rust-analyzer for non-Cargo projects (#4118)

* rust-analyzer in non-cargo projects

rust-analyzer can also be used in non-cargo projects. This requires a
rust-project.json file in the project root [1].

Make the rust-analyzer linter search for a rust-project.json file if no
Cargo.toml file could be found.

[1]: https://rust-analyzer.github.io/manual.html#non-cargo-based-projects

* Document rust-analyzer without cargo

* Test rust-analyzer with non-cargo projects

Change the other rust tests to match the new directory structure of the
test files.

test/linter/test_rust_analyzer.vader

@@ -7,12 +7,19 @@ After:
 Execute(The default executable path should be correct):
   AssertLinter 'rust-analyzer', ale#Escape('rust-analyzer')
 
-Execute(The project root should be detected correctly):
+Execute(The project root should be detected correctly in cargo projects):
   AssertLSPProject ''
 
-  call ale#test#SetFilename('../test-files/rust/test.rs')
+  call ale#test#SetFilename('../test-files/rust/cargo/testfile.rs')
 
-  AssertLSPProject ale#path#Simplify(g:dir . '/../test-files/rust')
+  AssertLSPProject ale#path#Simplify(g:dir . '/../test-files/rust/cargo')
+
+Execute(The project root should be detected correctly in non-cargo projects):
+  AssertLSPProject ''
+
+  call ale#test#SetFilename('../test-files/rust/rust-project/testfile.rs')
+
+  AssertLSPProject ale#path#Simplify(g:dir . '/../test-files/rust/rust-project')
 
 Execute(Should accept configuration settings):
   AssertLSPConfig {}

test/linter/test_rust_rls.vader

@@ -20,9 +20,9 @@ Execute(The toolchain should be ommitted if not given):
 Execute(The project root should be detected correctly):
   AssertLSPProject ''
 
-  call ale#test#SetFilename('../test-files/rust/test.rs')
+  call ale#test#SetFilename('../test-files/rust/cargo/testfile.rs')
 
-  AssertLSPProject ale#path#Simplify(g:dir . '/../test-files/rust')
+  AssertLSPProject ale#path#Simplify(g:dir . '/../test-files/rust/cargo')
 
 Execute(Should accept configuration settings):
   AssertLSPConfig {}

test/linter/test_rustc.vader

@@ -13,9 +13,9 @@ Execute(The options should be configurable):
   AssertLinter 'rustc', 'rustc --error-format=json --foo -'
 
 Execute(Some default paths should be included when the project is a Cargo project):
-  call ale#test#SetFilename('../test-files/cargo/test.rs')
+  call ale#test#SetFilename('../test-files/rust/cargo/testfile.rs')
 
   AssertLinter 'rustc', 'rustc --error-format=json --emit=mir -o /dev/null'
-  \   . ' -L ' . ale#Escape(ale#path#GetAbsPath(g:dir,  '../test-files/cargo/target/debug/deps'))
-  \   . ' -L ' . ale#Escape(ale#path#GetAbsPath(g:dir, '../test-files/cargo/target/release/deps'))
+  \   . ' -L ' . ale#Escape(ale#path#GetAbsPath(g:dir,  '../test-files/rust/cargo/target/debug/deps'))
+  \   . ' -L ' . ale#Escape(ale#path#GetAbsPath(g:dir, '../test-files/rust/cargo/target/release/deps'))
   \   . ' -'