Add method to convert ClassifiedBlock into completion locations. (#2316)

The completion engine maps completion locations to spans on a line, which
indicate whther to complete a command name, flag name, argument, and so on.

Initial implementation is simplistic, with some rough edges, since it relies
heavily on the parser's interpretation. For example

    du -

if asking for completions, `-` is considered a positional argument by the
parser, but the user is likely looking for a flag. These scenarios will be
addressed in a series of progressive enhancements to the engine.

crates/nu-parser/src/errors.rs

@@ -4,10 +4,10 @@ use std::fmt::Debug;
 #[derive(Debug)]
 pub struct ParseError<T: Debug> {
     /// An informative cause for this parse error
-    pub(crate) cause: nu_errors::ParseError,
+    pub cause: nu_errors::ParseError,
 
     /// What has been successfully parsed, if anything
-    pub(crate) partial: Option<T>,
+    pub partial: Option<T>,
 }
 
 pub type ParseResult<T> = Result<T, ParseError<T>>;

crates/nu-parser/src/lite_parse.rs

@@ -133,11 +133,6 @@ fn bare(src: &mut Input, span_offset: usize) -> ParseResult<Spanned<String>> {
         // correct information from the non-lite parse.
         bare.push(delimiter);
 
-        let span = Span::new(
-            start_offset + span_offset,
-            start_offset + span_offset + bare.len(),
-        );
-
         return Err(ParseError {
             cause: nu_errors::ParseError::unexpected_eof(delimiter.to_string(), span),
             partial: Some(bare.spanned(span)),

crates/nu-parser/src/parse.rs

@@ -1358,7 +1358,7 @@ fn classify_pipeline(
                     }),
                     positional: Some(args),
                     named: None,
-                    span: Span::unknown(),
+                    span: name_span,
                     external_redirection: if iter.peek().is_none() {
                         ExternalRedirection::None
                     } else {
@@ -1448,7 +1448,7 @@ fn classify_pipeline(
                     }),
                     positional: Some(args),
                     named: None,
-                    span: Span::unknown(),
+                    span: name_span,
                     external_redirection: if iter.peek().is_none() {
                         ExternalRedirection::None
                     } else {