refactor(completion): expression based variable/cell_path completion (nushell#15033)

# Description

fixes nushell#14643 , as well as some nested cell path cases:

```nushell
let foo = {a: [1 {a: 1}]}

$foo.a.1.#<tab>

const bar = {a: 1, b: 2}
$bar.#<tab>
```

So my plan of the refactoring process is that:
1. gradually move those rules of flattened shapes into expression match
branches, until they are gone
2. keep each PR focused, easier to review and track. 

# User-Facing Changes

# Tests + Formatting

+2

# After Submitting

Author: blindFS

crates/nu-cli/src/completions/cell_path_completions.rs

@@ -0,0 +1,97 @@
+use crate::completions::{Completer, CompletionOptions, SemanticSuggestion, SuggestionKind};
+use nu_engine::{column::get_columns, eval_variable};
+use nu_protocol::{
+    ast::{Expr, FullCellPath, PathMember},
+    engine::{Stack, StateWorkingSet},
+    eval_const::eval_constant,
+    Span, Value,
+};
+use reedline::Suggestion;
+
+use super::completion_options::NuMatcher;
+
+pub struct CellPathCompletion<'a> {
+    pub full_cell_path: &'a FullCellPath,
+}
+
+impl Completer for CellPathCompletion<'_> {
+    fn fetch(
+        &mut self,
+        working_set: &StateWorkingSet,
+        stack: &Stack,
+        _prefix: &[u8],
+        _span: Span,
+        offset: usize,
+        _pos: usize,
+        options: &CompletionOptions,
+    ) -> Vec<SemanticSuggestion> {
+        // empty tail is already handled as variable names completion
+        let Some((prefix_member, path_members)) = self.full_cell_path.tail.split_last() else {
+            return vec![];
+        };
+        let (mut prefix_str, span) = match prefix_member {
+            PathMember::String { val, span, .. } => (val.clone(), span),
+            PathMember::Int { val, span, .. } => (val.to_string(), span),
+        };
+        // strip the placeholder
+        prefix_str.pop();
+        let true_end = std::cmp::max(span.start, span.end - 1);
+        let span = Span::new(span.start, true_end);
+        let current_span = reedline::Span {
+            start: span.start - offset,
+            end: true_end - offset,
+        };
+
+        let mut matcher = NuMatcher::new(prefix_str, options.clone());
+
+        // evaluate the head expression to get its value
+        let value = if let Expr::Var(var_id) = self.full_cell_path.head.expr {
+            working_set
+                .get_variable(var_id)
+                .const_val
+                .to_owned()
+                .or_else(|| eval_variable(working_set.permanent_state, stack, var_id, span).ok())
+        } else {
+            eval_constant(working_set, &self.full_cell_path.head).ok()
+        }
+        .unwrap_or_default();
+
+        for suggestion in nested_suggestions(&value, path_members, current_span) {
+            matcher.add_semantic_suggestion(suggestion);
+        }
+        matcher.results()
+    }
+}
+
+// Find recursively the values for cell_path
+fn nested_suggestions(
+    val: &Value,
+    path_members: &[PathMember],
+    current_span: reedline::Span,
+) -> Vec<SemanticSuggestion> {
+    let value = val
+        .clone()
+        .follow_cell_path(path_members, false)
+        .unwrap_or_default();
+
+    let kind = SuggestionKind::Type(value.get_type());
+    let str_to_suggestion = |s: String| SemanticSuggestion {
+        suggestion: Suggestion {
+            value: s,
+            span: current_span,
+            ..Suggestion::default()
+        },
+        kind: Some(kind.to_owned()),
+    };
+    match value {
+        Value::Record { val, .. } => val
+            .columns()
+            .map(|s| str_to_suggestion(s.to_string()))
+            .collect(),
+        Value::List { vals, .. } => get_columns(vals.as_slice())
+            .into_iter()
+            .map(str_to_suggestion)
+            .collect(),
+        _ => vec![],
+    }
+}

crates/nu-cli/src/completions/completer.rs

@@ -1,6 +1,7 @@
 use crate::completions::{
-    CommandCompletion, Completer, CompletionOptions, CustomCompletion, DirectoryCompletion,
-    DotNuCompletion, FileCompletion, FlagCompletion, OperatorCompletion, VariableCompletion,
+    CellPathCompletion, CommandCompletion, Completer, CompletionOptions, CustomCompletion,
+    DirectoryCompletion, DotNuCompletion, FileCompletion, FlagCompletion, OperatorCompletion,
+    VariableCompletion,
 };
 use log::debug;
 use nu_color_config::{color_record_to_nustyle, lookup_ansi_color_style};
@@ -17,6 +18,10 @@ use std::{str, sync::Arc};
 
 use super::base::{SemanticSuggestion, SuggestionKind};
 
+/// Used as the function `f` in find_map Traverse
+///
+/// returns the inner-most pipeline_element of interest
+/// i.e. the one that contains given position and needs completion
 fn find_pipeline_element_by_position<'a>(
     expr: &'a Expression,
     working_set: &'a StateWorkingSet,
@@ -62,6 +67,15 @@ fn find_pipeline_element_by_position<'a>(
     }
 }
 
+/// Before completion, an additional character `a` is added to the source as a placeholder for correct parsing results.
+/// This function helps to strip it
+fn strip_placeholder<'a>(working_set: &'a StateWorkingSet, span: &Span) -> (Span, &'a [u8]) {
+    let new_end = std::cmp::max(span.end - 1, span.start);
+    let new_span = Span::new(span.start, new_end);
+    let prefix = working_set.get_span_contents(new_span);
+    (new_span, prefix)
+}
+
 #[derive(Clone)]
 pub struct NuCompleter {
     engine_state: Arc<EngineState>,
@@ -80,6 +94,28 @@ impl NuCompleter {
         self.completion_helper(line, pos)
     }
 
+    fn variable_names_completion_helper(
+        &self,
+        working_set: &StateWorkingSet,
+        span: Span,
+        offset: usize,
+    ) -> Vec<SemanticSuggestion> {
+        let (new_span, prefix) = strip_placeholder(working_set, &span);
+        if !prefix.starts_with(b"$") {
+            return vec![];
+        }
+        let mut variable_names_completer = VariableCompletion {};
+        self.process_completion(
+            &mut variable_names_completer,
+            working_set,
+            prefix,
+            new_span,
+            offset,
+            // pos is not required
+            0,
+        )
+    }
+
     // Process the completion for a given completer
     fn process_completion<T: Completer>(
         &self,
@@ -193,6 +229,37 @@ impl NuCompleter {
             return vec![];
         };
 
+        match &element_expression.expr {
+            Expr::Var(_) => {
+                return self.variable_names_completion_helper(
+                    &working_set,
+                    element_expression.span,
+                    fake_offset,
+                );
+            }
+            Expr::FullCellPath(full_cell_path) => {
+                // e.g. `$e<tab>` parsed as FullCellPath
+                if full_cell_path.tail.is_empty() {
+                    return self.variable_names_completion_helper(
+                        &working_set,
+                        element_expression.span,
+                        fake_offset,
+                    );
+                } else {
+                    let mut cell_path_completer = CellPathCompletion { full_cell_path };
+                    return self.process_completion(
+                        &mut cell_path_completer,
+                        &working_set,
+                        &[],
+                        element_expression.span,
+                        fake_offset,
+                        pos,
+                    );
+                }
+            }
+            _ => (),
+        }
+
         let flattened = flatten_expression(&working_set, element_expression);
         let mut spans: Vec<String> = vec![];
 
@@ -223,46 +290,13 @@ impl NuCompleter {
 
             // Complete based on the last span
             if is_last_span {
-                // Context variables
-                let most_left_var = most_left_variable(flat_idx, &working_set, flattened.clone());
-
                 // Create a new span
                 let new_span = Span::new(span.start, span.end - 1);
 
                 // Parses the prefix. Completion should look up to the cursor position, not after.
                 let index = pos - span.start;
                 let prefix = &current_span[..index];
 
-                // Variables completion
-                if prefix.starts_with(b"$") || most_left_var.is_some() {
-                    let mut variable_names_completer =
-                        VariableCompletion::new(most_left_var.unwrap_or((vec![], vec![])));
-
-                    let mut variable_completions = self.process_completion(
-                        &mut variable_names_completer,
-                        &working_set,
-                        prefix,
-                        new_span,
-                        fake_offset,
-                        pos,
-                    );
-
-                    let mut variable_operations_completer =
-                        OperatorCompletion::new(element_expression.clone());
-
-                    let mut variable_operations_completions = self.process_completion(
-                        &mut variable_operations_completer,
-                        &working_set,
-                        prefix,
-                        new_span,
-                        fake_offset,
-                        pos,
-                    );
-
-                    variable_completions.append(&mut variable_operations_completions);
-                    return variable_completions;
-                }
-
                 // Flags completion
                 if prefix.starts_with(b"-") {
                     // Try to complete flag internally
@@ -474,56 +508,6 @@ impl ReedlineCompleter for NuCompleter {
     }
 }
 
-// reads the most left variable returning it's name (e.g: $myvar)
-// and the depth (a.b.c)
-fn most_left_variable(
-    idx: usize,
-    working_set: &StateWorkingSet<'_>,
-    flattened: Vec<(Span, FlatShape)>,
-) -> Option<(Vec<u8>, Vec<Vec<u8>>)> {
-    // Reverse items to read the list backwards and truncate
-    // because the only items that matters are the ones before the current index
-    let mut rev = flattened;
-    rev.truncate(idx);
-    rev = rev.into_iter().rev().collect();
-
-    // Store the variables and sub levels found and reverse to correct order
-    let mut variables_found: Vec<Vec<u8>> = vec![];
-    let mut found_var = false;
-    for item in rev.clone() {
-        let result = working_set.get_span_contents(item.0).to_vec();
-
-        match item.1 {
-            FlatShape::Variable(_) => {
-                variables_found.push(result);
-                found_var = true;
-
-                break;
-            }
-            FlatShape::String => {
-                variables_found.push(result);
-            }
-            _ => {
-                break;
-            }
-        }
-    }
-
-    // If most left var was not found
-    if !found_var {
-        return None;
-    }
-
-    // Reverse the order back
-    variables_found = variables_found.into_iter().rev().collect();
-
-    // Extract the variable and the sublevels
-    let var = variables_found.first().unwrap_or(&vec![]).to_vec();
-    let sublevels: Vec<Vec<u8>> = variables_found.into_iter().skip(1).collect();
-
-    Some((var, sublevels))
-}
-
 pub fn map_value_completions<'a>(
     list: impl Iterator<Item = &'a Value>,
     span: Span,

crates/nu-cli/src/completions/mod.rs

@@ -1,4 +1,5 @@
 mod base;
+mod cell_path_completions;
 mod command_completions;
 mod completer;
 mod completion_common;
@@ -12,6 +13,7 @@ mod operator_completions;
 mod variable_completions;
 
 pub use base::{Completer, SemanticSuggestion, SuggestionKind};
+pub use cell_path_completions::CellPathCompletion;
 pub use command_completions::CommandCompletion;
 pub use completer::NuCompleter;
 pub use completion_options::{CompletionOptions, MatchAlgorithm};