual type ‘()’
+        //         • In the expression: ()
+        //           In an equation for ‘someFunc’: someFunc = ()
+        //       |
+        //     4 | someFunc = ()
+        //       |            ^^
+        //     Failed, three modules loaded.
+        //
+        //     ghci> :show targets
+        //     src/MyCoolLib.hs
+        //     ...
+        //
+        // I think this is OK, because the only reason we need to know which modules are loaded is
+        // to avoid the "module defined in multiple files" bug [1], so the potential outcomes of
+        // making this mistake are:
+        //
+        // 1. The next time the file is modified, we attempt to `:add` it instead of `:reload`ing
+        //    it. This is harmless, though it changes the order that `:show modules` prints output
+        //    in (maybe local binding order as well or something).
+        // 2. The next time the file is modified, we attempt to `:add` it by path instead of by
+        //    module name, but this function is only used when the modules aren't already in the
+        //    target set, so we know the module doesn't need to be referred to by its module name.
+        //
+        // [1]: https://gitlab.haskell.org/ghc/ghc/-/issues/13254#note_525037
+
+        self.targets.extend(modules);
 
         self.refresh_eval_commands_for_paths(paths).await?;
 
@@ -702,14 +762,15 @@ impl Ghci {
         path: &NormalPath,
         log: &mut CompilationLog,
     ) -> miette::Result<()> {
-        let module = self.targets.module_import_name(&self.search_paths, path)?;
+        let module = self.targets.get_import_name(path);
 
         self.stdin
-            .interpret_module(&mut self.stdout, &module.name, log)
+            .interpret_module(&mut self.stdout, &module, log)
             .await?;
 
-        if !module.loaded {
-            self.targets.insert_source_path(path.clone(), module.kind);
+        // Note: A borrowed path is only returned if the path is already present in the module set.
+        if let Cow::Owned(module) = module {
+            self.targets.insert_module(module);
         }
 
         self.refresh_eval_commands_for_paths(std::iter::once(path))
@@ -725,21 +786,24 @@ impl Ghci {
         paths: &[NormalPath],
         log: &mut CompilationLog,
     ) -> miette::Result<()> {
+        let modules = paths
+            .iter()
+            .map(|path| self.targets.get_import_name(path).into_owned())
+            .collect::<Vec<_>>();
+
         // Each `:unadd` implicitly reloads as well, so we have to `:unadd` all the modules in a
         // single command so that GHCi doesn't try to load a bunch of removed modules after each
         // one.
-        let modules = self.targets.format_modules(&self.search_paths, paths)?;
-
         self.stdin
-            .remove_modules(&mut self.stdout, &modules, log)
+            .remove_modules(&mut self.stdout, modules.iter().map(Borrow::borrow), log)
             .await?;
 
         for path in paths {
             self.targets.remove_source_path(path);
-            self.clear_eval_commands_for_paths(std::iter::once(path))
-                .await;
         }
 
+        self.clear_eval_commands_for_paths(paths).await;
+
         Ok(())
     }