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, 98 insertions, 0 deletions

diff --git a/editors/code/src/test/utils/diagnotics/vscode.test.ts b/editors/code/src/test/utils/diagnotics/vscode.test.ts
new file mode 100644
index 000000000..542dec1f5
--- /dev/null
+++ b/editors/code/src/test/utils/diagnotics/vscode.test.ts
@@ -0,0 +1,98 @@
+import * as assert from 'assert';
+import * as vscode from 'vscode';
+
+import { areDiagnosticsEqual } from '../../../utils/diagnostics/vscode';
+
+const range1 = new vscode.Range(
+    new vscode.Position(1, 2),
+    new vscode.Position(3, 4)
+);
+
+const range2 = new vscode.Range(
+    new vscode.Position(5, 6),
+    new vscode.Position(7, 8)
+);
+
+describe('areDiagnosticsEqual', () => {
+    it('should treat identical diagnostics as equal', () => {
+        const diagnostic1 = new vscode.Diagnostic(
+            range1,
+            'Hello, world!',
+            vscode.DiagnosticSeverity.Error
+        );
+
+        const diagnostic2 = new vscode.Diagnostic(
+            range1,
+            'Hello, world!',
+            vscode.DiagnosticSeverity.Error
+        );
+
+        assert(areDiagnosticsEqual(diagnostic1, diagnostic2));
+    });
+
+    it('should treat diagnostics with different sources as inequal', () => {
+        const diagnostic1 = new vscode.Diagnostic(
+            range1,
+            'Hello, world!',
+            vscode.DiagnosticSeverity.Error
+        );
+        diagnostic1.source = 'rustc';
+
+        const diagnostic2 = new vscode.Diagnostic(
+            range1,
+            'Hello, world!',
+            vscode.DiagnosticSeverity.Error
+        );
+        diagnostic2.source = 'clippy';
+
+        assert(!areDiagnosticsEqual(diagnostic1, diagnostic2));
+    });
+
+    it('should treat diagnostics with different ranges as inequal', () => {
+        const diagnostic1 = new vscode.Diagnostic(
+            range1,
+            'Hello, world!',
+            vscode.DiagnosticSeverity.Error
+        );
+
+        const diagnostic2 = new vscode.Diagnostic(
+            range2,
+            'Hello, world!',
+            vscode.DiagnosticSeverity.Error
+        );
+
+        assert(!areDiagnosticsEqual(diagnostic1, diagnostic2));
+    });
+
+    it('should treat diagnostics with different messages as inequal', () => {
+        const diagnostic1 = new vscode.Diagnostic(
+            range1,
+            'Hello, world!',
+            vscode.DiagnosticSeverity.Error
+        );
+
+        const diagnostic2 = new vscode.Diagnostic(
+            range1,
+            'Goodbye!, world!',
+            vscode.DiagnosticSeverity.Error
+        );
+
+        assert(!areDiagnosticsEqual(diagnostic1, diagnostic2));
+    });
+
+    it('should treat diagnostics with different severities as inequal', () => {
+        const diagnostic1 = new vscode.Diagnostic(
+            range1,
+            'Hello, world!',
+            vscode.DiagnosticSeverity.Warning
+        );
+
+        const diagnostic2 = new vscode.Diagnostic(
+            range1,
+            'Hello, world!',
+            vscode.DiagnosticSeverity.Error
+        );
+
+        assert(!areDiagnosticsEqual(diagnostic1, diagnostic2));
+    });
+});