[compiler] Improve snap workflow for debugging errors

Much nicer workflow for working through errors in the compiler:
* Run `yarn snap -w`, oops there are are errors
* Hit 'p' to select a fixture => the suggestions populate with recent failures, sorted alphabetically. No need to copy/paste the name of the fixture you want to focus on!
* tab/shift-tab to pick one, hit enter to select that one
* ...Focus on fixing that test...
* 'p' to re-enter the picker. Snap tracks the last state of each fixture and continues to show all tests that failed on their last run, so you can easily move on to the next one. The currently selected test is highlighted, making it easy to move to the next one.
* 'a' at any time to run all tests
* 'd' at any time to toggle debug output on/off (while focusing on a single test)

compiler/packages/snap/src/runner-watch.ts

@@ -9,7 +9,7 @@ import watcher from '@parcel/watcher';
 import path from 'path';
 import ts from 'typescript';
 import {FIXTURES_PATH, PROJECT_ROOT} from './constants';
-import {TestFilter} from './fixture-utils';
+import {TestFilter, getFixtures} from './fixture-utils';
 import {execSync} from 'child_process';
 
 export function watchSrc(
@@ -121,6 +121,12 @@ export type RunnerState = {
   // Input mode for interactive pattern entry
   inputMode: 'none' | 'pattern';
   inputBuffer: string;
+  // Autocomplete state
+  allFixtureNames: Array<string>;
+  matchingFixtures: Array<string>;
+  selectedIndex: number;
+  // Track last run status of each fixture (for autocomplete suggestions)
+  fixtureLastRunStatus: Map<string, 'pass' | 'fail'>;
 };
 
 function subscribeFixtures(
@@ -179,46 +185,187 @@ function subscribeTsc(
   );
 }
 
+/**
+ * Levenshtein edit distance between two strings
+ */
+function editDistance(a: string, b: string): number {
+  const m = a.length;
+  const n = b.length;
+
+  // Create a 2D array for memoization
+  const dp: number[][] = Array.from({length: m + 1}, () =>
+    Array(n + 1).fill(0),
+  );
+
+  // Base cases
+  for (let i = 0; i <= m; i++) dp[i][0] = i;
+  for (let j = 0; j <= n; j++) dp[0][j] = j;
+
+  // Fill in the rest
+  for (let i = 1; i <= m; i++) {
+    for (let j = 1; j <= n; j++) {
+      if (a[i - 1] === b[j - 1]) {
+        dp[i][j] = dp[i - 1][j - 1];
+      } else {
+        dp[i][j] = 1 + Math.min(dp[i - 1][j], dp[i][j - 1], dp[i - 1][j - 1]);
+      }
+    }
+  }
+
+  return dp[m][n];
+}
+
+function filterFixtures(
+  allNames: Array<string>,
+  pattern: string,
+): Array<string> {
+  if (pattern === '') {
+    return allNames;
+  }
+  const lowerPattern = pattern.toLowerCase();
+  const matches = allNames.filter(name =>
+    name.toLowerCase().includes(lowerPattern),
+  );
+  // Sort by edit distance (lower = better match)
+  matches.sort((a, b) => {
+    const distA = editDistance(lowerPattern, a.toLowerCase());
+    const distB = editDistance(lowerPattern, b.toLowerCase());
+    return distA - distB;
+  });
+  return matches;
+}
+
+const MAX_DISPLAY = 15;
+
+function renderAutocomplete(state: RunnerState): void {
+  // Clear terminal
+  console.log('\u001Bc');
+
+  // Show current input
+  console.log(`Pattern: ${state.inputBuffer}`);
+  console.log('');
+
+  // Get current filter pattern if active
+  const currentFilterPattern =
+    state.mode.filter && state.filter ? state.filter.paths[0] : null;
+
+  // Show matching fixtures (limit to MAX_DISPLAY)
+  const toShow = state.matchingFixtures.slice(0, MAX_DISPLAY);
+
+  toShow.forEach((name, i) => {
+    const isSelected = i === state.selectedIndex;
+    const matchesCurrentFilter =
+      currentFilterPattern != null &&
+      name.toLowerCase().includes(currentFilterPattern.toLowerCase());
+
+    let prefix: string;
+    if (isSelected) {
+      prefix = '> ';
+    } else if (matchesCurrentFilter) {
+      prefix = '* ';
+    } else {
+      prefix = '  ';
+    }
+    console.log(`${prefix}${name}`);
+  });
+
+  if (state.matchingFixtures.length > MAX_DISPLAY) {
+    console.log(
+      `  ... and ${state.matchingFixtures.length - MAX_DISPLAY} more`,
+    );
+  }
+
+  console.log('');
+  console.log('↑/↓/Tab navigate | Enter select | Esc cancel');
+}
+
 function subscribeKeyEvents(
   state: RunnerState,
   onChange: (state: RunnerState) => void,
 ) {
   process.stdin.on('keypress', async (str, key) => {
-    // Handle input mode (pattern entry)
+    // Handle input mode (pattern entry with autocomplete)
     if (state.inputMode !== 'none') {
       if (key.name === 'return') {
-        // Enter pressed - process input
-        const pattern = state.inputBuffer.trim();
+        // Enter pressed - use selected fixture or typed text
+        let pattern: string;
+        if (
+          state.selectedIndex >= 0 &&
+          state.selectedIndex < state.matchingFixtures.length
+        ) {
+          pattern = state.matchingFixtures[state.selectedIndex];
+        } else {
+          pattern = state.inputBuffer.trim();
+        }
+
         state.inputMode = 'none';
         state.inputBuffer = '';
-        process.stdout.write('\n');
+        state.allFixtureNames = [];
+        state.matchingFixtures = [];
+        state.selectedIndex = -1;
 
         if (pattern !== '') {
-          // Set the pattern as filter
           state.filter = {paths: [pattern]};
           state.mode.filter = true;
           state.mode.action = RunnerAction.Test;
           onChange(state);
         }
-        // If empty, just exit input mode without changes
         return;
       } else if (key.name === 'escape') {
         // Cancel input mode
         state.inputMode = 'none';
         state.inputBuffer = '';
-        process.stdout.write(' (cancelled)\n');
+        state.allFixtureNames = [];
+        state.matchingFixtures = [];
+        state.selectedIndex = -1;
+        // Redraw normal UI
+        onChange(state);
+        return;
+      } else if (key.name === 'up' || (key.name === 'tab' && key.shift)) {
+        // Navigate up in autocomplete list
+        if (state.matchingFixtures.length > 0) {
+          if (state.selectedIndex <= 0) {
+            state.selectedIndex =
+              Math.min(state.matchingFixtures.length, MAX_DISPLAY) - 1;
+          } else {
+            state.selectedIndex--;
+          }
+          renderAutocomplete(state);
+        }
+        return;
+      } else if (key.name === 'down' || (key.name === 'tab' && !key.shift)) {
+        // Navigate down in autocomplete list
+        if (state.matchingFixtures.length > 0) {
+          const maxIndex =
+            Math.min(state.matchingFixtures.length, MAX_DISPLAY) - 1;
+          if (state.selectedIndex >= maxIndex) {
+            state.selectedIndex = 0;
+          } else {
+            state.selectedIndex++;
+          }
+          renderAutocomplete(state);
+        }
         return;
       } else if (key.name === 'backspace') {
         if (state.inputBuffer.length > 0) {
           state.inputBuffer = state.inputBuffer.slice(0, -1);
-          // Erase character: backspace, space, backspace
-          process.stdout.write('\b \b');
+          state.matchingFixtures = filterFixtures(
+            state.allFixtureNames,
+            state.inputBuffer,
+          );
+          state.selectedIndex = -1;
+          renderAutocomplete(state);
         }
         return;
       } else if (str && !key.ctrl && !key.meta) {
-        // Regular character - accumulate and echo
+        // Regular character - accumulate, filter, and render
         state.inputBuffer += str;
-        process.stdout.write(str);
+        state.matchingFixtures = filterFixtures(
+          state.allFixtureNames,
+          state.inputBuffer,
+        );
+        state.selectedIndex = -1;
+        renderAutocomplete(state);
         return;
       }
       return; // Ignore other keys in input mode
@@ -240,10 +387,23 @@ function subscribeKeyEvents(
       state.debug = !state.debug;
       state.mode.action = RunnerAction.Test;
     } else if (key.name === 'p') {
-      // p => enter pattern input mode
+      // p => enter pattern input mode with autocomplete
       state.inputMode = 'pattern';
       state.inputBuffer = '';
-      process.stdout.write('Pattern: ');
+
+      // Load all fixtures for autocomplete
+      const fixtures = await getFixtures(null);
+      state.allFixtureNames = Array.from(fixtures.keys()).sort();
+      // Show failed fixtures first when no pattern entered
+      const failedFixtures = Array.from(state.fixtureLastRunStatus.entries())
+        .filter(([_, status]) => status === 'fail')
+        .map(([name]) => name)
+        .sort();
+      state.matchingFixtures =
+        failedFixtures.length > 0 ? failedFixtures : state.allFixtureNames;
+      state.selectedIndex = -1;
+
+      renderAutocomplete(state);
       return; // Don't trigger onChange yet
     } else {
       // any other key re-runs tests
@@ -279,6 +439,10 @@ export async function makeWatchRunner(
     debug: debugMode,
     inputMode: 'none',
     inputBuffer: '',
+    allFixtureNames: [],
+    matchingFixtures: [],
+    selectedIndex: -1,
+    fixtureLastRunStatus: new Map(),
   };
 
   subscribeTsc(state, onChange);

compiler/packages/snap/src/runner.ts

@@ -142,6 +142,14 @@ async function onChange(
       true, // requireSingleFixture in watch mode
     );
     const end = performance.now();
+
+    // Track fixture status for autocomplete suggestions
+    for (const [basename, result] of results) {
+      const failed =
+        result.actual !== result.expected || result.unexpectedError != null;
+      state.fixtureLastRunStatus.set(basename, failed ? 'fail' : 'pass');
+    }
+
     if (mode.action === RunnerAction.Update) {
       update(results);
       state.lastUpdate = end;