Merge #4178

4178: Validate the location of `crate` in paths r=matklad a=djrenren

**This solution does not fully handle `use` statements. See below**

This pull requests implements simple validation of usages of the `crate` keyword in `Path`s. Specifically it validates that:

- If a `PathSegment` is starts with the `crate` keyword, it is also the first segment of the `Path`
- All other usages of `crate` in `Path`s are considered errors.

This aligns with `rustc`'s rules. Unlike rustc this implementation does not issue a special error message in the case of `::crate` but it does catch the error.

Furthermore, this change does not cover all error cases. Specifically the following is not caught:

```rust
use foo::{crate}
```

This is because this check is context sensitive. From an AST perspective, `crate` is the root of the `Path`. Only by inspecting the full `UseItem` do we see that it is not in fact the root. This problem becomes worse because `UseTree`s are allowed to be arbitrarily nested:

```rust
use {crate, {{crate, foo::{crate}}}
```

So this is a hard problem to solve without essentially a breadth-first search. In a traditional compiler, I'd say this error is most easily found during the AST -> HIR conversion pass but within rust-analyzer I'm not sure where it belongs.  

Under the implementation in this PR, such errors are ignored so we're *more correct* just not *entirely correct*. 

Co-authored-by: John Renner <[email protected]>

2 files changed, 80 insertions, 0 deletions

diff --git a/crates/ra_syntax/test_data/parser/err/0040_illegal_crate_kw_location.rast b/crates/ra_syntax/test_data/parser/err/0040_illegal_crate_kw_location.rast
new file mode 100644
index 000000000..8306f7361
--- /dev/null
+++ b/crates/ra_syntax/test_data/parser/err/0040_illegal_crate_kw_location.rast
@@ -0,0 +1,76 @@
+[email protected]
+  [email protected]
+    [email protected] "use"
+    [email protected] " "
+    [email protected]
+      [email protected]
+        [email protected]
+          [email protected] "::"
+          [email protected] "crate"
+    [email protected] ";"
+  [email protected] "\n"
+  [email protected]
+    [email protected] "use"
+    [email protected] " "
+    [email protected]
+      [email protected]
+        [email protected] "{"
+        [email protected]
+          [email protected]
+            [email protected]
+              [email protected] "crate"
+        [email protected] ","
+        [email protected] " "
+        [email protected]
+          [email protected]
+            [email protected]
+              [email protected]
+                [email protected] "foo"
+          [email protected] "::"
+          [email protected]
+            [email protected] "{"
+            [email protected]
+              [email protected]
+                [email protected]
+                  [email protected] "crate"
+            [email protected] "}"
+        [email protected] "}"
+    [email protected] ";"
+  [email protected] "\n"
+  [email protected]
+    [email protected] "use"
+    [email protected] " "
+    [email protected]
+      [email protected]
+        [email protected]
+          [email protected]
+            [email protected]
+              [email protected] "hello"
+        [email protected] "::"
+        [email protected]
+          [email protected] "crate"
+    [email protected] ";"
+  [email protected] "\n"
+  [email protected]
+    [email protected] "use"
+    [email protected] " "
+    [email protected]
+      [email protected]
+        [email protected]
+          [email protected]
+            [email protected]
+              [email protected]
+                [email protected] "hello"
+          [email protected] "::"
+          [email protected]
+            [email protected] "crate"
+        [email protected] "::"
+        [email protected]
+          [email protected]
+            [email protected] "there"
+    [email protected] ";"
+  [email protected] "\n"
+error 6..11: The `crate` keyword is only allowed as the first segment of a path
+error 31..36: The `crate` keyword is only allowed as the first segment of a path
+error 51..56: The `crate` keyword is only allowed as the first segment of a path
+error 69..74: The `crate` keyword is only allowed as the first segment of a path
diff --git a/crates/ra_syntax/test_data/parser/err/0040_illegal_crate_kw_location.rs b/crates/ra_syntax/test_data/parser/err/0040_illegal_crate_kw_location.rs
new file mode 100644
index 000000000..bead4c0b6
--- /dev/null
+++ b/crates/ra_syntax/test_data/parser/err/0040_illegal_crate_kw_location.rs
@@ -0,0 +1,4 @@
+use ::crate;
+use {crate, foo::{crate}};
+use hello::crate;
+use hello::crate::there;