[circt-verilog-lsp] Add hover support

[uenoku]

Add hover functionality to the CIRCT Verilog LSP server.

The implementation adds hover support for Verilog symbols showing type information, definition location with source code, and links to source locations. It also implements hover for external source locations showing code snippets and links to external files.

[uenoku]

Suggested change

	// Find definition of `include/test.mlir:2:5`
	// Find hover of `include/test.mlir:2:5`

[bryceberger]

This has an out of bounds if the definition is on the first line of a file.

Reproduction:

module thing(input var[2:0] a, output var [2:0] b);
    logic [2:0] c;
    assign c = a;
    assign b = c;
endmodule

Hover on c (either at definition position or on use), works as expected.

Hover on a or b, and the server crashes:

terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string_view::substr: __pos (which is 18446744073709551615) > __size (which is 118)

Minimal solution:

diff --git a/lib/Tools/circt-verilog-lsp-server/VerilogServerImpl/VerilogServer.cpp b/lib/Tools/circt-verilog-lsp-server/VerilogServerImpl/VerilogServer.cpp
index 7bd80a6c47..a2933f1eeb 100644
--- a/lib/Tools/circt-verilog-lsp-server/VerilogServerImpl/VerilogServer.cpp
+++ b/lib/Tools/circt-verilog-lsp-server/VerilogServerImpl/VerilogServer.cpp
@@ -1297,6 +1297,9 @@
   auto offest = loc.offset();
   // Find line boundaries around the location
   auto start = text.find_last_of('\n', offest);
+  if (start == text.npos) {
+    start = 0;
+  }
   auto end = text.find_first_of('\n', offest);
   return StringRef(text.substr(start, end - start)).trim();
 }

[uenoku]

This has an out of bounds if the definition is on the first line of a file.

Great point, thanks! The fix makes sense, I'll include your fix, thanks!