Fix `cargo watch` code action filtering

There are two issues with the implementation of `provideCodeActions`
introduced in #1439:

1. We're returning the code action based on the file its diagnostic is
   in; not the file the suggested fix is in. I'm not sure how often
   fixes are suggested cross-file but it's something we should handle.

2. We're not filtering code actions based on the passed range. The means
   if there is any suggestion in a file we'll show an action for every
   line of the file. I naively thought that VS Code would filter for us
   but that was wrong.

Unfortunately the VS Code `CodeAction` object is very complex - it can
handle edits across multiple files, run commands, etc. This makes it
complex to check them for equality or see if any of their edits
intersects with a specified range.

To make it easier to work with suggestions this introduces a
`SuggestedFix` model object and a `SuggestFixCollection` code action
provider. This is a layer between the raw Rust JSON and VS Code's
`CodeAction`s. I was reluctant to introduce another layer of abstraction
here but my attempt to work directly with VS Code's model objects was
worse.

1 files changed, 74 insertions, 0 deletions

diff --git a/editors/code/src/utils/diagnostics/SuggestedFixCollection.ts b/editors/code/src/utils/diagnostics/SuggestedFixCollection.ts
new file mode 100644
index 000000000..3b0bf7468
--- /dev/null
+++ b/editors/code/src/utils/diagnostics/SuggestedFixCollection.ts
@@ -0,0 +1,74 @@
+import * as vscode from 'vscode';
+import SuggestedFix from './SuggestedFix';
+
+/**
+ * Collection of suggested fixes across multiple documents
+ *
+ * This stores `SuggestedFix` model objects and returns them via the
+ * `vscode.CodeActionProvider` interface.
+ */
+export default class SuggestedFixCollection
+    implements vscode.CodeActionProvider {
+    public static PROVIDED_CODE_ACTION_KINDS = [vscode.CodeActionKind.QuickFix];
+
+    private suggestedFixes: Map<string, SuggestedFix[]>;
+
+    constructor() {
+        this.suggestedFixes = new Map();
+    }
+
+    /**
+     * Clears all suggested fixes across all documents
+     */
+    public clear(): void {
+        this.suggestedFixes = new Map();
+    }
+
+    /**
+     * Adds a suggested fix for the given diagnostic
+     *
+     * Some suggested fixes will appear in multiple diagnostics. For example,
+     * forgetting a `mut` on a variable will suggest changing the delaration on
+     * every mutable usage site. If the suggested fix has already been added
+     * this method will instead associate the existing fix with the new
+     * diagnostic.
+     */
+    public addSuggestedFixForDiagnostic(
+        suggestedFix: SuggestedFix,
+        diagnostic: vscode.Diagnostic
+    ): void {
+        const fileUriString = suggestedFix.location.uri.toString();
+        const fileSuggestions = this.suggestedFixes.get(fileUriString) || [];
+
+        const existingSuggestion = fileSuggestions.find(s =>
+            s.isEqual(suggestedFix)
+        );
+
+        if (existingSuggestion) {
+            // The existing suggestion also applies to this new diagnostic
+            existingSuggestion.diagnostics.push(diagnostic);
+        } else {
+            // We haven't seen this suggestion before
+            suggestedFix.diagnostics.push(diagnostic);
+            fileSuggestions.push(suggestedFix);
+        }
+
+        this.suggestedFixes.set(fileUriString, fileSuggestions);
+    }
+
+    /**
+     * Filters suggested fixes by their document and range and converts them to
+     * code actions
+     */
+    public provideCodeActions(
+        document: vscode.TextDocument,
+        range: vscode.Range
+    ): vscode.CodeAction[] {
+        const documentUriString = document.uri.toString();
+
+        const suggestedFixes = this.suggestedFixes.get(documentUriString);
+        return (suggestedFixes || [])
+            .filter(({ location }) => location.range.intersection(range))
+            .map(suggestedEdit => suggestedEdit.toCodeAction());
+    }
+}