Attempt to locate `.cue` for PSX `.bin` before synthesising one

see #4146

src/BizHawk.Client.Common/RomLoader.cs

@@ -486,6 +486,21 @@ namespace BizHawk.Client.Common
 					}
 					break;
 				case VSystemID.Raw.PSX when ext is ".bin":
+					if (TryLoadSiblingCue(
+						nextComm,
+						binFilePath: file.Name,
+						forcedCoreName: forcedCoreName,
+						out var nextEmulator1,
+						out var game1,
+						out cancel))
+					{
+						nextEmulator = nextEmulator1;
+						rom = null;
+						game = game1;
+						return;
+					}
+					if (cancel) break; //TODO return? the cancel earlier in this method doesn't
+
 					const string FILE_EXT_CUE = ".cue";
 					var crc32Digest = CRC32Checksum.ComputeDigestHex(file.GetStream().ReadAllBytes()); // slow!
 					var cuePath = Path.Combine(Path.GetTempPath(), $"synthesised for {crc32Digest}{FILE_EXT_CUE}");
@@ -534,6 +549,70 @@ namespace BizHawk.Client.Common
 			nextEmulator = MakeCoreFromCoreInventory(cip, forcedCoreName);
 		}
 
+		private bool TryLoadSiblingCue(
+			CoreComm nextComm,
+			string binFilePath,
+			string forcedCoreName,
+			out IEmulator nextEmulator,
+			out GameInfo game,
+			out bool cancel)
+		{
+			nextEmulator = null;
+			game = null;
+			cancel = false;
+			const string FILE_EXT_CUE = ".cue";
+			const string FMT_STR_ASK = "Found \"{0}\".\nLoad that instead? Select \"No\" to synthesise a new .cue file for this game.";
+			HawkFile/*?*/ hfChosen = null;
+			//TODO can probably express this logic flow in a better way
+
+			HawkFile hfMatching = new(binFilePath.RemoveSuffix(".bin") + ".cue");
+			if (hfMatching.Exists)
+			{
+				var result = nextComm.Question(string.Format(FMT_STR_ASK, hfMatching.Name));
+				if (result is null)
+				{
+					cancel = true;
+					return false;
+				}
+				if (result is true)
+				{
+					hfChosen = hfMatching;
+				}
+			}
+
+			if (hfChosen is null)
+			{
+				var soleCueSiblingPath = Directory.EnumerateFiles(Path.GetDirectoryName(binFilePath))
+					.Where(static s => s.EndsWithOrdinal(FILE_EXT_CUE))
+					.Except([ hfMatching.Name ]) // seen but denied by user; don't prompt for the same file again
+					.SingleOrDefault();
+				HawkFile hfSoleSibling = soleCueSiblingPath is null ? null : new(soleCueSiblingPath);
+				if (hfSoleSibling is { Exists: true })
+				{
+					var result = nextComm.Question(string.Format(FMT_STR_ASK, hfSoleSibling.Name));
+					if (result is null)
+					{
+						cancel = true;
+						return false;
+					}
+					if (result is true)
+					{
+						hfChosen = hfSoleSibling;
+					}
+				}
+			}
+
+			if (hfChosen is null) return false;
+			return LoadDisc(
+				path: hfChosen.Name,
+				nextComm,
+				file: hfChosen,
+				ext: FILE_EXT_CUE,
+				forcedCoreName: forcedCoreName,
+				out nextEmulator,
+				out game);
+		}
+
 		private void LoadPSF(string path, CoreComm nextComm, HawkFile file, out IEmulator nextEmulator, out RomGame rom, out GameInfo game)
 		{
 			// TODO: Why does the PSF loader need CbDeflater provided?  Surely this is a matter internal to it.

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -258,6 +258,7 @@ namespace BizHawk.Client.EmuHawk
 			return new CoreComm(
 				message => this.ModalMessageBox(message, "Warning", EMsgBoxIcon.Warning),
 				AddOnScreenMessage,
+				text => this.ModalMessageBox3(icon: EMsgBoxIcon.Question, caption: "ROM loader", text: text),
 				cfp,
 				prefs,
 				new OpenGLProvider());

src/BizHawk.Emulation.Common/CoreComms.cs

@@ -8,9 +8,12 @@ namespace BizHawk.Emulation.Common
 	/// </summary>
 	public class CoreComm
 	{
+		public readonly Func<string, bool?> Question;
+
 		public CoreComm(
 			Action<string> showMessage,
 			Action<string, int?> notifyMessage,
+			Func<string, bool?> question,
 			ICoreFileProvider coreFileProvider,
 			CorePreferencesFlags prefs,
 			IOpenGLProvider oglProvider
@@ -18,6 +21,7 @@ namespace BizHawk.Emulation.Common
 		{
 			ShowMessage = showMessage;
 			Notify = notifyMessage;
+			Question = question;
 			CoreFileProvider = coreFileProvider;
 			CorePreferences = prefs;
 			OpenGLProvider = oglProvider;