Improve type hovers (#9515)

# Description

This PR does a few things to help improve type hovers and, in the
process, fixes a few outstanding issues in the type system. Here's a
list of the changes:

* `for` now will try to infer the type of the iteration variable based
on the expression it's given. This fixes things like `for x in [1, 2, 3]
{ }` where `x` now properly gets the int type.
* Removed old input/output type fields from the signature, focuses on
the vec of signatures. Updated a bunch of dataframe commands that hadn't
moved over. This helps tie things together a bit better
* Fixed inference of types from subexpressions to use the last
expression in the block
* Fixed handling of explicit types in `let` and `mut` calls, so we now
respect that as the authoritative type

I also tried to add `def` input/output type inference, but unfortunately
we only know the predecl types universally, which means we won't have
enough information to properly know what the types of the custom
commands are.

# User-Facing Changes

Script typechecking will get tighter in some cases
Hovers should be more accurate in some cases that previously resorted to
any.

# Tests + Formatting
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---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

crates/nu-protocol/src/ast/block.rs

@@ -1,5 +1,5 @@
-use super::Pipeline;
-use crate::{Signature, Span, VarId};
+use super::{Expr, Expression, Pipeline};
+use crate::{ast::PipelineElement, engine::StateWorkingSet, Signature, Span, Type, VarId};
 use serde::{Deserialize, Serialize};
 use std::ops::{Index, IndexMut};
 
@@ -65,6 +65,56 @@ impl Block {
             recursive: None,
         }
     }
+
+    pub fn input_type(&self, working_set: &StateWorkingSet) -> Type {
+        if let Some(first) = self.pipelines.first() {
+            if let Some(first) = first.elements.first() {
+                match first {
+                    PipelineElement::Expression(
+                        _,
+                        Expression {
+                            expr: Expr::Call(call),
+                            ..
+                        },
+                    ) => {
+                        let decl = working_set.get_decl(call.decl_id);
+
+                        decl.signature().get_input_type()
+                    }
+                    PipelineElement::Expression(
+                        _,
+                        Expression {
+                            expr: Expr::ExternalCall(..),
+                            ..
+                        },
+                    ) => Type::Any,
+                    _ => Type::Nothing,
+                }
+            } else {
+                Type::Nothing
+            }
+        } else {
+            Type::Nothing
+        }
+    }
+    pub fn output_type(&self) -> Type {
+        if let Some(last) = self.pipelines.last() {
+            if let Some(last) = last.elements.last() {
+                match last {
+                    PipelineElement::Expression(_, expr) => expr.ty.clone(),
+                    PipelineElement::Redirection(_, _, _) => Type::Any,
+                    PipelineElement::SeparateRedirection { .. } => Type::Any,
+                    PipelineElement::SameTargetRedirection { .. } => Type::Any,
+                    PipelineElement::And(_, expr) => expr.ty.clone(),
+                    PipelineElement::Or(_, expr) => expr.ty.clone(),
+                }
+            } else {
+                Type::Nothing
+            }
+        } else {
+            Type::Nothing
+        }
+    }
 }
 
 impl<T> From<T> for Block

crates/nu-protocol/src/engine/engine_state.rs

@@ -981,7 +981,7 @@ impl TypeScope {
     pub fn get_previous(&self) -> &Type {
         match self.outputs.last().and_then(|v| v.last()) {
             Some(input) => input,
-            None => &Type::Any,
+            None => &Type::Nothing,
         }
     }
 
@@ -1193,7 +1193,7 @@ impl<'a> StateWorkingSet<'a> {
 
     pub fn add_decl(&mut self, decl: Box<dyn Command>) -> DeclId {
         let name = decl.name().as_bytes().to_vec();
-        let input_type = decl.signature().input_type;
+        let input_type = decl.signature().get_input_type();
 
         self.delta.decls.push(decl);
         let decl_id = self.num_decls() - 1;

crates/nu-protocol/src/engine/overlay.rs

@@ -214,7 +214,23 @@ impl OverlayFrame {
         if let Some(decl) = self.decls.get(&(name, input) as &dyn DeclKey) {
             Some(*decl)
         } else {
-            self.decls.get(&(name, &Type::Any) as &dyn DeclKey).cloned()
+            // then fallback to not using the input type
+            for decl_key in self.decls.keys() {
+                if decl_key.0 == name {
+                    // FIXME: this fallback may give bad type information
+                    // in the case where no matching type is found. But, at
+                    // least we treat it as a found internal command rather
+                    // than an external command, which would cause further issues
+                    return Some(
+                        *self
+                            .decls
+                            .get(decl_key)
+                            .expect("internal error: found decl not actually found"),
+                    );
+                }
+            }
+
+            None
         }
     }
 }

crates/nu-protocol/src/plugin_signature.rs

@@ -138,14 +138,8 @@ impl PluginSignature {
     }
 
     /// Changes the input type of the command signature
-    pub fn input_type(mut self, input_type: Type) -> PluginSignature {
-        self.sig = self.sig.input_type(input_type);
-        self
-    }
-
-    /// Changes the output type of the command signature
-    pub fn output_type(mut self, output_type: Type) -> PluginSignature {
-        self.sig = self.sig.output_type(output_type);
+    pub fn input_output_type(mut self, input_type: Type, output_type: Type) -> PluginSignature {
+        self.sig.input_output_types.push((input_type, output_type));
         self
     }
 

crates/nu-protocol/src/signature.rs

@@ -114,8 +114,6 @@ pub struct Signature {
     pub rest_positional: Option<PositionalArg>,
     pub vectorizes_over_list: bool,
     pub named: Vec<Flag>,
-    pub input_type: Type,
-    pub output_type: Type,
     pub input_output_types: Vec<(Type, Type)>,
     pub allow_variants_without_examples: bool,
     pub is_filter: bool,
@@ -215,8 +213,6 @@ impl Signature {
             optional_positional: vec![],
             rest_positional: None,
             vectorizes_over_list: false,
-            input_type: Type::Any,
-            output_type: Type::Any,
             input_output_types: vec![],
             allow_variants_without_examples: false,
             named: vec![],
@@ -227,6 +223,46 @@ impl Signature {
         }
     }
 
+    // Gets the input type from the signature
+    pub fn get_input_type(&self) -> Type {
+        match self.input_output_types.len() {
+            0 => Type::Any,
+            1 => self.input_output_types[0].0.clone(),
+            _ => {
+                let first = &self.input_output_types[0].0;
+                if self
+                    .input_output_types
+                    .iter()
+                    .all(|(input, _)| input == first)
+                {
+                    first.clone()
+                } else {
+                    Type::Any
+                }
+            }
+        }
+    }
+
+    // Gets the output type from the signature
+    pub fn get_output_type(&self) -> Type {
+        match self.input_output_types.len() {
+            0 => Type::Any,
+            1 => self.input_output_types[0].1.clone(),
+            _ => {
+                let first = &self.input_output_types[0].1;
+                if self
+                    .input_output_types
+                    .iter()
+                    .all(|(_, output)| output == first)
+                {
+                    first.clone()
+                } else {
+                    Type::Any
+                }
+            }
+        }
+    }
+
     // Add a default help option to a signature
     pub fn add_help(mut self) -> Signature {
         // default help flag
@@ -423,14 +459,8 @@ impl Signature {
     }
 
     /// Changes the input type of the command signature
-    pub fn input_type(mut self, input_type: Type) -> Signature {
-        self.input_type = input_type;
-        self
-    }
-
-    /// Changes the output type of the command signature
-    pub fn output_type(mut self, output_type: Type) -> Signature {
-        self.output_type = output_type;
+    pub fn input_output_type(mut self, input_type: Type, output_type: Type) -> Signature {
+        self.input_output_types.push((input_type, output_type));
         self
     }