My setup

• vscode: 1.14.0
• OS: macOS 10.12.6
• rustup: 1.5.0
• rust: 1.20.0-nightly (f85579d 2017-07-12)
• vscode RLS plugin: 0.4.2
• vscode Perforce plugin: 2.1.1

vscode RLS settings:

	"rust.rustup": {
		"toolchain": "nightly-x86_64-apple-darwin",
		"nightlyToolchain": "nightly-x86_64-apple-darwin"
	},
	"rust.mode": "rls",
	"rust.rls": {
		"useRustfmt": true,
		"env": {
			"RUST_BACKTRACE": "1",
			"RUST_LOG": "trace"
		}
	}

Steps to reproduce

• open any Rust project folder (a simple bin or lib project generated with cargo new will do);
• open any .rs file in the project
• the Rust Language Server OUTPUT window will display the following crash messages:

TRACE:rls::server: parse_message > {"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"perforce://print/Users/rick/work/rust/doublecho/src/main.rs?-q","languageId":"rust","version":1,"text":""}}}
TRACE:rls::server: Handling DidOpenTextDocument (params: DidOpenTextDocumentParams { text_document: TextDocumentItem { uri: "perforce://print/Users/rick/work/rust/doublecho/src/main.rs?-q", language_id: Some("rust"), version: Some(1), text: "" } })
thread 'main' panicked at 'called Result::unwrap() on an Err value: StringError("URI scheme is not file")', src/libcore/result.rs:860:4

[snip]

[Error - 2:18:18 PM] Connection to server got closed. Server will not be restarted.

Apparently the perforce://print/... uri sent by the Perforce plugin was causing the "URI scheme is not file" error for RLS.

A workaround is to disable the Perforce plugin for rust workspaces.