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 <john@jrenner.net>

crates/ra_syntax/src/ast/generated/nodes.rs

@@ -1249,6 +1249,7 @@ pub struct PathSegment {
 }
 impl PathSegment {
     pub fn coloncolon_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![::]) }
+    pub fn crate_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![crate]) }
     pub fn l_angle_token(&self) -> Option<SyntaxToken> { support::token(&self.syntax, T![<]) }
     pub fn name_ref(&self) -> Option<NameRef> { support::child(&self.syntax) }
     pub fn type_arg_list(&self) -> Option<TypeArgList> { support::child(&self.syntax) }

crates/ra_syntax/src/validation.rs

@@ -96,6 +96,7 @@ pub(crate) fn validate(root: &SyntaxNode) -> Vec<SyntaxError> {
                 ast::RecordField(it) => validate_numeric_name(it.name_ref(), &mut errors),
                 ast::Visibility(it) => validate_visibility(it, &mut errors),
                 ast::RangeExpr(it) => validate_range_expr(it, &mut errors),
+                ast::PathSegment(it) => validate_crate_keyword_in_path_segment(it, &mut errors),
                 _ => (),
             }
         }
@@ -222,3 +223,41 @@ fn validate_range_expr(expr: ast::RangeExpr, errors: &mut Vec<SyntaxError>) {
         ));
     }
 }
+
+fn validate_crate_keyword_in_path_segment(
+    segment: ast::PathSegment,
+    errors: &mut Vec<SyntaxError>,
+) {
+    const ERR_MSG: &str = "The `crate` keyword is only allowed as the first segment of a path";
+
+    let crate_token = match segment.crate_token() {
+        None => return,
+        Some(it) => it,
+    };
+
+    // Disallow both ::crate and foo::crate
+    let path = segment.parent_path();
+    if segment.coloncolon_token().is_some() || path.qualifier().is_some() {
+        errors.push(SyntaxError::new(ERR_MSG, crate_token.text_range()));
+        return;
+    }
+
+    // We now know that the path variable describes a complete path.
+    // For expressions and types, validation is complete, but we still have
+    // to handle UseItems like this:
+    //      use foo:{crate};
+    // so we crawl upwards looking for any preceding paths on `UseTree`s
+    for node in path.syntax().ancestors().skip(1) {
+        match_ast! {
+            match node {
+                ast::UseTree(it) => if let Some(tree_path) = it.path() {
+                    if tree_path != path {
+                        errors.push(SyntaxError::new(ERR_MSG, crate_token.text_range()));
+                    }
+                },
+                ast::UseTreeList(_it) => continue,
+                _ => return,
+            }
+        };
+    }
+}

crates/ra_syntax/test_data/parser/err/0040_illegal_crate_kw_location.rast

@@ -0,0 +1,76 @@
+SOURCE_FILE@0..83
+  USE_ITEM@0..12
+    USE_KW@0..3 "use"
+    WHITESPACE@3..4 " "
+    USE_TREE@4..11
+      PATH@4..11
+        PATH_SEGMENT@4..11
+          COLON2@4..6 "::"
+          CRATE_KW@6..11 "crate"
+    SEMICOLON@11..12 ";"
+  WHITESPACE@12..13 "\n"
+  USE_ITEM@13..39
+    USE_KW@13..16 "use"
+    WHITESPACE@16..17 " "
+    USE_TREE@17..38
+      USE_TREE_LIST@17..38
+        L_CURLY@17..18 "{"
+        USE_TREE@18..23
+          PATH@18..23
+            PATH_SEGMENT@18..23
+              CRATE_KW@18..23 "crate"
+        COMMA@23..24 ","
+        WHITESPACE@24..25 " "
+        USE_TREE@25..37
+          PATH@25..28
+            PATH_SEGMENT@25..28
+              NAME_REF@25..28
+                IDENT@25..28 "foo"
+          COLON2@28..30 "::"
+          USE_TREE_LIST@30..37
+            L_CURLY@30..31 "{"
+            USE_TREE@31..36
+              PATH@31..36
+                PATH_SEGMENT@31..36
+                  CRATE_KW@31..36 "crate"
+            R_CURLY@36..37 "}"
+        R_CURLY@37..38 "}"
+    SEMICOLON@38..39 ";"
+  WHITESPACE@39..40 "\n"
+  USE_ITEM@40..57
+    USE_KW@40..43 "use"
+    WHITESPACE@43..44 " "
+    USE_TREE@44..56
+      PATH@44..56
+        PATH@44..49
+          PATH_SEGMENT@44..49
+            NAME_REF@44..49
+              IDENT@44..49 "hello"
+        COLON2@49..51 "::"
+        PATH_SEGMENT@51..56
+          CRATE_KW@51..56 "crate"
+    SEMICOLON@56..57 ";"
+  WHITESPACE@57..58 "\n"
+  USE_ITEM@58..82
+    USE_KW@58..61 "use"
+    WHITESPACE@61..62 " "
+    USE_TREE@62..81
+      PATH@62..81
+        PATH@62..74
+          PATH@62..67
+            PATH_SEGMENT@62..67
+              NAME_REF@62..67
+                IDENT@62..67 "hello"
+          COLON2@67..69 "::"
+          PATH_SEGMENT@69..74
+            CRATE_KW@69..74 "crate"
+        COLON2@74..76 "::"
+        PATH_SEGMENT@76..81
+          NAME_REF@76..81
+            IDENT@76..81 "there"
+    SEMICOLON@81..82 ";"
+  WHITESPACE@82..83 "\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

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;

xtask/src/ast_src.rs

@@ -595,7 +595,7 @@ pub(crate) const AST_SRC: AstSrc = AstSrc {
             qualifier: Path,
         }
         struct PathSegment {
-            T![::], T![<], NameRef, TypeArgList, ParamList, RetType, PathType, T![>]
+            T![::], T![crate], T![<], NameRef, TypeArgList, ParamList, RetType, PathType, T![>]
         }
         struct TypeArgList {
             T![::],